INTRODUCTION · 2019-11-19 · Press tools Heavy duty presses for bulk metal formation such as sheet metal, forging, bending, punching, etc. Printing industry For paper feeding, packaging

Department of Mechanical Engineering Prepared By: Jainik Makwana Darshan Institute of Engineering & Technology, Rajkot Page 1.1

1 INTRODUCTION

Course Contents

1.1 Introduction

1.2 Fluid power and its scope

1.3 Classification of fluid power systems

1.4 Hydrostatic and hydrodynamic

systems

1.5 Advantages of a fluid power system

1.6 General layout of hydraulic system

1.7 Advantages of a hydraulic system

1.8 Disadvantages of a hydraulic system

1.9 Applications of hydraulic systems

1.10 Principles of hydraulic fluid power

1.11 Basic electrical devices

Oil Hydraulics and Pneumatics (2171912) 1. Introduction


1.1 INTRODUCTION

In the industry we use three methods for transmitting power from one point to another.

Mechanical transmission is through shafts, gears, chains, belts, etc. Electrical transmission is

through wires, transformers, etc. Fluid power is through liquids or gas in a confined space. In

this chapter, we shall discuss a structure of hydraulic systems and pneumatic systems. We

will also discuss the advantages and disadvantages and compare hydraulic, pneumatic,

electrical and mechanical systems.

1.2 FLUID POWER AND ITS SCOPE

− Fluid power is the technology that deals with the generation, control and transmission of

forces and movement of mechanical element or system with the use of pressurized fluids in

a confined system. Both liquids and gases are considered fluids. Fluid power system

includes a hydraulic system (hydra meaning water in Greek) and a pneumatic system

(pneuma meaning air in Greek). Oil hydraulic employs pressurized liquid petroleum oils

and synthetic oils, and pneumatic employs compressed air that is released to the atmosphere

after performing the work.

− Perhaps it would be in order that we clarify our thinking on one point. By the term “fluid”

we refer to air or oil, for it has been shown that water has certain drawbacks in the

transmission of hydraulic power in machine operation and control. Commercially, pure

water contains various chemicals (some deliberately included) and also foreign matter, and

unless special precautions are taken when it is used, it is nearly impossible to maintain

valves and working surfaces in satisfactory condition. In the cases where the hydraulic

system is closed (i.e., the one with a self-contained unit that serves one machine or one

small group of machines), oil is commonly used, thus providing, in addition to power

transmission, benefits of lubrication not afforded by water as well as increased life and

efficiency of packings and valves. It should be mentioned that in some special cases, soluble

oil diluted with water is used for safety reasons. The application of fluid power is limited

only by the ingenuity of the designer, production engineer or plant engineer. If the

application pertains to lifting, pushing, pulling, clamping, tilting, forcing, pressing or any

other straight line (and many rotary) motions, it is possible that fluid power will meet the

requirement.

− Fluid power applications can be classified into two major segments:

Stationary hydraulics: Stationary hydraulic systems remain firmly fixed in one position.

The characteristic feature of stationary hydraulics is that valves are mainly solenoid operated.

The applications of stationary hydraulics are as follows:

Production and assembly of vehicles of all types.

Machine tools and transfer lines.

Lifting and conveying devices.

Metal-forming presses.

Plastic machinery such as injection-molding machines.

Rolling machines.

Lifts.



Food processing machinery.

Automatic handling equipment and robots.

Mobile hydraulics: Mobile hydraulic systems move on wheels or tracks such as a tower

crane or excavator truck to operate in many different locations or while moving. A

characteristic feature of mobile hydraulics is that the valves are frequently manually operated.

The applications of mobile hydraulics are as follows:

Automobiles, tractors, aeroplanes, missile, boats, etc.

Construction machinery.

Tippers, excavators and elevating platforms.

Lifting and conveying devices.

Agricultural machinery.

− Hydraulics and pneumatics have almost unlimited application in the production of goods

and services in nearly all sectors of the country. Several industries are dependent on the

capabilities that fluid power affords. Table 1.1 summarizes few applications of fluid power.

Table 1.1 - More applications of fluid power

Agriculture Tractors; farm equipment such as

mowers, ploughs,

chemical and water sprayers, fertilizer

spreaders, harvesters

Automation Automated transfer lines, robotics

Automobiles Power steering, power brakes,

suspension systems,

hydrostatic transmission

Aviation Fluid power equipment such as

landing wheels in aircraft.

Helicopters, aircraft trolleys, aircraft

test beds, luggage loading and

unloading systems, ailerons, aircraft

servicing, flight simulators

Construction

industry/equipment

For metering and mixing of concrete

rudders, excavators,

lifts, bucket loaders, crawlers, post-

hole diggers, road graders, road

cleaners, road maintenance vehicles,

tippers

Defense Missile-launching systems, navigation

controls

Entertainment Amusement park entertainment rides

such as roller coasters

Fabrication industry Hand tools such as pneumatic drills,

grinders, borers,

riveting machines, nut runners

Food and beverage All types of food processing

equipment, wrapping, bottling,

Foundry Full and semi-automatic molding

machines, tilting of furnaces, die-



casting machines

Glass industry Vacuum suction cups for handling

Instrumentation Used to create/operate complex

instruments in space

rockets, gas turbines, nuclear power

plants, industrial labs

Jigs and fixtures Work holding devices, clamps,

stoppers, indexers

Machine tools Automated machine tools,

numerically controlled(NC)

machine tools

Materials handling Jacks, hoists, cranes, forklifts,

conveyor systems

Medical Medical equipment such as breathing

assistors, heart assist

devices, cardiac compression

machines, dental drives and human

patient simulator

Movies Special-effect equipment use fluid

power; movies such as

Jurassic park, Jaws, Anaconda,

Titanic

Mining Rock drills, excavating equipment,

ore conveyors, loaders

Newspapers and periodicals Edge trimming, stapling, pressing,

bundle wrapping

Oil industry Off-shore oil rigs

Paper and packaging Process control systems, special-

purpose machines for

rolling and packing

Pharmaceuticals Process control systems such as bottle

filling, tablet placement, packaging

Plastic industry Automatic injection molding

machines, raw material

feeding, jaw closing, movement of

slides of blow molder

Press tools Heavy duty presses for bulk metal

formation such as sheet

metal, forging, bending, punching,

etc.

Printing industry For paper feeding, packaging

Robots Fluid power operated robots,

pneumatic systems

Ships Stabilizing systems, unloading and

loading unit, gyroscopic

instruments, movement of flat forms,

lifters, subsea inspection equipment

Textiles Web tensioning devices, trolleys,

process controllers



Transportation Hydraulic elevators, winches,

overhead trams

Under sea Submarines, under sea research

vehicles, marine drives and control of

ships

Wood working Tree shearers, handling huge logs,

feeding clamping and

saw operations

− The following are the two types of hydraulic systems:

1. Fluid transport systems: Their sole objective is the delivery of a fluid from one location

to another to accomplish some useful purpose. Examples include pumping stations for

pumping water to homes, cross-country gas lines, etc.

2. Fluid power systems: These are designed to perform work. In fluid power systems, work

is obtained by pressurized fluid acting directly on a fluid cylinder or a fluid motor. A cylinder

produces a force resulting in linear motion, whereas a fluid motor produces a torque resulting

in rotary motion.

1.3 CLASSIFICATION OF FLUID POWER SYSTEMS

The fluid power system can be categorized as follows:

1. Based on the control system

Open-loop system: There is no feedback in the open system and performance is

based on the characteristics of the individual components of the system. The open-

loop system is not accurate and error can be reduced by proper calibration and

control.

Closed-loop system: This system uses feedback. The output of the system is fed back

to a comparator by a measuring element. The comparator compares the actual output

to the desired output and gives an error signal to the control element. The error is used

to change the actual output and bring it closer to the desired value. A simple closed-

loop system uses servo valves and an advanced system uses digital electronics.

2. Based on the type of control

Fluid logic control: This type of system is controlled by hydraulic oil or air. The system

employs fluid logic devices such as AND, NAND, OR, NOR, etc. Two types of fluid logic

systems are available:

(a) Moving part logic (MPL): These devices are miniature fluid elements using

moving parts such as diaphragms, disks and poppets to implement various logic gates.

(b) Fluidics: Fluid devices contain no moving parts and depend solely on interacting

fluid jets to implement various logic gates.

Electrical control: This type of system is controlled by electrical devices. Four basic

electrical devices are used for controlling the fluid power systems: switches, relays,



timers and solenoids. These devices help to control the starting, stopping, sequencing,

speed, positioning, timing and reversing of actuating cylinders and fluid motors.

Electrical control and fluid power work well together where remote control is

essential.

Electronic control: This type of system is controlled by microelectronic devices. The

electronic brain is used to control the fluid power muscles for doing work. This

system uses the most advanced type of electronic hardware including programmable

logic control (PLC) or microprocessor (P). In the electrical control, a change in

system operation results in a cumbersome process of redoing hardware connections.

The difficulty is overcome by programmable electronic control. The program can be

modified or a new program can be fed to meet the change of operations. A number of

such programs can be stored in these devices, which makes the systems more flexible.

1.4 HYDROSTATIC AND HYDRODYNAMIC SYSTEMS

− A hydrostatic system uses fluid pressure to transmit power. Hydrostatics deals with the

mechanics of still fluids and uses the theory of equilibrium conditions in fluid. The system

creates high pressure, and through a transmission line and a control element, this pressure

drives an actuator (linear or rotational).

− The pump used in hydrostatic systems is a positive displacement pump. The relative spatial

position of this pump is arbitrary but should not be very large due to losses (must be less

than 50 m). An example of pure hydrostatics is the transfer of force in hydraulics.

− Hydrodynamic systems use fluid motion to transmit power. Power is transmitted by the

kinetic energy of the fluid. Hydrodynamics deals with the mechanics of moving fluid and

uses flow theory. The pump used in hydrodynamic systems is a non-positive displacement

pump.

− The relative spatial position of the prime mover (e.g., turbine) is fixed. An example of pure

hydrodynamics is the conversion of flow energy in turbines in hydroelectric power plants.

− In oil hydraulics, we deal mostly with the fluid working in a confined system, that is, a

hydrostatic system.

1.5 ADVANTAGES OF A FLUID POWER SYSTEM

Oil hydraulics stands out as the prime moving force in machinery and equipment designed to

handle medium to heavy loads. In the early stages of industrial development, mechanical

linkages were used along with prime movers such as electrical motors and engines for

handling loads. But the mechanical efficiency of linkages was very low and the linkages

often failed under critical loading conditions. With the advent of fluid power technology and

associated electronics and control, it is used in every industry now.

− The advantages of a fluid power system are as follows:

1. Fluid power systems are simple, easy to operate and can be controlled accurately:

Fluid power gives flexibility to equipment without requiring a complex mechanism. Using

fluid power, we can start, stop, accelerate, decelerate, reverse or position large



forces/components with great accuracy using simple levers and push buttons. For example, in

earth-moving equipment, bucket carrying load can be raised or lowered by an operator using

a lever. The landing gear of an aircraft can be retrieved to home position by the push button.

2. Multiplication and variation of forces: Linear or rotary force can be multiplied by a

fraction of a kilogram to several hundreds of tons.

3. Multifunction control: A single hydraulic pump or air compressor can provide power and

control for numerous machines using valve manifolds and distribution systems. The fluid

power controls can be placed at a central station so that the operator has, at all times, a

complete control of the entire production line, whether it be a multiple operation machine or a

group of machines. Such a setup is more or less standard in the steel mill industry.

4. Low-speed torque: Unlike electric motors, air or hydraulic motors can produce a large

amount of torque while operating at low speeds. Some hydraulic and pneumatic motors can

even maintain torque at a very slow speed without overheating.

5. Constant force or torque: Fluid power systems can deliver constant torque or force

regardless of speed changes.

6. Economical: Not only reduction in required manpower but also the production or

elimination of operator fatigue, as a production factor, is an important element in the use of

fluid power.

7. Low weight to power ratio: The hydraulic system has a low weight to power ratio

compared to electromechanical systems. Fluid power systems are compact.

8. Fluid power systems can be used where safety is of vital importance: Safety is of vital

importance in air and space travel, in the production and operation of motor vehicles, in

mining and manufacture of delicate products. For example, hydraulic systems are responsible

for the safety of take-off, landing and flight of aeroplanes and space craft. Rapid advances in

mining and tunnelling are the results of the application of modern hydraulic and pneumatic

systems.

1.6 GENERAL LAYOUT OF HYDRAULIC SYSTEM

1.6.1 Components of Hydraulic System

− Basic hydraulic system has the following components:

1) Oil reservoir 2) Rotary pump

3) Pressure relief valve 4) Direction control valve

5) Flow control valve 6) Double acting cylinder

7) Pressure gauge 8) Filter

1) Oil Reservoir

− Main function of "oil reservoir" is to store sufficient amount of hydraulic oil in the system.

− Apart from this, it has other important functions such as:

(a) To cool the hot return oil.

(b) To settle down the contaminants.



Figure 1.1 - Layout of general hydraulic system

(c) To remove air bubbles.

(d) To separate water from the oil etc.

2) Rotary pump

− The function of rotary pump is to pump hydraulic oil to the hydraulic circuit'

− It converts the mechanical energy (rotation of shaft) into hydraulic energy

− Rotary pump is a positive displacement pump. It can deliver constant flow even at high

pressure

3) Pressure relief valve

− It is an important component which is required for every positive displacement pump

− This valve is connected at the outlet of pump. Its main function is to release the oil back

tank when the pressure increases beyond pre-set value.

4) Direction control valve

− It controls the direction of flow of oil, by which it performs extension and retraction of

actuator

5) Flow control valve

− It Controls the rate of flow of oil by which speed of extension or retraction of actuator is

controlled.

6) Actuator

− Actuator produces work. There are two types, linear actuator and rotary actuator

− Linear actuator is called cylinder, rotary actuator is called motor.



− Double acting cylinder develops force and motion. It converts hydraulic energy in to

mechanical energy

Force developed = Pressure of oil x Area of piston

7) Pressure gauge

− It is an important component of hydraulic system.

− It shows the pressure reading.

− Pressure settings are made by looking to the pressure gauge.

− Without pressure gauge, it is not possible to make the pressure relief valve setting,

unloading valve settings etc.

8) Filter

− Its main function is to remove suspended solid contaminates from the oil and to provide

clean hydraulic oil to the system.

1.7 ADVANTAGES OF A HYDRAULIC SYSTEM

The basic advantages offered by a hydraulic system are as follows:

l. Hydraulic power is easy to produce, transmit, store, regulate and control, maintain and

transform

2. Weight to power ratio of a hydraulic system is comparatively less than that for an electro-

mechanical system. (About 8.5 kg/kw for electrical motors and 0.g5 kg/kw for a hydro

system).

3. It is possible to generate high gain in force and power amplification.

4. Hydraulic systems are uniform and smooth, generate step less motion and variable speed

and force to a greater accuracy.

5. Division and distribution of hydraulic power is simpler and easier than other forms of

energy.

6. Limiting and balancing of hydraulic forces are easily performed.

7. Frictional resistance is much less in a hydraulic system as compared to a mechanical

movement.

8. Hydraulic elements can be located at any place and controlled reversely.

9. The noise and vibration produced by hydraulic pumps is minimal.

10. Hydraulic systems are cheaper if one considers the high efficiency -of power

transmission.

I l. Easy maintenance of hydraulic system is another advantage.

12. Hydraulics is mechanically safe, compact and is adaptable-to other forms of power and

can be easily controlled.

13. Hydraulic output can be both linear, rotational and angular. Use of flexible connection in

hydraulic system permits generation of compound motion without gears etc.



14. Hydraulics is a better over-load safe power system. This can be easily achieved by using a

pressure relief valve.

15. Absolutely accurate feedback of load, position, etc. can be achieved in a hydraulic system

as in electro hydraulic and digital electronic servo system. Because of high power and

accurate control possibility, in modem engineering language hydraulics is termed as the

muscle of the system and electronics its nerves.

1.8 DISADVANTAGES OF A HYDRAULIC SYSTEM

In spite of all the above advantages, hydraulic systems have some drawbacks which are

mentioned below.

The disadvantages are:

1. Hydraulic elements have to be machined to a high degree of precision which increases the

manufacturing cost of the system.

2. Certain hydraulic systems are exposed to unfriendly climate and dirty atmosphere as the in

case of mobile hydraulics like dumpers, loaders, etc.

3. Leakage of hydraulic oil poses a problems to hydraulic users.

4. Hydraulic elements have to be specially treated to protect them against rust, corrosion, dirt,

etc.

5. Hydraulic oil may pose problems if it disintegrates due to aging and chemical

deterioration.

6. Petroleum based hydraulic oil may pose fire hazards thus limiting the upper level of

working temperature. However, due to availability of synthetic fire resistant oils this problem

is of academic interest now-a-days. To combat environmental effects of petroleum and

chemical based oils, efforts are on to use bio degradable oils now.

1.9 APPLICATIONS OF HYDRAULIC SYSTEMS

1. Machine tools: CNC (computerized Numerical control) machines, hydraulic presses,

hydraulic shaper, etc.

2. Material handling equipment: Elevators, forklifts, cranes, lifts and hoists erc.

3. Construction field: Earth moving machines such as excavators, cranes, dozers, loaders,

dumpers, tippers, trucks, tractors etc.

4. Automobiles: Hydraulic brakes, hydraulic steering, hydraulic suspension, hydraulic

clutch, hydraulic power transmission, hydraulic coupling,

5. Material testing laboratory: UTM (universal testing machine) and other destructive testing

Machines, BP (burst pressure) testing machine etc.

6. Aerospace: Landing gear, brakes, flight controls (such as), cargo loading door, rudder,

elevator, flap, aileron, etc.

7. Railways: Hydraulic brakes, hydraulic steering, hydraulic suspension, hydraulic clutch,

hydraulic power transmission, hydraulic coupling hydraulic torque converter, etc.



8. Marine field: Ship steering system, ship yards, ship building.

9. Medical equipment: Medical chairs and operating tables.

10. Agricultural equipment: Harvesters, tractors, field sprayers, seeding machine, fertilizer

Machine etc.

Table 1.2 - Difference between Hydraulic System and pneumatic System

No. Hydraulic system Pneumatic system

1 Working fluid is hydraulic oil. Working fluid is compressed air

2

As oil is incompressible, oil can be

pressurized to very high pressure. (500 bar

or even more)

Air is compressible; hence air can be

pressurized to lesser pressure. (Only up

to l0 bar approx.)

3 Since pressure is high, force developed is

also very high (thousands of tone).

Since pressure is very less, force

developed is very less (up to I ton)

4 Since pressure is high, components are

very strong, made of steel and are heavy.

Components of pneumatic system are

lighter in weight, are made of

aluminium.

5

As oil has more viscosity, it cannot flow

fast. Hence hydraulic systems are slower in

operation.

Air has very less viscosity, it can flow

fast. Hence pneumatic systems are

quicker in operation.

6 Due to continuous recirculation,

temperature of oil increases.

Harder it runs, cooler it works. Free

expansion of air in cylinders and

motors causes chilling effect.

7

Hydraulic oils are petroleum based oils;

they are inflammable and there is every

chance of fire hazard, if neglected.

No chance of fire hazard. Hence

pneumatic tools are preferably used

inside mines, where flammable gasses

may present.

8 leakage of oil results in dirty and slippery

Surroundings, may lead to accidents.

Very clean and dry surrounding is

maintained.

9 Pump used is positive displacement pump.

So, pressure relief valve is necessary. No need of pressure relief valve.

10

There is no need of separable lubrication

System, because, hydraulic oil itself is a

lubricant.

Lubricator is necessary. Oil is mixed

with the compressed air in lubricator

and then supplied to the system.

11 Applications: CNC. Machine tools, earth

moving machines, automobiles. aviation

etc.

Applications : Material handling

systems,

hand tools mining works, automation,

automobiles etc.



1.10 PRINCIPLES OF HYDRAULIC FLUID POWER

− In the language of physical science anything that can flow is a fluid. As Per the definition of

fluid, air, oil and water are nothing but fluids since all of them can flow. In our discussion

here, we will concentrate only on oil as the present day hydraulic system uses oil. The

operation of a fluid power system, i.e. a hydraulic system using oil is governed by the basic

physical laws of fluid flow as developed by a great-scientist Blaise Pascal (1648). This law

is known as "Pascal's law".

Potential Head

Figure 1.2 (a) - Potential head is independent of shape and size

Figure 1.2 (b) - Potential head is independent of container shape

1.10.1 Law of Hydrostatics

− The law of hydrostatics states that the pressure p of a fluid at rest increases on increasing

the depth. This means that,

p = Pressure = w h where,

w = specific weight of the liquid



h = depth or 'head' of the fluid

− It refers to a specific datum, the head defined by position L is called the potential head, h

and the total head represented by (h + z) is the piezometric head.

− From the above we understand that every liquid exerts a pressure on its base area by its own

weight. The pressure is dependent on the height of the liquid column and its density

irrespective of the shape or geometry of the container.

− Mathematically Hydrostatic Pressure is equal to

P = h x ρ x g

h = height of fluid column

ρ = density of liquid

g = acceleration of free fall.

1.11 BASIC ELECTRICAL DEVICES

− Seven basic electrical devices commonly used in the control of fluid power systems are:

1. Manually actuated push button switches

2. Limit switches

3. Pressure switches

4. Solenoids

5. Relays

6. Timers

7. Temperature switches

− Other devices used in electro pneumatics are

1. Proximity sensors

2. Electric counters

1.11.1 Push button switches

− A push button is a switch used to close or open an electric control circuit. They are

primarily used for starting and stopping of operation of machinery.

− They also provide manual override when the emergency arises. Push button switches are

actuated by pushing the actuator into the housing. This causes set of contacts to open or close.

− Push buttons are of two types

i) Momentary push button

ii) Maintained contact or detent push button



− Momentary push buttons return to their unactuated position when they are released.

Maintained (or mechanically latched) push buttons has a latching mechanism to hold it in

the selected position.

− The contact of the push buttons, distinguished according to their functions,

i) Normally open (NO) type

ii) Normally closed (NC) type

iii) Change over (CO) type.

1.11.2 Limit switches

− Any switch that is actuated due to the position of a fluid power component (usually a piston

rod or hydraulic motor shaft or the position of load is termed as limit switch. The actuation

of a limit switch provides an electrical signal that causes an appropriate system response.

− Limit switches perform the same function as push button switches. Push buttons are

manually actuated whereas limit switches are mechanically actuated.

− There are two types classification of Limit switches depending upon method of actuations

of contacts

a) Lever actuated contacts

b) Spring loaded contacts

− In lever type limit switches, the contacts are operated slowly. In spring type limit switches,

the contacts are operated rapidly. Figure 1.3 shows a simplified cross sectional view of a

limit switch and its symbol.

Figure 1.3 - Cross sectional view of a limit switch

1.11.3 Pressure switches

− A pressure switch is a pneumatic-electric signal converter. Pressure switches are used to

sense a change in pressure, and opens or closes an electrical switch when a predetermined

pressure is reached. Bellow or diaphragm is used to sense the change of pressure.



− Figure 1.4 shows a diaphragm type of pressure switch. When the pressure is applied at the

inlet and when the pre-set pressure is reached, the diaphragm expands and pushes the spring

loaded plunger to make/break contact.

Figure 1.4 - Cross sectional view of a pressure switch

1.11.4 Solenoids

− Electrically actuated directional control valves form the interface between the two parts of

an electro-pneumatic control. The most important tasks of electrically actuated DCVs

include.

i) Switching supply air on or off

ii) Extension and retraction of cylinder drives

− Electrically actuated directional control valves are switched with the aid of solenoids. They

can be divided into two groups:

i) Spring return valves only remain in the actuated position as long as current

flows through the solenoid

ii) In the initial position, all solenoids of an electrically actuated DCVs are de-

energized and the solenoids are inactive. A double valve has no clear initial

position, as it does not have a return spring. The possible voltage levels for

solenoids are 12 V DC, 12V AC, 12 V 50/60 Hz, 24V 50/60 Hz, 110/120V

50/60 Hz, 220/230V 50/60 Hz.

iii) Double solenoid valves retain the last switched position even when no current flows

through the solenoid.



1.11.5 Relays

− A relay is an electro magnetically actuated switch. It is a simple electrical device used for

signal processing. Relays are designed to withstand heavy power surges and harsh

environment conditions. When a voltage is applied to the solenoid coil, an electromagnet

field results. This causes the armature to be attracted to the coil core.

− The armature actuates the relay contacts, either closing or opening them, depending on the

design. A return spring returns the armature to its initial position when the current to the coil

is interrupted. Cross sectional view of a relay is shown in Figure 1.5.

− A large number of control contacts can be incorporated in relays in contrast to the case of a

push button station. Relays are usually designated as K1, K2, and K3 etc. Relays also

possess interlocking capability that is an important safety feature in control circuits.

Interlocking avoids simultaneous switching of certain coils.

1.11.6 Timer or Time delay relays

− Timers are required in control systems to effect time delay between work operations. This is

possible by delaying the operation of the associated control element through a timer. Most

of the timers we use is Electronic timers. There are two types of time relay

Figure 1.5 - Cross sectional view of a relay

i) Pull in delay ( on –delay timer)

ii) Drop –out delay (off delay timer)

− In the on-delay timer, shown in Figure cc, when push button PB is pressed (ON), capacitor

C is charged through potentiometer R1 as diode D is reverse –biased. The time taken to

charge the capacitor, depends on the resistance of the potentiometer (R1) and the

capacitance(C) of the capacitor. By adjusting the resistance of the potentiometer, the

required time delay can be set. When the capacitor is charged sufficiently, coil K is

energized, and its contacts are operated after the set time delay. When the push button is

released (OFF), the capacitor discharges quickly through a small resistance (R2) as the

diode by passes resistor R1, and the contacts of relay (K) return to their normal position

without any delay.



− In the off-delay timer, the contacts are operated without any delay when the push button is

pressed (ON). The contacts return to the normal position after the set delay when the push

button is released (OFF).

− The construction and symbols of the on-delay and off-relay timers are given in Figure1.6.

Figure 1.6 - Construction features of timer and its symbols

1.11.7 Temperature Switch

− Temperature switches automatically senses a change in temperature and opens or closes an

electrical switch when a predetermined temperature is reached. This switch can be wired

either normally open or normally closed.

− Temperature switches can be used to protect a fluid power system from serious damage

when a component such as a pump or strainer or cooler begins to malfunction.


2 SYSTEM COMPONENTS, HYDRAULIC OILS, FLUID PROPERTIES AND FILTER

Course Contents

2.1 Hydraulic oils and fluid properties

2.2 Types of hydraulic fluids

2.3 Properties of Fluids

2.4 Physical characteristics of

hydraulic fluid

2.5 Filters and strainers

2.6 Types of filters

Oil Hydraulics and Pneumatics (2171912) 2. System Components, Hydraulic Oils, Fluid Properties and Filter


2.1 HYDRAULIC OILS AND FLUID PROPERTIES

A hydraulic fluid power system may be defined as a means of power transmission in which a

relatively incompressible fluid is used as the power transmitting media. The primary purpose

of a hydraulic system is the transfer of energy from one location to another and the

conversion of this energy to useful work. Hydraulic power is usually generated by pumps and

the energy generated is converted to useful work by hydraulic cylinders or other actuators

(linear or rotary). The transmission of this energy is accomplished by movement of the

hydraulic fluid through metal tubes or elastomeric hoses, while the control of the power is

achieved by means of values. As no hydraulic system can perform the assigned task without

the hydraulic fluid, this fluid is of utmost importance in a hydraulic system.

The broad tasks of hydraulic oil can be classified broadly as follows:

l. To transfer hydraulic energy

2. To lubricate all parts

3. To avoid corrosion

4. To remove impurities and abrasion

5. To dissipate heat

2.2 TYPES OF HYDRAULIC FLUIDS

There are different types of hydraulic fluids that have the required properties. In general

while selecting suitable fluid, a few important factors are to be considered.

Its compatibility with seals, bearing and other components.

Its viscosity and environmental stability are also considered.

1. Petroleum-based fluids: Mineral oils are the petroleum-based oils that are the most

commonly used hydraulic fluids. Basically, they possess most of the desirable characteristics:

they are easily available and are economical.

- In addition, they offer the best lubrication ability, least corrosion problems and are

compatible with most seal materials. The only major disadvantage of these fluids is their

flammability.

- They pose fire hazards, mainly from the leakages, in high-temperature environments such as

steel industries, etc. Mineral oils are good for operating temperatures below 50°C, at higher

temperatures, these oils lose their chemical stability and form acids, varnishes, etc.

- All these lead to the loss of lubrication characteristics, increased wear and tear, corrosion

and related problems. Fortunately, additives are available that improve chemical stability,

reduce oxidation, foam formation and other problems.

- A petroleum oil is still by far the most highly used base for hydraulic fluids. In general,

petroleum oil has the following properties:

Excellent lubricity

Higher demulsibility



More oxidation resistance

Higher viscosity index

Protection against rust

Good sealing characteristics

Easy dissipation of heat

Easy cleaning by filtration

Most of the desirable properties of the fluid, if not already present in the crude oil, can

be incorporated through refining or adding additives.

A principal disadvantage of petroleum oil is that it burns easily. For applications where fire

could be a hazard, such as heat treating, hydroelectric welding, die casting, forging and many

others, there are several types of fire-resistant fluids available.

2. Emulsions: Emulsions are a mixture of two fluids that do not chemically react with others.

Emulsions of petroleum-based oil and water are commonly used. An emulsifier is normally

added to the emulsion, which keeps liquid as small droplets and remains suspended in the

other liquid. Two types of emulsions are in use:

A. Oil-in-water emulsions: This emulsion has water as the main phase, while small

droplets of oil are dispersed in it. Generally, the oil dilution is limited, about 5%;

hence, it exhibits the characteristics of water.

- Its limitations are poor viscosity, leading to leakage problems, loss in volumetric

efficiency and poor lubrication properties. These problems can be overcome to a

greater extent by using certain additives. Such emulsions are used in high-

displacement, low-speed pumps (such as in mining applications).

B. Water-in-oil emulsions: Water-in-oil emulsions, also called inverse emulsions, are

basically oil based in which small droplets of water are dispersed throughout the oil

phase.

- They are most popular fire-resistant hydraulic fluids. They exhibit more of an oil-

like characteristic; hence, they have good viscosity and lubrication properties.

- The commonly used emulsion has a dilution of 60% oil and 40% water. These

emulsions are good for operations at 25°C, as at a higher temperature, water

evaporates and leads to the loss of fire-resistant properties.

3. Water glycol: Water glycol is another non-flammable fluid commonly used in aircraft

hydraulic systems.

- It generally has a low lubrication ability as compared to mineral oils and is not suitable for

high-temperature applications. It has water and glycol in the ratio of 1:1. Because of its

aqueous nature and presence of air, it is prone to oxidation and related problems. It needs to

be added with oxidation inhibitors.

- Enough care is essential in using this fluid as it is toxic and corrosive toward certain metals

such as zinc, magnesium and aluminium. Again, it is not suitable for high-temperature

operations as the water may evaporate.

- However, it is very good for low-temperature applications as it possesses high antifreeze

characteristics.



4. Synthetic fluids: Synthetic fluid, based on phosphate ester, is another popular fire-

resistant fluid. It is suitable for high-temperature applications, since it exhibits good viscosity

and lubrication characteristics.

- It is not suitable for low-temperature applications.

- It is not compatible with common sealing materials such as nitrile. Basically being

expensive, it requires expensive sealing materials (viton).

- In addition, phosphate ester is not an environmental-friendly fluid. It also attacks aluminum

and paints.

5. Vegetable oils: The increase in the global pollution has led to the use of more

environmental-friendly fluids.

- Vegetable-based oils are biodegradable and are environmental safe. They have good

lubrication properties, moderate viscosity and are less expensive. They can be formulated to

have good fire resistance characteristics with certain additives.

- Vegetable oils have a tendency to easily oxidize and absorb moisture.

- The acidity, sludge formation and corrosion problems are more severe in vegetable oils than

in mineral oils. Hence, vegetable oils need good inhibitors to minimize oxidation problems.

6. Biodegradable: As more and more organizations are understanding their social

responsibility and are turning toward eco-friendly machinery and work regime, a

biodegradable hydraulic fluid is too becoming a sought after product in the dawn of an

environmentalist era.

- Biodegradable hydraulic fluids, alternatively known as bio-based hydraulic fluids, Bio-

based hydraulic fluids use sunflower, rapeseed, soybean, etc., as the base oil and hence cause

less pollution in the case of oil leaks or hydraulic hose failures.

- These fluids carry similar properties as that of a mineral oil–based anti-wear hydraulic fluid,

Hypothetically, if a company plans to introduce bio-based fluids into the hydraulic

components of the machinery and the permissible operating pressure of hydraulic

components is reduced to 80%, then it would inversely lead to a 20% reduction in breaking-

out force owing to the 20% reduction in excavator’s operating pressure.

- It is so because a reduction in the operating pressure of a system leads to a reduction in

actuator force.

- Besides, the transformation would not only include the cost of fluid and flushing of

machinery to transcend from a mineral oil to vegetable oil repeatedly but also include the

derating costs of machinery.

7. Fire Resistance: There are many hazardous applications where human safety requires the

use of a fire-resistant fluid. Examples include coal mines, hot metal processing equipment,

aircraft and marine fluid power systems.

- A fire-resisting fluid is one that can be ignited but does not support combustion when the

ignition source is removed.

- Flammability is defined as the ease of ignition and ability to propagate the flame.



- The following are the usual characteristics tested in order to determine the flammability of

hydraulic fluids:

1. Flash point: The temperature at which an oil surface gives off sufficient vapours

to ignite when a flame is passed over the surface.

2. Fire point: The temperature at which an oil releases sufficient vapours to support

combustion continuously for 5 s when a flame is passed over the surface.

3. Autogenously ignition temperature: The temperature at which ignition occurs

spontaneously.

- The fire-resistant fluids are designated as follows:

1. HFA: A high-water-content fluid or HWCF (80% or more), for example, water–oil

emulsions.

2. HFB: This is water–oil emulsion containing petroleum oil and water.

3. HFC: This is a solution of water and glycol.

4. HFD: This is a synthetic fluid, for example, phosphates or phosphate–petroleum

blends.

- The commonly used hydraulic liquids are petroleum derivatives; consequently, they burn

vigorously once they reach a fire point. For critical applications, artificial or synthetic

hydraulic fluids are used that have fire resistance.

- Fire-resistant fluids have been developed to reduce fire hazards. There are basically four

different types of fire-resistant hydraulic fluids in common use:

1. Water–glycol solution: This type consists of an actual solution of 40% water and

60% glycol. These solutions have high-viscosity-index values, but as the viscosity rises, the

water evaporates. The operating temperature ranges run from −20°C to about 85°C.

2. Water-in-oil emulsions: This type consists of about 40% water completely

dispersed in a special oil base. It is characterized by small droplets of water completely

surrounded by oil. The operating temperature range runs from −30°C to about 80°C. As is the

case with water–glycol solutions, it is necessary to replenish evaporated water to maintain

proper viscosity.

3. Straight synthetics: This type is chemically formulated to inhibit combustion and

in general has the highest fire-resistant temperature. The disadvantages of straight synthetics

include low viscosity index, incompatibility with most natural or synthetic rubber seals and

high cost.

4. High-water-content fluids (HWCFs): This type consists of about 90% water and

10% concentrate (designated as 90/10). The concentrate consists of fluid additives that

improve viscosity, lubrication, rust protection and protection against bacteria growth. The

maximum operating temperature should be held to 50°C to minimize evaporation.

- The advantages of HWCF are as follows:

1. Fire resistance due to a high flash point of about 150°C.



2. Lower system operating temperature due to good heat dissipation.

3. Biodegradable and environmental-friendly additives.

4. High viscosity index.

5. Cleaner operation of the system.

6. Low cost of concentrate and storage.

- The disadvantages of HWCF are as follows:

1. Greater contamination due to higher densities of fluids.

2. High evaporation loss.

3. Faster corrosion due to oxidation.

4. pH value to be maintained between 7.5 and 9.0.

5. Promotion of bacterial growth and filtration difficulties due to the acidic nature of

fluids.

Performance of HWCFs can be improved using additives such as

1. Anti-wear additives. 2. Anti-foaming additives.

3. Corrosion inhibitors. 4. Biocides to kill water-borne bacteria.

5. Emulsifying agents. 6. Flocculation promoters.

7. Deionization agents. 8. Oxidation inhibitors.

9. Anti-vaporizing agents.

2.3 PROPERTIES OF FLUIDS

For a fluid to perform efficiently, it must possess certain properties. The various properties

required for an ideal hydraulic fluid are as follows:

1. Ideal viscosity.

2. Good lubrication capability.

3. Demulsibility.

4. Good chemical and environmental stability.

5. Incompressibility.

6. Fire resistance.

7. Low flammability.

8. Foam resistance.

9. Low volatility.

10. Good heat dissipation.

11. Low density.

12. System compatibility.



- It is almost impossible to achieve all these properties in a hydraulic fluid. Although we can

select a good fluid with desirable properties, some of the characteristics of a fluid change

with usage.

- For example, it is common for the temperature of a fluid to rise due to friction in the system,

which reduces the viscosity of the fluid, which in turn increases leakage and reduces

lubrication ability.

- A fluid gets oxidized and becomes acidic with usage. Certain additives are added to

preserve the desirable properties and to make the fluid more stable.

- Some of the desirable properties and their influence on a hydraulic fluid are discussed

briefly in the following sub-sections.

Ideal Viscosity

- The most basic desirable property of a hydraulic fluid is optimum viscosity. It is a measure

of a fluid’s resistance to flow.

- When viscosity is low, the fluid flows easily. On the other hand, when viscosity is high, the

fluid flows with difficulty. A low viscous fluid is thin and can flow easily, whereas a high

viscous fluid is thick and cannot flow easily.

- The viscosity of a fluid should be high enough to seal the working gap between the parts

and prevent leakage but should be low enough to cause easy flow throughout the system.

- A high-viscosity fluid requires high energy to overcome the internal friction, resulting in

excess heat generation.

- On the other hand, a low-viscosity fluid flows easily but causes leakages and reduces the

volumetric and overall efficiency. Therefore the hydraulic fluid should have an optimum

viscosity.

1. High viscosity:

High resistance to flow.

Increased power consumption due to frictional loss.

High temperature caused by friction.

Increased pressure drop because of the resistance.

Possibility of sluggish or slow operation.

Difficulty in separating air from oil in a reservoir.

Greater vacuum at the pump inlet, causing cavitation. Higher system noise level.

2. Low viscosity:

Increased internal leakage.

Excessive water.

Possibility of decreased pump efficiency, causing slower operation of the

actuator. Increased temperature resulting from leakage losses.

- There are two basic methods of specifying the viscosity of fluids: absolute and kinematic

viscosity. Viscosity index is an arbitrary measure of a fluid resistance to viscosity change

with temperature changes. Thus, viscosity is affected by temperature changes.



- As temperature increases, the viscosity of a fluid decreases. A fluid that has a relatively

stable viscosity at temperature extremes has a high viscosity index. A fluid that is very thick

while cold and very thin while hot has a low viscosity index.

Lubrication Capability

- Hydraulic fluids must have good lubricity to prevent friction and wear between the closely

fitted working parts such as vanes of pumps, valve spools, piston rings and bearings.

- Wear is the removal of surface material due to the frictional force between two metal-to-

metal contacts of surfaces. This can result in a change in dimensional tolerances, which can

lead to improper functioning and failure of the components.

- Hydraulic oil should have a good lubricating property. That is, the film so formed should be

strong enough that it is not wiped out by the moving parts.

Demulsibility

- The ability of a hydraulic fluid to separate rapidly from moisture and successfully resist

emulsification is known as “demulsibility.”

- If an oil emulsifies with water, the emulsion promotes the destruction of lubricating and

sealant properties. Highly refined oils are basically water resistant by nature.

Good Chemical and Environmental Stability (Oxidation and Corrosion Resistance)

- For a good hydraulic fluid, a good chemical and environmental stability is desirable. Most

fluids are vulnerable to oxidation, as they come in contact with oxygen in air.

- Mineral oils or petroleum-based oils (widely used in hydraulic systems) contain carbon and

hydrogen molecules, which easily react with oxygen.

- The oxidation products are highly soluble in oil and being acidic in nature they can easily

corrode metallic parts.

- The soluble acidic products cause corrosion, whereas insoluble products make the operation

sluggish. Oxidation leads to deterioration in the chemical nature of fluid, which may form

some chemical sledges, gum or varnish at low velocity or stagnation points in the system.

- Many factors influence the rate of oxidation, such as temperature, pressure, moisture and so

on. Temperature is the most affecting one, as the rate of oxidation increases severely with rise

in temperature.

- The moisture entering the hydraulic system with air causes the parts made of ferrous

materials to rust. Rust is a chemical reaction between iron or steel and oxygen.

- Corrosion, on the other hand, is the chemical reaction between a metal and an acid. The

result of rusting and corrosion is the “eating away” of the metal surfaces of the hydraulic

components. Rust and corrosion cause excessive leakage between moving parts.

- Rust and corrosion can be prevented by incorporating additives that plate on the metal

surface to prevent chemical reaction.



Neutralization Numbers

- Neutralization number is a measure of the acidity or alkalinity of hydraulic oil. This is

referred to as the pH value of the oil. High acidity causes the oxidation rate in oil to increase

rapidly.

Low Flammability

- A fire-resistant fluid is one that can get ignited in the presence of an ignition source but does

not support combustion when the source is removed. This characteristic is defined as

flammability.

- It refers to the ease with which a fluid gets ignited and propagates the flame. Hence, it is

desirable to have a low flammability for a hydraulic fluid.

Foam Resistance

- Air can be present in a hydraulic fluid in two forms: dissolved and entrained. For example,

if the return line to the reservoir is not submerged, the jet of oil entering the liquid surface

will carry air with it.

- This causes air bubbles to form in the oil. If these bubbles rise to the surface too slowly,

they will be drawn into the pump intake.

- This can cause pump damage due to cavitation.

- Another adverse effect of entrained and dissolve air is a great reduction in the bulk

modulus of the hydraulic fluid.

Low Volatility

- A fluid should possess low vapour pressure or high boiling point. The vapour pressure of a

fluid varies with temperature and hence the operating temperature range of the system is

important in determining the stability of the fluid.

Good Heat Dissipation

- A hydraulic fluid should have a high heat dissipation capability. The temperature of a fluid

shoots up if its heat dissipation characteristics are poor.

- Too high fluid temperature can cause a system to malfunction. If the fluid overheats, it may

cause the following:

1. Give off vapor and cause cavitation of the pump.

2. Increase the rate of oxidation causing its rapid deterioration by producing sledges,

varnishes, etc., thus shortening its useful life.

3. Reduce viscosity of the fluid resulting in increased leakage, both internal and

external.

4. Cause thermal distortion in components.

5. Damage seals and packaging owing to embrittlement.

- Hydraulic systems should be designed so that a heat balance occurs at a satisfactory

operating temperature.



Low Density

- The relative density of a mineral oil is 0.9 (the exact value depends on the base oil and the

additive used). Synthetic fluids can have a relative density greater than 1. T

- he relative density is important when designing the layout of pumps and reservoir.

System Compatibility

- A hydraulic fluid should be inert to materials used in or near the hydraulic equipment. If the

fluid in anyway attacks, destroys, dissolves or changes the parts of hydraulic system, the

system may lose its functional efficiency and may start malfunction.

Stable Chemically and Physically

- Fluid characteristics should remain unchanged during an extended useful life and during

storage.

- The fluid in a working hydraulic system is subjected to violent usage-large pressure

fluctuations, shock, turbulence, aeration, cavitation, water and particulate contamination, high

shear rates, and large temperature variations.

- Since many aspects of stability are chemical in nature, the temperatures to which the fluid

will be exposed is an important criterion in the selection of a. hydraulic fluid.

Good Heat Dissipation

- An important requirement of the fluid is to carry heat away from the working parts. Pressure

drops, mechanical friction, fluid friction, leakages, all generate heat.

- The fluid must carry the generated heat away and readily dissipate it to the atmosphere or

coolers. Therefore high thermal conductivity and high specific heat values are desirable in the

fluid chosen.

High Bulk Modulus

- In general, oil is taken as incompressible. However, in practice, all materials are

compressible and so is oil.

- The bulk modulus is a measure of the degree of compressibility of the fluid and is the

reciprocal of compressibility.

- The higher the bulk modulus, the lesser the material will be compressed with increasing

pressure.

- Bulk modulus is an important characteristic of a hydraulic fluid because of control

problems, especially in servo hydraulics.

Low Coefficient of Expansion

- A low coefficient of expansion is usually desirable in a hydraulic fluid to minimise the total

volume of the system required at the operating temperature.



2.4 PHYSICAL CHARACTERISTICS OF HYDRAULIC FLUID

Density

- Density may be defined as the mass of oil per unit volume. The unit will be kg/cm3 or

kg/m3.

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 (𝜌) =𝑚𝑎𝑠𝑠

𝑣𝑜𝑙𝑢𝑚𝑒

- Density of any liquid is generally measured by an instrument called hydrometer. If one dips

the instrument into the liquid or oil, the density can be directly read.

- Hydraulic oils which are used in industrial hydraulic systems may have a density of 0.8 to

0.9 gm/cm3.

Table 2.1 - Density characteristics of hydraulic fluids

Fluid Density (kg/m3)

Shell Tellus ISO 32 mineral oil

875

Shell HFB 60%oil,40%oil

933

Shell HFC 60%glycol,40%water

1084

Shell HFD phosphate ester 1125

Shell Naturelle HFE 32

918

Specific Gravity

- Specific gravity of oil is defined as the ratio of densities of oil and water. Specific gravity of

a fluid is important in those cases where the overall system weight must be kept minimum.

𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐺𝑟𝑎𝑣𝑖𝑡𝑦 =𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑜𝑖𝑙

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟

- As per standards, density of water is accepted as 1. Hence if we say that the specific gravity

of oil is 0.80, then the logical inference is that the oil in question has a density of 0.8 gm/cm3.

- Specific gravity and density may seem to be the same; it is necessary that we understand the

mathematical relationship so that while solving problems we are not confused.

- A heavy fluid can cause pump cavitation and resultant malfunction of the system.

Specific Weight

- Specific weight of hydraulic oil is calculated by multiplying density of oil by acceleration

due to gravity.

𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑊𝑒𝑖𝑔ℎ𝑡 (𝛾) =𝑊𝑒𝑖𝑔ℎ𝑡

𝑉𝑜𝑙𝑢𝑚𝑒



Viscosity

- Viscosity is a very important property of oil. It is the measure of the ability of e liquid to

flow. It can actually be defined as the resistance to flow.

- To understand the physical concept of viscosity, let us take few simple examples. When or -

swims in a pool of water, one experiences a resistance to the motion.

- One might have noticed that when a liquid kept in a container is stirred and left to itself.6c

motion will disappear after sometime. This indicates that there is some kind d frictional force

in all types of fluids. This force is called the viscous force.

2.5 FILTERS AND STRAINERS

- For proper operation and long service life of a hydraulic system, oil cleanliness is of prime

importance. Hydraulic components are very sensitive to contamination.

- The cause of majority of hydraulic system failures can be traced back to contamination.

Hence, filtration of oil leads to proper operation and long service life of a hydraulic system.

- Strainers and filters are designed to remove foreign particles from the hydraulic fluid. They

can be differentiated by the following definitions:

1. Filters: They are devices whose primary function is the retention, by some fine porous

medium, of insoluble contaminants from fluid.

- Filters are used to pick up smaller contaminant particles because they are able to

accumulate them better than a strainer.

- Generally, a filter consists of fabricated steel housing with an inlet and an outlet. The filter

elements are held in position by springs or other retaining devices.

- Because the filter element is not capable of being cleaned, that is, when the filter becomes

dirty, it is discarded and replaced by a new one. Particle sizes removed by filters are

measured in microns.

- The smallest sized particle that can be removed is as small as 1 μm. A strainer is a device

whose function is to remove large particles from a fluid using a wire screen.

- The smallest sized particle that can be removed by a strainer is as small as 0.15 mm or 150

μm.

2. Hydraulic strainers: A strainer is a coarse filter. Fluid flows more or less straight through

it. A strainer is constructed of a fine wire mesh screen or of screening consisting of a

specially processed wire of varying thickness wrapped around metal frames.

- It does not provide as fine a screening action as filters do, but offers less resistance to flow

and is used in pump suction lines where pressure drop must be kept to a minimum.

- A strainer should be as large as possible or wherever this is not practical, two or more may

be used in parallel.

2.5.1 Causes of Contamination

The causes of contamination are as follows:

Contaminants left in the system during assembly or subsequent maintenance work.



Contaminants generated when running the system such as wear particles, sludge and

varnish due to fluid oxidation and rust and water due to condensation.

Contaminants introduced into the system from outside. These include using the wrong

fluid when topping up and dirt particles introduced by contaminated tools or repaired

components.

2.6 TYPES OF FILTERS

Filters may be classified as follows:

1. According to the filtering methods:

Mechanical filters: This type normally contains a metal or cloth screen or a series of

metal disks separated by thin spacers. Mechanical filters are capable of removing only

relatively coarse particles from the fluid.

Absorption filters: These filters are porous and permeable materials such as paper,

wood pulp, diatomaceous earth, cloth, cellulose and asbestos. Paper filters are

impregnated with a resin to provide added strength. In this type of filters, the particles

are actually absorbed as the fluid permeates the material. Hence, these filters are used

for extremely small particle filtration.

Adsorbent filters: Adsorption is a surface phenomenon and refers to the tendency of

particles to cling to the surface of the filters. Thus, the capacity of such a filter

depends on the amount of surface area available. Adsorbent materials used include

activated clay and chemically treated paper.

2. According to the size of pores in the material:

Surface filters: These are nothing but simple screens used to clean oil passing

through their pores. The screen thickness is very thin and dirty unwanted particles are

collected at the top surface of the screen when the oil passes, for example, strainer.

Depth filters: These contain a thick-walled filter medium through which the oil is

made to flow and the undesirable foreign particles are retained. Much finer particles

are arrested and the capacity is much higher than surface filters.

3. According to the location of filters:

Intake or inline filters (suction strainers): These are provided first before the pump

to protect the pump against contaminations in the oil as shown in Fig. 2.1. These

filters are designed to give a low pressure drop, otherwise the pump will not be able to

draw the fluid from the tank. To achieve low pressure drop across the filters, a coarse

mesh is used. These filters cannot filter out small particles.

Advantages of suction filters:

(a) A suction filter protects the pump from dirt in the reservoir. Because the suction

filter is outside the reservoir, an indicator telling when the filter element is dirty can

be used.



Figure 2.1 - Suction filter

(b) The filter element can be serviced without dismantling the suction line or

reservoir (easy to maintain).

Disadvantages of suction filters:

(a) A suction filter may starve the pump if not sized properly.

Pressure line filters (high-pressure filters): These are placed immediately after the

pump to protect valves and actuators and can be a finer and smaller mesh (Fig. 2.2).

They should be able to withstand the full system pressure. Most filters are pressure

line filters.

Figure 2.2 - Pressure filter



Advantages of a pressure line filter:

(a) A pressure filter can filter very fine contaminants because the system pressure is

available to push the fluid through the element.

(b) A pressure filter can protect a specific component from the harm of deteriorating

particles generated from an upstream component.

Disadvantages of a pressure line filter:

(a) The housing of a pressure filter must be designed for high pressure because it

operates at full system pressure. This makes the filter expensive.

(b) If pressure differential and fluid velocity are high enough, dirt can be pushed

through the element or the element may tear or collapse.

Return line filters (low-pressure filters): These filters filter the oil returning from

the pressure-relief valve or from the system, that is, the actuator to the tank (Fig. 2.3).

They are generally placed just before the tank. They may have a relatively high

pressure drop and hence can be a fine mesh. These filters have to withstand low

pressure only and also protect the tank and pump from contamination.

Figure 2.3 - Return line filter

Advantages of a return line filter:

(a) A return line filter catches the dirt in the system before it enters the reservoir.

(b)The filter housing does not operate under full system pressure and is therefore less

expensive than a pressure filter.



Disadvantages of a return line filter:

(a) There is no direct protection for circuit components.

(b) In return line full flow filters, flow surges from discharging cylinders, actuators

and accumulators must be considered when sizing.

4. Depending on the amount of oil filtered by a filter:

Full flow filters: In this type, complete oil is filtered. Full flow of oil must enter the

filter element at its inlet and must be expelled through the outlet after crossing the

filter element fully (Fig. 2.4). This is an efficient filter. However, it incurs large

pressure drops. This pressure drop increases as the filter gets blocked by

contamination.

Figure 2.4 - Full flow filter

Proportional filters (bypass filters): In some hydraulic system applications, only a portion

of oil is passed through the filter instead of entire volume and the main flow is directly passed

without filtration through a restricted passage (Fig. 2.5).

Figure 2.5 - Proportional filter

2.6.1 Beta Ratio of Filters

- Filters are rated according to the smallest size of particles they can trap. Filter ratings are

identified by nominal and absolute values in micrometers.

- A filter with a nominal rating of 10 μm is supposed to trap up to 95% of the entering

particles greater than 10 μm in size.

- The absolute rating represents the size of the largest pore or opening in the filter and thus

indicates the largest size particle that could go through. Hence, absolute rating of a 10 μm

nominal size filter would be greater than 10 μm.

- A better parameter for establishing how well a filter traps particles is called the beta ratio or

beta rating.



- The beta ratio is determined during laboratory testing of a filter receiving a steady-state

flow containing a fine dust of selected particle size.

- The test begins with a clean filter and ends when pressure drop across the filter reaches a

specified value indicating that the filter has reached the saturation point. This occurs when

contaminant capacity has been reached.

- By mathematical definition, the beta ratio equals the number of upstream particles of size

greater than Nμm divided by the number of downstream particles having size greater than

Nμm where N is the selected particle size for the given filter. The ratio is represented by the

following equation:

Beta ratio = No. of upstream particles of size > N μm

No. of downstream particles of size > N μm

- A beta ratio of 1 would mean that no particle above specified N are trapped by the filter. A

beta ratio of 50 means that 50 particles are trapped for every one that gets through. Most

filters have a beta ratio greater than 75:

Beta ratio = No. of upstream particles − No. of downstre am particles

No. of upstream particles



Table 2.2 – Hydraulic and Pneumatic symbols as per

Lines

Continuous line (for) flow line

Dashed line (for) pilot, drain

Envelope (for) long and short dashes

around two or more component

symbols.

Circular

Large circle - pump, motor

Small circle - Measuring devices

Semi-circle - rotary actuator

Square

-One square - pressure control function -Two or three adjacent squares - directional control

Diamond

Diamond - Fluid conditioner (filter, separator, lubricator, heat exchanger)

Miscellaneous Symbols

Spring

Flow Restriction

Triangle

Solid - Direction of Hydraulic Fluid Flow

Open - Direction of Pneumatic flow

Pumps and Compressors

Fixed Displacement hydraulic pump symbols

Unidirectional

Bidirectional

Unidirectional

Bidirectional



Compressor symbol

Motors

Fixed displacement hydraulic motor symbol

Unidirectional

Bidirectional

Variable displacement hydraulic motor symbol

Unidirectional

Bidirectional

Pneumatic motor symbol

Unidirectional

Bidirectional

Rotary Actuator symbol

Hydraulic

Pneumatic

Cylinders

Single acting cylinder symbols

Returned by external force

Returned by spring or extended by spring

force

Double acting cylinder symbols

Single piston rod (fluid required to

extend and retract)

Double ended piston rod

Cylinders with cushions symbols

Single fixed cushion

Double fixed cushion

Single adjustable cushion

Double adjustable cushion

Directional Control Valve symbols

Directional control valve (2 ports / 2 positions)



Normally closed directional control valve

with 2 ports and 2 finite positions Normally open directional control valve

with 2 ports and 2 finite positions


Normally closed directional control valve

with 3 ports and 2 finite positions Normally open directional control valve

with 3 ports and 2 finite positions


Directional control valve with 4 ports and 2 finite positions


Directional control valve with 4 ports and 3 finite positions (center position can have various flow paths)

Directional control valve (5 ports / 2 positions) normally a pneumatic valve


Directional control valve (5 ports / 3 positions) Normally a pneumatic valve


Control Method Operator symbols for valves

Manual Control

General symbol of a valve's manual

operator (without showing the control

type)

Pushbutton

Lever

Foot pedal

Mechanical Valve Control

Plunger or tracer

Spring (used on one side of a valve to

hold it in the normally open or normally

closed state)



Roller

Roller (one direction only)

Electrical/Solenoid Valve Control

Solenoid (the one side's winding shown)

Pilot Operation

(uses pressure to actuate valve)

Pneumatic actuated pilot

Hydraulic actuated pilot

Pilot operated two-stage valve (uses a second lesser force to actuate the pilot

actuation of the valve)

Pneumatic: Solenoid first stage

Pneumatic: Air pilot second stage

Hydraulic: Solenoid first stage

Hydraulic: Hydraulic pilot second stage

Check valves, Shuttle valves, Rapid Exhaust valves

Check valve symbol-free flow one

direction, blocked flow in other direction

Pilot operated check valve symbol, pilot

to close

Pilot operated check valve symbol, pilot

to open

Shuttle valve

Rapid exhaust valve/Pneumatic

Pressure Control Valves

Pressure Relief Valve (safety valve) normally closed

Line pressure is limited to the setting of

the valve, secondary part is directed to

tank.

Proportional Pressure Relief Valve



Line pressure is limited to and

proportional to an electronic signal

Sequence Valve

When the line pressure reaches the setting

of the valve, valve opens permitting flow

to the secondary port. The pilot must be

externally drained to tank.

Pressure Reducing valve (Hydraulic Pressure Regulator)

Pressure downstream of valve is limited to the setting of the valve

Flow Control Valves

Throttle valve

Adjustable output flow

Flow Control valves

Flow control valve with fixed

output (variations in inlet pressure do not

affect rate of flow

Flow control valve with fixed output and

relief port to reservoir with relief for

excess flow (variations in inlet pressure

do not affect rate of flow)

Flow control valve with variable output

Flow control valve with fixed orifice

Flow control valve with metered flow

toward right free flow to left

Flow control valve with pressure

compensated flow control fixed output

flow regardless of load

Flow control valve with pressure and

temperature compensated

Shut-Off Valve

Shut-Off Valve Simplified symbol



Accumulators

Accumulator symbol (Stores Pressure)

Reservoir (Tank)

Reservoir symbol (Holds Fluid medium of

your system)

Filters, Water Traps, Lubricators and Miscellaneous Apparatus

Filter or Strainer

Water Trap

Filter with water trap

Air Dryer

Lubricator

Conditioning unit (FRL, Pressure Regulator)

Heat Exchangers


3 HYDRAULIC PUMPS, MOTORS AND ACTUATORS

Course Contents

3.1 Hydraulic pumps

3.2 Linear actuators

3.3 Types of cylinder

3.4 Rotary actuators

3.5 Classification of Hydraulic

Motors

3.6 Hydrostatic transmission

system

Oil Hydraulics and Pneumatics (2171912) 3. Hydraulic Pumps, Motors and Actuators


3.1 HYDRAULIC PUMPS

A pump will have an inlet called suction and an outlet called delivery. It converts mechanical

energy in to hydraulic energy. Pump is also known as heart of hydraulic system. The basic

principle is that Due to mechanical action, the pump created partial vacuum at its inlet. This

permits atmospheric pressure to force the fluid through the inlet line and in to the pump, then

pump pushes the fluid into the hydraulic system.

− These are mainly classified into two categories:

(a) Non-positive displacement pumps (Hydrodynamic/Rotodynamic)

(b) Positive displacement pumps (Hydrostatic)

(a) Non-positive displacement pumps (Hydrodynamic/Rotodynamic)

− In these pumps the fluid is pressurized by the rotation of the propeller and the fluid pressure

is proportional to the rotor speed. These pumps can not withstanding high pressures and

generally used for low-pressure and high-volume flow applications.

− The fluid pressure and flow generated due to inertia effect of the fluid. The fluid motion is

generated due to rotating propeller. These pumps provide a smooth and continuous flow but

the flow output decreases with increase in system resistance (load).

− Simply external work is imparted to fluid in the control volume in the form of kinetic

energy and change in pressure energy is almost negligible.

− The important advantages of non-positive displacement pumps are lower initial cost, less

operating maintenance because of less moving parts, simplicity of operation, higher

reliability and suitability with wide range of fluid etc. These pumps are primarily used for



transporting fluids and find little use in the hydraulic or fluid power industries. Centrifugal

pump is the common example of non-positive displacement pumps.

(b) Positive displacement pumps (Hydrostatic)

− These pumps deliver a constant volume of fluid in a cycle. The discharge quantity per

revolution is fixed in these pumps and they produce fluid flow proportional to their

displacement and rotor speed. These pumps are used in most of the industrial fluid power

applications. The output fluid flow is constant and is independent of the system pressure

(load).

− Simply the energy transferred to the fluid purely in pressure energy form and change in

kinetic energy is almost negligible. It is important to note that the positive displacement

pumps do not produce pressure but they only produce fluid flow.

− The important advantage associated with these pumps is that the high-pressure and low-

pressure areas (means input and output region) are separated and hence the fluid cannot leak

back due to higher pressure at the outlets. These features make the positive displacement

pump most suited and universally accepted for hydraulic systems.

− The important advantages of positive displacement pumps over non-positive displacement

pumps include capability to generate high pressures, high volumetric efficiency, high power

to weight ratio, change in efficiency throughout the pressure range is small and wider

operating range pressure and speed. Important positive displacement pumps are gears pumps,

vane pumps and piston pumps. The details of these pumps are discussed in the following

sections.

3.1.1 Gear Pumps

− Gear pump is a robust and simple positive displacement pump. It has two meshed gears

revolving about their respective axes. These gears are the only moving parts in the pump.

They are compact, relatively inexpensive and have few moving parts. The rigid design of the

gears and houses allow for very high pressures and the ability to pump highly viscous fluids.

These pump includes helical and herringbone gear sets (instead of spur gears), lobe shaped

rotors similar to Roots blowers (commonly used as superchargers), and mechanical designs

that allow the stacking of pumps. Based upon the design, the gear pumps are classified as:

(a) External gear pumps

(b) Lobe pumps

(c) Internal gear pumps

− Generally gear pumps are used to pump:

• Petrochemicals: Pure or filled bitumen, pitch, diesel oil, crude oil, lube oil etc.

• Chemicals: Sodium silicate, acids, plastics, mixed chemicals, isocyanates etc.

• Paint and ink

• Resins and adhesives

• Pulp and paper: acid, soap, lye, black liquor, kaolin, lime, latex, sludge etc.



• Food: Chocolate, cacao butter, fillers, sugar, vegetable fats and oils, molasses,

animal food etc.

(a) External gear pumps

− The external gear pump consists of externally meshed two gears housed in a pump case as

shown in figure 3.1 one of the gears is coupled with a prime mover and is called as driving

gear and another is called as driven gear.

− The rotating gear carries the fluid from the tank to the outlet pipe. The suction side is

towards the portion whereas the gear teeth come out of the mesh.

− When the gears rotate, volume of the chamber expands leading to pressure drop below

atmospheric value. Therefore the vacuum is created and the fluid is pushed into the void due

to atmospheric pressure. The fluid is trapped between housing and rotating teeth of the gears.

− The discharge side of pump is towards the portion where the gear teeth run into the mesh

and the volume decreases between meshing teeth.

− The clearance between gear teeth and housing and between side plate and gear face is very

important and plays an important role in preventing leakage. In general, the gap distance is

less than 10 micrometres. The amount of fluid discharge is determined by the number of gear

teeth, the volume of fluid between each pair of teeth and the speed of rotation.

− The important drawback of external gear pump is the unbalanced side load on its bearings.

It is caused due to high pressure at the outlet and low pressure at the inlet which results in

slower speeds and lower pressure ratings in addition to reducing the bearing life.

− Gear pumps are most commonly used for the hydraulic fluid power applications and are

widely used in chemical installations to pump fluid with a certain viscosity.

Figure 3.1 - External gear pump



(b) Lobe pumps

− Lobe pumps work on the similar principle of working as that of external gear pumps.

However in Lobe pumps, the lobes do not make any contact like external gear pump (see

Figure 3.2).

− Lobe contact is prevented by external timing gears located in the gearbox. Similar to the

external gear pump, the lobes rotate to create expanding volume at the inlet.

− Now, the fluid flows into the cavity and is trapped by the lobes. Fluid travels around the

interior of casing in the pockets between the lobes and the casing. Finally, the meshing of the

lobes forces liquid to pass through the outlet port. The bearings are placed out of the pumped

liquid. Therefore the pressure is limited by the bearing location and shaft deflection.

− Lobe pumps are widely used in industries such as pulp and paper, chemical, food,

beverage, pharmaceutical and biotechnology etc.

− These pumps can handle solids (e.g., cherries and olives), slurries, pastes, and a variety of

liquids. A gentle pumping action minimizes product degradation.

− Lobe pumps are frequently used in food applications because they handle solids without

damaging the product. Large sized particles can be pumped much effectively than in other

positive displacement types. As the lobes do not make any direct contact therefore, the

clearance is not as close as in other Positive displacement pumps.

(c) Internal gear pumps

− Internal gear pumps are exceptionally versatile. They are often used for low or medium

viscosity fluids such as solvents and fuel oil and wide range of temperature. This is non-

pulsing, self-priming and can run dry for short periods. It is a variation of the basic gear

pump.

Figure 3.2 - Lobe pump



− It comprises of an internal gear, a regular spur gear, a crescent-shaped seal and an external

housing. The schematic of internal gear pump is shown in Figure 3.3.

− Liquid enters the suction port between the rotor (large exterior gear) and idler (small

interior gear) teeth. Liquid travels through the pump between the teeth and crescent. Crescent

divides the liquid and acts as a seal between the suction and discharge ports.

− When the teeth mesh on the side opposite to the crescent seal, the fluid is forced out

through the discharge port of the pump. This clearance between gears can be adjusted to

accommodate high temperature, to handle high viscosity fluids and to accommodate the wear.

− However, these pumps are not suitable for high speed and high pressure applications. Only

one bearing is used in the pump therefore overhung load on shaft bearing reduces the life of

the bearing.

Figure 3.3 - Internal gear pump

3.1.2 Vane Pumps

− In the previous topic we have studied the gear pumps. These pumps have a disadvantage of

small leakage due to gap between gear teeth and the pump housing. This limitation is

overcome in vane pumps.

− The leakage is reduced by using spring or hydraulically loaded vanes placed in the slots of

driven rotor.

− Vane pumps are available in a number of vane configurations including sliding vane,

flexible vane, swinging vane, rolling vane, and external vane etc.

− The operating range of these pumps varies from -32 °C to 260 °C.

− The schematic of vane pump working principle is shown in figure 3.4 Vane pumps generate

a pumping action by tracking of vanes along the casing wall.



− The vane pumps generally consist of a rotor, vanes, ring and a port plate with inlet and

outlet ports.

Figure 3.4 - Schematic of working principle of vane pump

− The rotor in a vane pump is connected to the prime mover through a shaft. The vanes are

located on the slotted rotor.

− The rotor is eccentrically placed inside a cam ring as shown in the figure. The rotor is

sealed into the cam by two side plates.

− When the prime mover rotates the rotor, the vanes are thrown outward due to centrifugal

force. The vanes track along the ring.

− It provides a tight hydraulic seal to the fluid which is more at the higher rotation speed due

to higher centrifugal force. This produces a suction cavity in the ring as the rotor rotates. It

creates vacuum at the inlet and therefore, the fluid is pushed into the pump through the inlet.

The fluid is carried around to the outlet by the vanes whose retraction causes the fluid to be

expelled.

− The capacity of the pump depends upon the eccentricity, expansion of vanes, and width of

vanes and speed of the rotor. It can be noted that the fluid flow will not occur when the

eccentricity is zero.

− These pumps can handle thin liquids (low viscosity) at relatively higher pressure.

− However, these pumps are not suitable for high speed applications and for the high

viscosity fluids or fluids carrying some abrasive particles.

− The maintenance cost is also higher due to many moving parts. These pumps have various

applications for the pumping of following fluids:

• Aerosol and Propellants

• Aviation Service - Fuel Transfer, Deicing

• Auto Industry - Fuels, Lubes, Refrigeration Coolants

• Bulk Transfer of LPG and NH3

• LPG Cylinder Filling



• Alcohols

• Refrigeration - Freons, Ammonia

• Solvents

Unbalanced Vane pump

Figure 3.5 - Unbalanced vane pump

− In practice, the vane pumps have more than one vane as shown in figure 3.5. The rotor is

offset within the housing, and the vanes are constrained by a cam ring as they cross inlet and

outlet ports.

− Although the vane tips are held against the housing, still a small amount of leakage exists

between rotor faces and body sides.

− Also, the vanes compensate to a large degree for wear at the vane tips or in the housing

itself. The pressure difference between outlet and inlet ports creates a large amount of load on

the vanes and a significant amount of side load on the rotor shaft which can lead to bearing

failure. This type of pump is called as unbalanced vane pump.

Adjustable vane pump

− The proper design of pump is important and a challenging task. In ideal condition, the

capacity of a pump should be exactly same to load requirements. A pump with larger capacity

wastes energy as the excess fluid will pass through the pressure relief valve.

− It also leads to a rise in fluid temperature due to energy conversion to the heat instead of

useful work and therefore it needs some external cooling arrangement. Therefore, the higher

capacity pump increases the power consumption and makes the system bulky and costly.

− Pumps are generally available with certain standard capacities and the user has to choose

the next available capacity of the pump. Also, the flow rate from the pump in most hydraulic

applications needs to be varying as per the requirements.

− Therefore, some vane pumps are also available with adjustable capacity as shown in figure

3.6. This can be achieved by adjusting a positional relationship between rotor and the inner

casing by the help of an external controlling screw.



Figure

3.6 - Adjustable vane pump

− These pumps basically consist of a rotor, vanes, cam ring, port plate, thrust bearing for

guiding the cam ring and a discharge control screw by which the position of the cam ring

relative to the rotor can be varied. In general, the adjustable vane pumps are unbalanced

pump type.

− The amount of fluid that is displaced by a vane pump running at a constant speed is

determined by the maximum extension of the vanes and the vanes width.

− However, for a pump running in operation, the width of vanes cannot be changed but the

distance by which the vanes are extended can be varied. This is possible by making a

provision for changing the position of the cam ring (adjustable inner casing) relative to the

rotor as shown in figure.

− The eccentricity of rotor with respect to the cam ring is adjusted by the movement of the

screw. The delivery volume increases with increase in the eccentricity. This kind of

arrangement can be used to achieve a variable volume from the pump and is known as

variable displacement vane pump.

Balanced vane pump

− Figure 3.7 shows the schematic of a balanced vane pump.

− This pump has an elliptical cam ring with two inlet and two outlet ports. Pressure loading

still occurs in the vanes but the two identical pump halves create equal but opposite forces on

the rotor. It leads to the zero net force on the shaft and bearings. Thus, lives of pump and

bearing increase significantly. Also the sounds and vibrations decrease in the running mode

of the pump.



Figure 3.7 - Balanced Vane Pump

3.1.3 Piston Pumps

− Piston pumps are meant for the high-pressure applications. These pumps have high-

efficiency and simple design and needs lower maintenance. These pumps convert the rotary

motion of the input shaft to the reciprocating motion of the piston. These pumps work similar

to the four stroke engines.

− They work on the principle that a reciprocating piston draws fluid inside the cylinder when

the piston retracts in a cylinder bore and discharge the fluid when it extends. These pumps are

positive displacement pump and can be used for both liquids and gases. Piston pumps are

basically of two types:

(a) Axial piston pumps

(b) Radial piston pumps

(a) Axial piston pumps

− Axial piston pumps are positive displacement pumps which converts rotary motion of the

input shaft into an axial reciprocating motion of the pistons. These pumps have a number of

pistons (usually an odd number) in a circular array within a housing which is commonly

referred to as a cylinder block, rotor or barrel. These pumps are used in jet aircraft.

− These pumps have sub-types as:

1. Bent axis piston pump

2. Swash plate axial piston pump

1. Bent axis piston pump

− Figure 3.8 shows the schematic of bent axis piston pump. In these pumps, the reciprocating

action of the pistons is obtained by bending the axis of the cylinder block.

− The cylinder block rotates at an angle which is inclined to the drive shaft. The cylinder

block is turned by the drive shaft through a universal link.



− The cylinder block is set at an offset angle with the drive shaft. The cylinder block contains

a number of pistons along its periphery. These piston rods are connected with the drive shaft

flange by ball-and-socket joints.

− The volumetric displacement (discharge) of the pump is controlled by changing the offset

angle. It makes the system simple and inexpensive.

− These pistons are forced in and out of their bores as the distance between the drive shaft

flange and the cylinder block changes. A universal link connects the block to the drive shaft,

to provide alignment and a positive drive.

− The discharge does not occur when the cylinder block is parallel to the drive shaft. The

offset angle can vary from 0° to 40°.

Figure 3.8 - Bent axis piston pump

2. Swash plate axial piston pump

− A swash plate is a device that translates the rotary motion of a shaft into the reciprocating

motion. It consists of a disk attached to a shaft as shown in Figure 3.9. If the disk is aligned

perpendicular to the shaft; the disk will turn along with the rotating shaft without any

reciprocating effect.

− Similarly, the edge of the inclined shaft will appear to oscillate along the shaft's length.

This apparent linear motion increases with increase in the angle between disk and the shaft

(offset angle). The apparent linear motion can be converted into an actual reciprocating

motion by means of a follower that does not turn with the swash plate.

− In swash plate axial piston pump a series of pistons are aligned coaxially with a shaft

through a swash plate to pump a fluid. The schematic of swash plate piston pump is shown in

Figure 3.9. The axial reciprocating motion of pistons is obtained by a swash plate that is

either fixed or has variable degree of angle.

− As the piston barrel assembly rotates, the piston rotates around the shaft with the piston

shoes in contact with the swash plate. The piston shoes follow the angled surface of the

swash plate and the rotational motion of the shaft is converted into the reciprocating motion

of the pistons.

− This reciprocating motion of the piston results in the drawing in and pumping out of the

fluid. Pump capacity can be controlled by varying the swash plate angle with the help of a



separate hydraulic cylinder. The pump capacity (discharge) increases with increase in the

swash plate angle and vice-versa.

Figure 3.9 - Swash plate axial piston pump

(b) Radial piston pumps

− The typical construction of radial piston pump is shown in Figure 3.10. The piston pump

has pistons aligned radially in a cylindrical block. It consists of a pintle, a cylinder barrel with

pistons and a rotor containing a reaction ring. The pintle directs the fluid in and out of the

cylinder. Pistons are placed in radial bores around the rotor.

− The piston shoes ride on an eccentric ring which causes them to reciprocate as they rotate.

The eccentricity determines the stroke of the pumping piston. Each piston is connected to

inlet port when it starts extending while it is connected to the outlet port when start retracting.

This connection to the inlet and outlet port is performed by the timed porting arrangement in

the pintle.

− As the cylinder barrel rotates, the pistons on one side travel outward. This draws the fluid

in as the cylinder passes the suction port of the pintle. It is continued till the maximum

eccentricity is reached. When the piston passes the maximum eccentricity, pintle is forced

inwards by the reaction ring. This forces the fluid to flow out of the cylinder and enter in the

discharge (outlet) port of the pintle.



Figure 3.10 - Radial piston pump

3.2 LINEAR ACTUATORS

Reliability of hydraulic system not only depends on the system design but also on factors

such as component design and manufacturing and their correct choice. This is also correct

while selecting the cylinder.

A correct cylinder in a hydraulic system contributes to:

1. Optimize system maintainability

2. Ensure minimum down time

3. Ease the process of repairing and trouble shooting

4. Ensure maximum work accuracy

5. Maintain least economic liability and financial losses

3.3 TYPES OF CYLINDER

Functionally cylinders are classified as:

1. Single acting cylinders



2. Double acting cylinder

3. Telescopic cylinders

4. Tandem cylinders

1. Single-Acting Cylinders

− A single-acting cylinder is simplest in design and is shown schematically in Fig 3.11. It

consists of a piston inside a cylindrical housing called barrel. On one end of the piston there

is a rod, which can reciprocate. At the opposite end, there is a port for the entrance and exit of

oil.

− Single-acting cylinders produce force in one direction by hydraulic pressure acting on the

piston. (Single-acting cylinders can exert a force in the extending direction only.) The return

of the piston is not done hydraulically. In single-acting cylinders, retraction is done either by

gravity or by a spring.

Figure 3.11 - Single-acting cylinders

According to the type of return, single-acting cylinders are classified as follows:

Gravity-return single-acting cylinder

Spring-return single-acting cylinder

Gravity-Return Single-Acting Cylinder

− Figure 3.12 shows gravity-return-type single-acting cylinders. In the push type [Fig. 3.12

(a)], the cylinder extends to lift a weight against the force of gravity by applying oil pressure

at the blank end.

− The oil is passed through the blank-end port or pressure port. The rod-end port or vent port

is open to atmosphere so that air can flow freely in and out of the rod end of the cylinder.

− To retract the cylinder, the pressure is simply removed from the piston by connecting the

pressure port to the tank. This allows the weight of the load to push the fluid out of the

cylinder back to the tank. In pull-type gravity-return-type single-acting cylinder, the cylinder

[Fig.3.12 (b)] lifts the weight by retracting.

− The blank-end port is the pressure port and blind-end port is now the vent port. This

cylinder automatically extends whenever the pressure port is connected to the tank.



Figure 3.12 - Gravity-return single-acting cylinder: (a) Push type; (b) pull type

Spring-Return Single-Acting Cylinder

− A spring-return single-acting cylinder is shown in Fig.3.13.In push type [Fig. 3.13 (a)], the

pressure is sent through the pressure port situated at the blank end of the cylinder.

− When the pressure is released, the spring automatically returns the cylinder to the fully

retracted position. The vent port is open to atmosphere so that air can flow freely in and out

of the rod end of the cylinder.

− Figure 3.13 (b) shows a spring-return single-acting cylinder. In this design, the cylinder

retracts when the pressure port is connected to the pump flow and extends whenever the

pressure port is connected to the tank. Here the pressure port is situated at the rod end of the

cylinder.

Figure 3.13 - (a) Push- and (b) pull-type single-acting cylinders

2. Double-Acting Cylinder

There are two types of double-acting cylinders:

Double-acting cylinder with a piston rod on one side.

Double-acting cylinder with a piston rod on both sides.

Double-Acting Cylinder with a Piston Rod on One Side

− Figure 3.14 shows the operation of a double-acting cylinder with a piston rod on one side.

To extend the cylinder, the pump flow is sent to the blank-end port as in Fig. 3.14 (a). The

fluid from the rod-end port returns to the reservoir.

− To retract the cylinder, the pump flow is sent to the rod-end port and the fluid from the

blank-end port returns to the tank as in Fig. 3.14 (b).



Figure 3.14 - Double-acting cylinder with a piston rod on one side

Double-Acting Cylinder with a Piston Rod on Both Sides

Figure 3.15 - Double-acting cylinder with a piston rod on one side

− A double-acting cylinder with a piston rod on both sides (Fig. 3.15) is a cylinder with a rod

extending from both ends. This cylinder can be used in an application where work can be

done by both ends of the cylinder, thereby making the cylinder more productive.

− Double-rod cylinders can withstand higher side loads because they have an extra bearing,

one on each rod, to withstand the loading.

3. Telescopic Cylinder

− A telescopic cylinder (shown in Fig. 3.16) is used when a long stroke length and a short

retracted length are required. The telescopic cylinder extends in stages, each stage consisting

of a sleeve that fits inside the previous stage.

− One application for this type of cylinder is raising a dump truck bed. Telescopic cylinders

are available in both single-acting and double-acting models. They are more expensive than

standard cylinders due to their more complex construction.



− They generally consist of a nest of tubes and operate on the displacement principle. The

tubes are supported by bearing rings, the innermost (rear) set of which have grooves or

channels to allow fluid flow.

Figure 3.16 - Telescopic cylinder

− For a given input flow rate, the speed of operation increases in steps as each successive

section reaches the end of its stroke. Similarly, for a specific pressure, the load-lifting

capacity decreases for each successive section.

4. Tandem Cylinder

Figure 3.17 - Tandem cylinder

− A tandem cylinder, shown in Fig. 3.17, is used in applications where a large amount of

force is required from a small-diameter cylinder. Pressure is applied to both pistons, resulting

in increased force because of the larger area.

− The drawback is that these cylinders must be longer than a standard cylinder to achieve an

equal speed because flow must go to both pistons.



5. Diaphragrn Cylinder

− These are used for very small displacements. There will be a diaphragm instead of piston

inside the cylinder. The diaphragm deflects when working fluid is admitted into the cylinder.

− The diaphragm is fitted with a piston rod and hence the piston rod extends or retracts.

− Applications: Food industries, Milk dairies, Air brake etc.

3.4 ROTARY ACTUATORS

− Hydraulic motors are rotary actuators. However, the name rotary actuator is reserved for a

particular type of unit that is limited in rotation to less than 360 degree.

− A hydraulic motor is a device which converts fluid power into rotary power or converts

fluid pressure into torque.

− Torque is a function of pressure or, in other words, the motor input pressure level is

determined by the resisting torque at the output shaft. A hydraulic pump is a device which

converts mechanical force and motion into fluid power.

− A hydraulic motor is not a hydraulic pump when run backward. A design that is completely

acceptable as a motor may operate very poorly as a pump in a certain applications.

Table 3.1 - Differences between a hydraulic motor and a hydraulic pump

Hydraulic Motor Hydraulic Pump

It is a device for delivering torque at a given

pressure. The main emphasis is on

mechanical efficiency and torque that can be

transmitted.

It is a device for delivering flow at a given

pressure. The main emphasis is on

volumetric efficiency and flow.

Motors usually operate over a wide range of

speed, from a low RPM to high RPM.

Pumps usually operate at high RPM.

Most motors are designed for bidirectional

applications such as braking loads, rotary

tables.

In most situations, pumps usually operate in

one direction.

Motors may be idle for long time (as in

index table).

Pumps usually operate continuously.

Motors are subjected to high side loads

(from gears, chains, belt-driven pulleys).

Majority of pumps are not subjected to side

loads. Usually pumps are pad mounted on

power pack top and shaft is connected to the

prime mover directly.



3.3.1 Applications

− Hydraulic motors have become popular in industries. Hydraulic motors can be applied

directly to the work. They provide excellent control for acceleration, operating speed,

deceleration, smooth reversals and positioning.

− They also provide flexibility in design and eliminate much of bulk and weight of

mechanical and electrical power transmission.

− The applications of hydraulic motors in their various combinations with pumping units are

termed hydrostatic transmission.

− A hydrostatic transmission converts mechanical power into fluid power and then reconverts

fluid power into shaft power. The advantages of hydrostatic transmissions include power

transmission to remote areas, infinitely variable speed control, self-overload protection,

reverse rotation capability, dynamic braking and a high power-to-weight ratio. Applications

include material-handling equipment, farm tractors, railway locomotives, buses, lawn

mowers and machine tools.

− New fields of applications are being discovered constantly for hydrostatic transmissions.

Farm implements, road machinery, material-handling equipment, Numerical Control(NC)

machines high-performance aircrafts, military uses and special machinery are only a few of

new fields expanding through the use of fluid power transmission. Many automobiles,

railway locomotives and buses use a hydrostatic transmission.

Table 3.2 - Comparison between a hydraulic motor and an electric motor

Electric Motor Hydraulic Motor

Electric motors cannot be stopped instantly.

Their direction of rotation cannot be

reversed instantly. This is because of air gap

between the rotor and stator and the weak

magnetic field.

Hydraulic motors can be stalled for any

length of time. Their direction of rotation

can be instantly reversed and their rotational

speed can be infinitely varied without

affecting their torque. They can be braked

instantly and have immense torque

capacities.

Electric motors are heavy and bulky.

Hydraulic motors are very compact

compared to electric motors. For the same

power, they occupy about 25% of the space

required by electric motors and weigh about

10% of electric motors.

Moment of inertia-to-torque ratio is nearly

100.

Moment of inertia-to-torque ratio is nearly

1.



3.5 CLASSIFICATION OF HYDRAULIC MOTORS

− There are two types of hydraulic motors: (a) High-speed low-torque motors and (b) low–

speed high-torque motors. In high-speed low-torque motors, the shaft is driven directly from

either the barrel or the cam plate, whereas in low-speed high-torque motors, the shaft is

driven through a differential gear arrangement that reduces the speed and increases the

torque.

− Depending upon the mechanism employed to provide shaft rotation, hydraulic motors can

be classified as follows:

1. Gear motors.

2. Vane motors.

3. Piston motors:

Axial piston-type motors.

Radial piston-type motors.

− Gear motors are the least efficient, most dirt-tolerant and have the lowest pressure rating of

3. Piston motors are the most efficient, least dirt-tolerant and have high pressure ratings.

Vane and piston motors can be fixed or variable displacement, but gear motors are available

with only fixed displacement.

1. Gear Motors:

− A gear motor develops torque due to hydraulic pressure acting against the area of one tooth.

There are two teeth trying to move the rotor in the proper direction, while one net tooth at the

center mesh tries to move it in the opposite direction.

− In the design of a gear motor, one of the gears is keyed to an output shaft, while the other is

simply an idler gear. Pressurized oil is sent to the inlet port of the motor. Pressure is then

applied to the gear teeth, causing the gears and output shaft to rotate.

− The pressure builds until enough torque is generated to rotate the output shaft against the

load. The side load on the motor bearing is quite high, because all the hydraulic pressure is on

one side. This limits the bearing life of the motor. Schematic diagram of gear motor is shown

in Fig.

− Gear motors are normally limited to 150 bar operating pressures and 2500 RPM operating

speed. They are available with a maximum flow capacity of 600 LPM.

− The gear motors are simple in construction and have good dirt tolerance, but their

efficiencies are lower than those of vane or piston pumps and they leak more than the piston

units. Generally, they are not used as servo motors.

− Hydraulic motors can also be of internal gear design. These types can operate at higher

pressures and speeds and also have greater displacements than external gear motors.



Figure 3.18 - Gear motor

2. Vane Motors

− Figure 3.19 shows an unbalanced vane motor consisting of a circular chamber in which

there is an eccentric rotor carrying several spring or pressure-loaded vanes. Because the fluid

flowing through the inlet port finds more area of vanes exposed in the upper half of the

motor, it exerts more force on the upper vanes, and the rotor turns counter clockwise. Close

tolerances are maintained between the vanes and ring to provide high efficiencies.

− The displacement of a vane hydraulic motor is a function of eccentricity. The radial load on

the shaft bearing of an unbalanced vane motor is also large because all its inlet pressure is on

one side of the rotor.

Figure 3.19 - Vane motor



3. Piston Motors

Piston motors are classified into the following types:

1. According to the piston of the cylinder block and the drive shaft, piston motors are

classified as follows:

Axial piston motors.

Radial piston motors.

2. According to the basis of displacement, piston motors are classified as follows:

Fixed-displacement piston motors.

Variable-displacement piston motors

Axial Piston Motors

− In axial piston motors, the piston reciprocates parallel to the axis of the cylinder block.

These motors are available with both fixed-and variable-displacement feature types. They

generate torque by pressure acting on the ends of pistons reciprocating inside a cylinder

block.

− Figure 3.20 illustrates the inline design in which the motor, drive shaft and cylinder block

are centered on the same axis. Pressure acting on the ends of the piston generates a force

against an angled swash plate. This causes the cylinder block to rotate with a torque that is

proportional to the area of the pistons.

− The torque is also a function of the swash-plate angle. The inline piston motor is designed

either as a fixed- or a variable-displacement unit.

− In variable-displacement units, the swash plate is mounted on the swinging yoke. The angle

can be varied by various means such as a lever, hand wheel or servo control. If the offset

angel is increased, the displacement and torque capacity increase but the speed of the drive

shaft decreases. Conversely, reducing the angle reduces the torque capability but increases

the drive shaft speed.

Figure 3.20 - Straight axis piston pump

Bent-Axis Piston Motors

− A bent-axis piston motor is shown in Fig 3.21. This type of motor develops torque due to

pressure acting on the reciprocating piston. In this motor, the cylinder block and drive shaft

mount at an angel to each other so that the force is exerted on the drive shaft flange.



− Speed and torque depend on the angle between the cylinder block and the drive shaft. The

larger the angle, the greater the displacement and torque, and the smaller the speed. This

angle varies from 7.5° (minimum) to 30° (maximum). This type of motor is available in two

types, namely fixed-displacement type and variable-displacement type.

Figure 3.21 - Bent axis piston motor

3.6 HYDROSTATIC TRANSMISSION SYSTEM

In this type of drives a hydrostatic pump and a motor is used.

− The engine drives the pump and it generates hydrostatic pressure on the fluid.

− The pressurized fluid then fed to the motor and the motor drives the wheel.

− In these transmissions mechanical power is generated in the motor as a result of

displacement under hydraulic pressure.

− The fluid, of course, also carries kinetic energy, but since it leaves the motor at the same

velocity as that at which it enters, there is no change in its kinetic- energy content, and kinetic

energy plays no part in the transmission of power.

− It consists of a pump, which converts torque and rotation of mechanical shaft into flow of

pressurized fluid combined with a hydraulic motor, which converts fluid flow under pressure

into rotating torque on the output shaft.

− The pump and motor are identical in construction but they may vary in size and

displacement, particularly when torque multiplication is needed.

− By employing variable delivery of hydraulic units, it is possible to obtain a wide range of

output ratios.

Various types of hydrostatic systems

1. Constant displacement pump and constant displacement motor

2. Variable displacement pump and constant displacement motor



Figure 3.22 – Hydrostatic transmission system

3. Constant displacement pump and variable displacement motor

4. Variable displacement pump and variable displacement motor

Advantages of hydrostatic drive

− Hydrostatic drive eliminates the need for mechanical transmission components like clutch

and gearbox as well as allied controls.

− It provides for smooth and precise control of vehicle speed and travel.

− This system ensures faster acceleration and deceleration of vehicle

− It offers better flexibility in vehicle installation because of wide range in choice of pumps

and motors of different capacities and of fixed or variable displacement type. Besides

hydraulic fluid pipes lines replace mechanical transmission drive line components

− The ease with which the reverse drive can be obtained makes the hydrostatic drive more

attractive. This drive is fully reversible from maximum speed in one direction to zero speed

and to maximum speed in the reverse direction.

Limitations of hydrostatic drive

− Noisy in operation

− Heavier in weight and larger in bulk

− Costlier when compared to other types of transmission

− Manufacturing of pump and motor requires high precision machining of components and

skilled workmanship

− In view of high pressure employed in system, the working components are heavier.

− It also possesses problem of oil leakage through oil seals.

Applications of hydrostatic drive

− It is used to move the machine tools accurately.

− Used in steering gears of ship.

− Used in war ships to operate gun turrets.



− Used in road rollers, tractors, earth movers, heavy duty trucks.

Comparison of hydrostatic drive with hydrodynamic drives

− Torque ratio is lesser in hydrostatic drives for different speed ratios.

− Hydrostatic offers high efficiency over a wide range of speeds when compared to

hydrodynamic drives.

− Vehicle with hydrostatic drive has no tendency to creep unlike hydrodynamic drive during

idling.

− Dynamic braking of vehicle is an inherent feature of hydrostatic drive. This feature helps to

eliminate conventional shoe or disc type of brakes. Creep is caused to drag torque, movement

of vehicle during idling


4 HYDRAULIC VALVES AND ACCESSORIES

Course Contents

4.1 Control valves

4.2 Direction control valve

4.3 Pressure control valve

4.4 Flow control valves

4.5 Accumulators

4.6 Reservoirs

4.7 Hoses

4.8 Heat exchangers (heating and cooling devices)

Oil Hydraulics and Pneumatics (2171912) 4. Hydraulic Valves and Accessories


4.1 CONTROL VALVES

In a hydraulic system, the hydraulic energy available from a pump is converted into motion

and force by means of an actuator. The control of these mechanical outputs (motion and

force) is one of the most important functions in a hydraulic system. The proper selection of

control selection ensures the desired output and safe function of the system. In order to

control the hydraulic outputs, different types of control valves are required. It is important to

know various types of control valves and their functions. This not only helps to design a

proper hydraulic system but also helps to discover the innovative ways to improve the

existing systems.

There are basically three types of valves employed in hydraulic systems:

1. Directional control valves

2. Flow control valves

3. Pressure control valves

− These control valves contain ports that are external openings for the fluid to enter and

leave. The number of ports is usually identified by the term ‘way’. For example, a valve with

four ports is named as four-way valve.

− The fluid flow rate is responsible for the speed of actuator (motion of the output) and

should controlled in a hydraulic system. This operation can be performed by using flow

control valves. The pressure may increase gradually when the system is under operation. The

pressure control valves protect the system by maintaining the system pressure within the

desired range. Also, the output force is directly proportional to the pressure and hence, the

pressure control valves ensure the desired force output at the actuator.

4.2 DIRECTION CONTROL VALVE

− Directional control valves are used to control the distribution of energy in a fluid power

system. They provide the direction to the fluid and allow the flow in a particular direction.

− These valves are used to control the start, stop and change in direction of the fluid flow.

These valves regulate the flow direction in the hydraulic circuit.

− Directional control valves can be classified in the following manner:

1. Type of construction:

• Poppet valves

• Spool valves

2. Number of ports:

• Two- way valves

• Three – way valves

• Four- way valves.

3. Number of switching position:

• Two – position



• Three - position

4. Actuating mechanism:

• Manual actuation

• Mechanical actuation

• Solenoid actuation

• Hydraulic actuation

• Pneumatic actuation

• Indirect actuation

1. Type of construction

Check Valves

Figure 4.1 - Check valve

− These are unidirectional valves and permit the free flow in one direction only. These valves

have two ports: one for the entry of fluid and the other for the discharge.

− They are consists of a housing bore in which ball or poppet is held by a small spring force.

The valve having ball as a closing member is known as ball check valve.

− The various types of check valves are available for a range of applications. These valves

are generally small sized, simple in construction and inexpensive.

− Generally, the check valves are automatically operated. Human intervention or any external

control system is not required.

− These valves can wear out or can generate the cracks after prolonged usage and therefore

they are mostly made of plastics for easy repair and replacements. The check valve is

designed for a specific cracking pressure which is the minimum upstream pressure at which

the valve operates.

− The ball is held against the valve seat by a spring force. It can be observed from the figure

that the fluid flow is not possible from the spring side but the fluid from opposite side can

pass by lifting the ball against.



− However, there is some pressure drop across the valve due to restriction by the spring force.

Therefore these valves are not suitable for the application of high flow rate. When the

operating pressure increases the valve becomes more tightly seated in this design.

− When the closing member is not a ball but a poppet energized by a spring is known as

poppet valve. The advantages of the poppet valves include no leakage, long life and

suitability with high pressure applications. These valves are commonly used in liquid or gel

mini-pump dispenser spigots, spray devices, some rubber bulbs for pumping air, manual air

pumps, and refillable dispensing syringes. The pressure drop is comparatively less in right

angle check valve.

− Some valves are meant for an application where free flow is required in one direction and

restricted flow required in another direction. These types of valves are called as restriction

check valve. These valves are used when a direction sensitive flow rate is required. For

example, the different actuator speeds are required in both the directions. The flow

adjustment screw can be used to set the discharge (flow rate) in the restricted direction.

Figure 4.2 - Restriction check valve

− Another important type of check valve known as pilot operated check valve which is

shown in figure 4.3. The function of the pilot operated check valve is similar to a normal

check valve unless it gets an extra pressure signal through a pilot line.

− Pilot allows free flow in one direction and prevents the flow in another direction until the

pilot pressure is applied. But when pilot pressure acts, the poppet opens and the flow is

blocked from both the sides. These valves are used to stop the fluid suddenly.

Spool valve

− The spool valves derive their name from their appearance. It consists of a shaft sliding in a

bore which has large groove around the circumference.

− This type of construction makes it look like a spool. The spool is sealed along the clearance

between moving spool and housing (valve body).

− The quality of seal or the amount of leakage depends on the amount of clearance, viscosity

of fluid and the level of the pressure.



Figure 4.3 - Pilot operated check valve

− The grooves guide the fluid flow by interconnecting or blocking the holes (ports). The

spool valves are categorized according to the number of operating positions and the way

hydraulic lines interconnections.

− One of the simplest two way spool valve is shown in Figure 4.4. The standard terms are

referred as Port ‘P’ is pressure port, Port ‘T’ is tank port and Port ‘A’ and Port ‘B’ are the

actuator (or working) ports. The actuators can move in forward or backward direction

depending on the connectivity of the pressure and tank port with the actuators port.

Figure 4.4 - Valve closed

Figure 4.5 - Valve opened by actuation

2. Number of ports

Two way valves

− Two way valves have only two ports as shown in Figure 4.4 and Figure 4.5. These valves

are also known as on-off valves because they allow the fluid flow only in direction.



− Normally, the valve is closed. These valves are available as normally open and normally

closed function. These are the simplest type of spool valves.

− When actuating force is not applied to the right, the port P is not connected with port A as

shown in figure. Therefore, the actuation does not take place.

− Similarly, Figure 4.5 shows the two-way spool valve in the open condition. Here, the

pressure port P is connected with the actuator port A.

Three way valves

− When a valve has one pressure port, one tank port and one actuating port as shown in

Figures 4.6, it is known as three way valve.

− In this valve, the pressure port pressurizes one port and exhausts another one. As shown in

figures 4.6, only one actuator port is opened at a time.

− In some cases a neutral position is also available when both the ports are blocked.

Generally, these valves are used to operate single acting cylinders.

Figure 4.6 - Three way valve: P to A connected and T is blocked

Figure 4.7 - Three way valve in closed position

Four way valves

− It is generally used to operate the cylinders and fluid motors in both the directions. The four

ways are: pump port P, tank port T, and two working ports A and B connected to the actuator.

− The primary function of a four way valve is to pressurize and exhaust two working ports A

and B alternatively.

− Figures 4.9 and show three position four way valves. These types of valves have three

switching positions. They have a variety of possible flow path configurations but have

identical flow path configuration.



Figure 4.8 - Three position four way valve in open center mode

− When the centered path is actuated, port A and B are connected with both the ports P and T

respectively. In this case, valve is not active because all the ports are open to each other. The

fluid flows to the tank at atmospheric pressure. In this position work cannot be done by any

part of the system. This configuration helps to prevent heat build-up.

Figure 4.9 - Three position four way valve: P to B and A to T

− When left end (port B) is actuated, the port P is connected with ports B and T is connected

with port A as shown in Figure 4.9. Similarly, when the right end is actuated the port P is

connected to A and working port B is connected to port T as shown in Figure 4.10. The three

position valves are used when the actuator is needed to stop or hold at some intermediate

position.

Figure 4.10 - Three position four way valve: P to A and B to T

Figure 4.11 - Three position four way valve: closed center



− Figure 4.11 shows a three position four way valve in the closed center position. The

working of the valve is similar to open center DCV. In closed center DCV all user ports (port

A and port B) are closed. Therefore, these ports are hydraulically locked and the actuator

cannot be moved by the external load.

− The pumped fluid flows through the relief valve. The pump works under the high pressure

condition which not only wastes the pump power but also causes wear of the pump parts. The

fluid temperature also rises due to heat generation by the pump energy transformation. The

increase in fluid temperature may lead to the oxidation and viscosity drop of the fluid. The

oxidation and viscosity drop reduces the pump life and leakage in the system.

Figure 4.12 - Tandem centered valve

− Figure 4.12 shows a tandem center three position four way direction control valve. In this

configuration, the working ports A and B are blocked and the pump port P is connected to the

tank port T. Tandem center results in the locked actuator.

− However, pump to tank flow takes place at the atmospheric temperature. This kind of

configuration can be used when the load is needed to hold. Disadvantages of high pressure

pumping in case of closed center can be removed by using this configuration.

− The regenerative center is another important type of common center configuration used in

hydraulic circuits. Regenerative means the flow is generated from the system itself.

− Regenerative center is used when the actuator movement in one direction requires two

different speeds. For example, the half-length of the stroke requires fast movement during no-

load condition and remaining half-length requires slow motion during load conditions. The

regenerative center saves the pump power.

Figure 4.13 - Regenerative Center



− Figure 4.13 shows the regenerative configuration for the three position four way (3/4) DCV

in its mid position. This configuration increases the piston speed. In the mid position pump

Port P is connected to A and B, and tank port T is blocked.

− Figure 4.14 shows the floating center 3/4 DCV in its mid position. In this configuration, the

pump port is blocked and both the working ports A and B are connected to the tank port T.

Therefore, the working ports A and B can be moved freely which is reason they are called as

floating center. The pumped fluid passes through the relief valve. Therefore, pump works in

the high pressure condition. This configuration is used only in some special cases.

Figure 4.14 - Floating Center

Two position four way (2/4) valves

− The two position four way valves have only two switching positions and do not have any

mid position. Therefore, they are also known as impulse valves. The typical connections of

2/4 valves is shown in Figures 4.15 and 4.16. These valves can be used to operate double

acting cylinders. These are also used to reciprocate or hold an actuator.

− The operation is faster because the distance between ports of these valves is smaller.

Hence, these valves are used on machines where fast reciprocation cycles are needed such as

punching and stamping etc.

Figure 4.15 - Two position four way DCV: P to B and A to T

Figure 4.16 - Two position four way DCV: P to A and B to T



4. Classification based on actuation mechanism

Manual actuation

In this type, the spool is operated manually. Manual actuators are hand lever, push button and

pedals etc.

Mechanical actuation

− The DCV spool can be operated by using mechanical elements such as roller and cam,

roller and plunger and rack and pinion etc. In these arrangements, the spool end is of roller or

a pinion gear type. The plunger or cam or rack gear is attached to the actuator. Thus, the

mechanical elements gain some motion relative to the actuator (cylinder piston) which can be

used for the actuation.

Solenoid actuation

− The solenoid actuation is also known as electrical actuation. The schematic of solenoid

actuation is shown in Figure 4.17. The energized solenoid coil creates a magnetic force which

pulls the armature into the coil. This movement of armature controls the spool position. The

main advantage of solenoid actuation is its less switching time.

Figure 4.17 - Working of solenoid to shift spool of valve

Hydraulic actuation

− This type actuation is usually known as pilot-actuated valve and a schematic is shown in

Figure 4.18. In this type of actuation, the hydraulic pressure is directly applied on the spool.

The pilot port is located on one end of the valve. Fluid entering from pilot port operates

against the piston and forces the spool to move forward. The needle valve is used to control

the speed of the actuation.

Figure 4.18 - Pilot actuated DCV



Pneumatic actuation

− DCV can also be operated by applying compressed air against a piston at either end of the

valve spool. The construction of the system is similar to the hydraulic actuation as shown in

Figure 4.18. The only difference would be the actuation medium. The actuation medium is

the compressed air in pneumatic actuation system.

Indirect actuation of directional control valve

− The direction control valve can be operated by manual, mechanical, solenoidal (electrical),

hydraulic (pilot) and pneumatic actuations. The mode of actuation does not have any

influence on the basic operation of the hydraulic circuits.

− Mostly, the direct actuation is restricted to use with smaller valves only because usually lot

of force is not available. The availability of limited force is the greatest disadvantage of the

direct actuation systems. In practice, the force required to shift the spool is quiet higher.

Therefore, the larger valves are often indirectly actuated in sequence.

− First, the smaller valve is actuated directly and the flow from the smaller valve is directed

to either side of the larger valve.

− The control fluid can be supplied by the same circuit or by a separate circuit. The pilot

valve pressure is usually supplied internally.

− These two valves are often incorporated as a single unit. These valves are also called as

Electro-hydraulic operated DCV.

4.3 PRESSURE CONTROL VALVE

The pressure control valves are used to protect the hydraulic components from excessive

pressure. This is one of the most important components of a hydraulic system and is

essentially required for safe operation of the system. Its primary function is to limit the

system pressure within a specified range. It reduces the system pressure and as the pressure

reduces to the set limit again the valve closes. Various types of pressure control valves are

discussed in the following sections:

1. Direct type of relief valve

− Schematic of direct pressure relief valve is shown in figure 4.19. This type of valves has

two ports; one of which is connected to the pump and another is connected to the tank. It

consists of a spring chamber where poppet is placed with a spring force.

− Generally, the spring is adjustable to set the maximum pressure limit of the system. The

poppet is held in position by combined effect of spring force and dead weight of spool.

− As the pressure exceeds this combined force, the poppet raises and excess fluid bypassed to

the reservoir (tank). The poppet again reseats as the pressure drops below the pre-set value. A

drain is also provided in the control chamber. It sends the fluid collected due to small leakage

to the tank and thereby prevents the failure of the valve.



Figure 4.19 - Pressure Relief Valve

2. Unloading Valve

− The construction of unloading valve is shown in Figure 4.20. This valve consists of a

control chamber with an adjustable spring which pushes the spool down.

− The valve has two ports: one is connected to the tank and another is connected to the pump.

The valve is operated by movement of the spool. Normally, the valve is closed and the tank

port is also closed.

Figure 4.20 - Unloading Valve

− These valves are used to permit a pump to operate at the minimum load. It works on the

same principle as direct control valve that the pump delivery is diverted to the tank when

sufficient pilot pressure is applied to move the spool.

The pilot pressure maintains a static pressure to hold the valve opened. The pilot pressure

holds the valve until the pump delivery is needed in the system.

− As the pressure is needed in the hydraulic circuit; the pilot pressure is relaxed and the spool

moves down due to the self-weight and the spring force. Now, the flow is diverted to the

hydraulic circuit.



− The drain is provided to remove the leaked oil collected in the control chamber to prevent

the valve failure. The unloading valve reduces the heat build-up due to fluid discharge at a

pre-set pressure value.

3. Sequence valve

− The primary function of this type of valve is to divert flow in a predetermined sequence. It

is used to operate the cycle of a machine automatically. A sequence valve may be of direct-

pilot or remote-pilot operated type.

Schematic of the sequence valve is shown in Figure 4.21. Its construction is similar to the

direct relief valve.

− It consists of the two ports; one main port connecting the main pressure line and another

port (secondary port) is connected to the secondary circuit. The secondary port is usually

closed by the spool.

− The pressure on the spool works against the spring force. When the pressure exceeds the

pre-set value of the spring; the spool lifts and the fluid flows from the primary port to the

secondary port.

Figure 4.21 - Sequence valve

− For remote operation; the passage used for the direct operation is closed and a separate

pressure source for the spool operation is provided in the remote operation mode.

4. Counterbalance Valve

− The schematic of counterbalance valve is shown in Figure 4.22. It is used to maintain the

back pressure and to prevent a load from failing. The counterbalance valves can be used as

breaking valves for decelerating heavy loads.



− These valves are used in vertical presses, lift trucks, loaders and other machine tools where

position or hold suspended loads are important.

− Counterbalance valves work on the principle that the fluid is trapped under pressure until

pilot pressure overcomes the pre-set value of spring force. Fluid is then allowed to escape,

letting the load to descend under control. This valve is normally closed until it is acted upon

by a remote pilot pressure source. Therefore, a lower spring force is sufficient. It leads to the

valve operation at the lower pilot pressure and hence the power consumption reduces, pump

life increases and the fluid temperature decreases.

Figure 4.22 - Counter Balance Valve

5. Pressure Reducing Valve

− Sometimes a part of the system may need a lower pressure. This can be made possible by

using pressure reducing valve as shown in Figure 4.23. These valves are used to limit the

outlet pressure.

− Generally, they are used for the operation of branch circuits where the pressure may vary

from the main hydraulic pressure lines.

− These are open type valve and have a spring chamber with an adjustable spring, a movable

spool as shown in figure. A drain is provided to return the leaked fluid in the spring (control)

chamber.

− A free flow passage is provided from inlet port to the outlet port until a signal from the

outlet port tends to throttle the passage through the valve. The pilot pressure opposes the

spring force and when both are balanced, the downstream is controlled at the pressure setting.

− When the pressure in the reduced pressure line exceeds the valve setting, the spool moves

to reduce the flow passage area by compressing the spring. It can be seen from the figure that

if the spring force is more, the valve opens wider and if the controlled pressure has greater

force, the valves moves towards the spring and throttles the flow.



Figure 4.23 - Pressure Reducing Valve

Table 4.1 – Difference between pressure relief valve and pressure reducing valve

Sr.

No;

Pressure Relief valve Pressure Reducing Valve

1 This is NC (Normally Closed) type

valve This is NO (Normally Open) tvpe valve

2 Fitted in by-pass line to reservoir tank Fitted in main line to system

3 Outlet is connected to reservoir tank. Outlet is connected to system

4 Inlet pressure is system pressure. Outlet pressure is system pressure

5 Inlet pressure is the pilot pressure, Outlet pressure is pilot pressure.

6

It opens when inlet pressure (system

pressure or pilot pressure) becornes

more than pre-set

value

It closes when outlet pressure (System

pressure or pilot pressure) becomes

more then pre-set value

Table 4.2 – Difference between pressure relief valve and Unloading valve

Sr.

No, Pressure relief valve Unloading valve

1 Its inlet itself is its pilot connection. It has a separate Pilot connection.

2 It is set for maximum pressure

required for operation of the system.

It is set for the minimum pressure

required during idle period of the

system.



3 It opens when its inlet pressure

(maximum system pressure) increases

above pre-set value

It opens when the pilot pressure

(minimum

idle-period pressure) increases above

pre-set value.

4 It is a safety valve which avoids

damage to the system components due

to high pressure.

It avoids over heating of working fluid,

and it saves power to a greater extent.

4.4 FLOW CONTROL VALVES

− In practice, the speed of actuator is very important in terms of the desired output and needs

to be controlled. The speed of actuator can be controlled by regulating the fluid flow.

− A flow control valve can regulate the flow or pressure of the fluid. The fluid flow is

controlled by varying area of the valve opening through which fluid passes.

− The fluid flow can be decreased by reducing the area of the valve opening and it can be

increased by increasing the area of the valve opening.

− A very common example to the fluid flow control valve is the household tap. Figure 4.24

shows the schematic diagram of a flow control valve. The pressure adjustment screw varies

the fluid flow area in the pipe to control the discharge rate.

Figure 4.24 - Flow Control Valve

− The pressure drop across the valve may keep on fluctuating. In general, the hydraulic

systems have a pressure compensating pump.

− The inlet pressure remains almost constant but the outlet pressure keeps on fluctuating

depending on the external load. It creates fluctuating pressure drop. Thus, the ordinary flow

control valve will not be able to maintain a constant fluid flow.

− A pressure compensated flow control valve maintains the constant flow throughout the

movement of a spool, which shifts its position depending on the pressure.

− Flow control valves can also be affected by temperature changes. It is because the viscosity

of the fluid changes with temperature. Therefore, the advanced flow control valves often have

the temperature compensation. The temperature compensation is achieved by the thermal

expansion of a rod, which compensates for the increased coefficient of discharge due to

decreasing viscosity with temperature.



Types of Flow Control Valves

The flow control valves work on applying a variable restriction in the flow path. Based on the

construction; there are mainly four types viz. plug valve, butterfly valve, ball valve and

balanced valve.

1. Plug or glove valve

− The plug valve is quite commonly used valve. It is also termed as glove valve. Schematic

of plug or glove valve is shown in Figure 4.25.

− This valve has a plug which can be adjusted in vertical direction by setting flow adjustment

screw. The adjustment of plug alters the orifice size between plug and valve seat. Thus the

adjustment of plug controls the fluid flow in the pipeline.

− The characteristics of these valves can be accurately predetermined by machining the taper

of the plug. The typical example of plug valve is stopcock that is used in laboratory

glassware.

− The valve body is made of glass or teflon. The plug can be made of plastic or glass. Special

glass stopcocks are made for vacuum applications. Stopcock grease is used in high vacuum

applications to make the stopcock air-tight.

Figure 4.25 - Plug or glove valve

2. Butterfly valve

− A butterfly valve is shown in Figure 4.26. It consists of a disc which can rotate inside the

pipe. The angle of disc determines the restriction.

− Butterfly valve can be made to any size and is widely used to control the flow of gas. These

valves have many types which have for different pressure ranges and applications.

− The resilient butterfly valve uses the flexibility of rubber and has the lowest pressure rating.

The high performance butterfly valves have a slight offset in the way the disc is positioned. It

increases its sealing ability and decreases the wear.



− For high-pressure systems, the triple offset butterfly valve is suitable which makes use of a

metal seat and is therefore able to withstand high pressure. It has higher risk of leakage on the

shut-off position and suffer from the dynamic torque effect.

− Butterfly valves are favoured because of their lower cost and lighter weight. The disc is

always present in the flow therefore a pressure drop is induced regardless of the valve

position.

Figure 4.26 Butterfly valve

3. Balanced valve

− Schematic of a balanced valve is shown in figure 4.27. It comprises of two plugs and two

seats. The opposite flow gives little dynamic reaction onto the actuator shaft. It results in the

negligible dynamic torque effect.

− However, the leakage is more in these kind of valves because the manufacturing tolerance

can cause one plug to seat before the other.

− The pressure-balanced valves are used in the houses. They provide water at nearly constant

temperature to a shower or bathtub despite of pressure fluctuations in either the hot or cold

supply lines.

Figure 4.27 - Balanced valve

4.5 ACCUMULATORS

A hydraulic accumulator is a device that stores the potential energy of an incompressible

fluid held under pressure by an external source against some dynamic force. This dynamic

force can come from different sources. The stored potential energy in the accumulator is a

quick secondary source of fluid power capable of doing useful work.



4.5.1 Types of accumulators

1. Weight-loaded or gravity accumulator:

− Schematic diagram of weight loaded accumulator is shown in Fig. 4.28. It is a vertically

mounted cylinder with a large weight.

− When the hydraulic fluid is pumped into it, the weight is raised. The weight applies a force

on the piston that generates a pressure on the fluid side of piston.

− The advantage of this type of accumulator over other types is that it applies a constant

pressure on the fluid throughout its range of motion.

− The main disadvantage is its extremely large size and heavy weight. This makes it

unsuitable for mobile application.

2. Spring-loaded accumulator:

− A spring-loaded accumulator stores energy in the form of a compressed spring. A hydraulic

fluid is pumped into the accumulator, causing the piston to move up and compress the spring

as shown in Fig. 4.29. The compressed spring then applies a force on the piston that exerts a

pressure on the hydraulic fluid.

− This type of accumulator delivers only a small volume of oil at relatively low pressure.

Furthermore, the pressure exerted on the oil is not constant as in the dead-weight-type

accumulator. As the springs are compressed, the accumulator pressure reaches its peak, and

as the springs approach their free lengths, the accumulator pressure drops to a minimum.

Figure 4.28 - Dead weight accumulator



Figure 4.29 - Spring-loaded accumulator

3. Gas-loaded accumulator:

− A gas-loaded accumulator is popularly used in industries. Here the force is applied to the

oil using compressed air. Schematic diagram of a gas loaded accumulator is shown in Fig.

4.30. A gas accumulator can be very large and is often used with water or high water-based

fluids using air as a gas charge.

− Typical application is on water turbines to absorb pressure surges owing to valve closure

and on ram pumps to smooth out the delivery flow. The exact shape of the accumulator

characteristic curve depends on pressure–volume relations:

Isothermal (constant temperature): This occurs when the expansion or compression

of the gas is very slow. The relationship between absolute pressure p and volume V of

the gas is constant:

pV = constant

Isentropic (adiabatic processes): This is where there is no flow of energy into or out

of the fluid. The law that the gas obeys is given by pVγ = constant, where γ is ratio of

specific heat and is approximately equal to 1.4.

Polytropic: This is somewhere between isothermal and isentropic. This gas change is

governed by the law pVn = constant, where n is somewhere between 1 and 1.4 and is

known as the polytropic coefficient.

Figure 4.30 - Gas-loaded accumulator.



4.6 RESERVOIRS

The functions of a fluid reservoir in a power hydraulic system are as follows:

1. To provide a chamber in which any change in the volume of fluid in a hydraulic circuit can

be accommodated. When the cylinder extends, there is an increased volume of fluid in the

circuit and consequently there is a decrease in the reservoir level.

2. To provide a filling point for the system.

3. To serve as a storage space for the hydraulic fluid used in the system.

4. It is used as the location where the fluid is conditioned.

5. To provide a volume of fluid which is relatively stationery to allow entrained air to

separate out and heavy contaminants to settle. The reservoir is where sludge, water and metal

slips settle.

6. It is a place where the entrained air picked up by the oil is allowed to escape.

7. To accomplish the dissipation of heat by its proper design and to provide a radiating and

convective surface to allow the fluid to cool.

A reservoir is constructed with steel plates. The inside surface is painted with a sealer to

prevent rust due to condensed moisture.

At the bottom, it contains a drain plug to allow the complete drainage of the tank when

required. A removable head can be provided for easy access during cleaning. A vented

breather cap is also included that contains an air filtering screen. This allows the tank to

breathe as the oil level changes due to system demand requirements.

A baffle plate extends lengthwise across the center of the tank. The purpose of the baffle

plate is to separate the pump inlet line from the return line to prevent the same fluid from

recirculating continuously within the tank. The functions of a baffle plate are as follows:

1. To permit foreign substances to settle to bottom.

2. To allow entrained air to escape from oil.

3. To prevent localized turbulence in the reservoir.

4. To promote heat dissipation through reservoir walls. The return line should enter the

reservoir on the side of the baffle plate that is opposite to the pump suction line.

4.6.1 Features of a Hydraulic Reservoir:

Schematic diagram of hydraulic reservoir is shown in Fig.4.31. There are many components

mounted on reservoir and each one of them having specific features. Following are the

features of a hydraulic reservoir:

1. Filler cap (breather cap): It should be airtight when closed but may contain the air vent

that filters air entering the reservoir to provide a gravity push for proper oil flow.

2. Oil level gauge: It shows the level of oil in the reservoir without having to open the

reservoir.



Figure 4.31 - Hydraulic reservoir

3. Baffle plate: It is located lengthwise through the center of the tank and is two-third the

height of the oil level. It is used to separate the outlet to the pump from the return line. This

ensures a circuitous flow instead of the same fluid being recirculated. The baffle prevents

local turbulence in the tank and allows foreign material to settle, get rid of entrapped air and

increase heat dissipation.

4. Suction and return lines: They are designed to enter the reservoir at points where air

turbulence is the least. They can enter the reservoir at the top or at the sides, but their ends

should be near the bottom of the tank. If the return line is above the oil level, the returning oil

can foam and draw in air.

5. Intake filter: It is usually a screen that is attached to the suction pipe to filter the hydraulic

oil.

6. Drain plug: It allows all oil to be drained from the reservoir. Some drain plugs are

magnetic to help remove metal chips from the oil.

7. Strainers and filters: Strainers and filters are designed to remove foreign particles from

the hydraulic fluid.

4.6.2 Types of Reservoirs

Industrial reservoirs come in a variety of styles. Some of them are the following:

1. Non-pressurized: The reservoir may be vented to atmosphere using an air filter or a

separating diaphragm. The type most commonly used in industry, normally, has an air

breather filter, although in very dirty environments, diaphragms or air bags are used.



2. Pressurized: A pressurized reservoir usually operates between 0.35 and 1.4 bar and has to

be provided with some method of pressure control; this may be a small air compressor

maintaining a set charge pressure.

− In motor circuits where there is a little change in fluid volume in the reservoir, a simple

relief valve may be used to limit the air pressure that alters with changes in temperature.

− The advantages of a pressurized reservoir are that it provides boost pressure to the main

pump and prevents the ingress of atmospheric dirt.

4.6.3 Sizing of the Reservoir.

The reservoir capacity should be adequate to cater for changes in fluid volume within the

system, and with sufficient surface area to provide system cooling. An oversize reservoir can

present some disadvantages such as increased cost, size and longer warming-up periods when

starting from cold. There are many empirical rules for sizing reservoirs.

The sizing of a reservoir is based on the following criteria:

1. The minimum reservoir capacity should be twice the pump delivery per minute. This must

be regarded as an absolute minimum and may not be sufficient to allow for the volume

changes in the system.

2. The reservoir capacity should be three to four times the pump delivery per minute. This

may well be too high a volume for mobile application.

3. The reservoir capacity should be 2–15 L per installed horse power. This may result in very

large reservoirs when high-pressure systems are used.

4. It must make allowance for dirt and chips to settle and for air to escape.

5. It must be able to hold all the oil.

6. It must maintain the oil level high enough to prevent the whirlpool effect.

7. It should have a large surface area to dissipate heat generated in the system.

8. It should have an adequate air space to allow for the thermal expansion of oil.

4.6.4 Reservoir Design and Construction (Accessories of reservoir):

− The reservoir must be equipped with a filling point and an air vent point. These are often

combined as filler breather unit complete with a filling mesh.

− Combined units of this nature tend to have a slow filling rate and the filling mesh is often

deliberately punctured to speed up the inflow. This defeats its purpose and allows large dirt

into the reservoir.

− It is also very easy to leave the air-filter cap off after filling thereby permitting

contaminants to enter. It is better to have a separate filling point complete with a quick

release coupling and integral filler unit, in which case all the fluid is pumped into the

reservoir through the filter, spin-on air breather are available with a fine filter range: 25 m

absolute is typical ( 25 = 75).



− The pump suction line should be fitted with an adequately sized strainer. The strainer

should be located so that there is a well distributed flow and its lowest point should be at least

75 mm above the bottom of the reservoir (to prevent any debris on the bottom of the tank

being pulled into the suction line). The top of the strainer should be at least 150 mm below

the lowest fluid level to prevent a vortex being formed and air drawn into the suction line.

− System return lines and drain lines should terminate below the minimum fluid level to

prevent aeration of the fluid. The main returns should be fitted with diffusers that reduce the

return oil velocity and prevent turbulence. Where diffusers are not used, the end of return

lines should be cut at 45%. Return outlets should be carefully positioned to minimize

interaction between the return flow and the reservoir boundaries. If the return pipes have an

anti-syphon whole, aeration may be caused by the jet of fluid emanating from them during

normal operation. This can be reduced by fitting deflector plates.

− The reservoir must be fitted with some form of level indicator, generally a sight glass that

should have the minimum and maximum levels clearly marked. A float switch can be

incorporated to give an alarm or shut the pump down if the oil level falls below a certain

value. This is very important in large systems where there is a possibility of a pipe failure or

other sudden leak. Shutting the pump down protects the pump from cavitation and reduces

the spillage of fluid.

− Baffles are fitted in the reservoir to separate the return lines from the suction lines. They

create a long flow path, which reduces surging effects, encourages dirt separation and

improves cooling.

− A fine wire mesh baffle between 60 and 200 mesh size (250–75 m aperture), set at an

angle of 20–40to horizontal, with the top edge terminating below the fluid surface will

considerably aid the separation of entrained air.

− Wherever the hydraulic fluid is an oil and water emulsion, baffles should not be used as

they tend to separate emulsion. With these fluids, good circulation in the reservoir is

important to maintain emulsion.

− Figure 4.32 shows a reservoir designed to include most of these features. Although it is not

the best shape for heat dissipation, it has other merits.

− If the fluid temperature inside the reservoir varies outside specified limits, temperature

switches can be used to give a warning, switch in coolers or heaters, shut down pumps,

prevent start up, etc.

− Wherever systems operate outdoors in cold temperatures or, for example, servo systems,

which in order to preserve accuracy need to work at a constant temperature, heaters may be

incorporated. These are a special type of immersion heaters having a low wattage relative to

the surface area (maximum 6 W/m2).



Figure 4.32 - Reservoir design and construction.

4.7 HOSES

In many hydraulic systems where it is necessary to move the drive unit qlong with the oil

lines. In such places hydraulic hoses made of synthetic rubber are preferred. Plastic tubing are

also used in hydraulic systems to convey the fluid. The advantage of using rubber hose are:

1. Rubber hoses are flexible

2. Their capacity to absorb or withstand shock and vibration is much higher than the

tubes

3. They are easy to install and dismantle

4. They can be manufactured in long lengths

5. They are capable to take high pressure

However the following disadvantages are also to be noted:

1. They are poor in abrasion resistance

2. Their initial cost is higher

3. They are poor in weathering resistance

4.7.1 Construction of hydraulic hoses

− The constructional view of types of medium and high pressure flexible hoses is shown in

figure 4.33 and 4.34.

− They are generally made of rubber reinforced with fibre or steel braiding. Usually hoses are

constructed with an inner tube, re-reinforcement and an outer protective cover.

− The steel wire may be either spirally woven or cross woven. Spiral reinforced hoses are

stronger but the fitting are to be supplied by hose manufacturer.

− Cross woven braids tend to fail by brittle fracture at the place where wire cross. The

advantage of cross woven braids are that they are re-usable and can be assembled by the

users.



− A hydraulic hose consists of an inner tube through which the fluid is conveyed and thus it

comes into direct contact with the hydraulic fluid.

− The rubber or other synthetic material used for this must be compatible as well as should be

able to withstand the range of working temperature without losing its chemical and physical

stability. Various types of synthetic material could be used for hoses for fluid power system.

Figure 4.33 - Types of flexible hoses, a, b, c – medium pressure d, e, f – medium high pressure

Figure 4.34 - Types of flexible hose – super high and heavy duty type including teflon and metallic

hoses

4.7.2 Hose material

1. Plastic:

− Homogeneous plastics can be used for 7-35 bar pressure only. They are temperature

dependent. The most commonly used plastics are nylon, polyethylene vinyl etc.



− A brief discussion about some of the materials used for hoses is done below.

a) Nylon

Good abrasion and impact resistance

Cannot be used above 15 bar pressure

Satisfactory working temp. range = 75 to 105 degree Celsius

Unsafe due to high temp. and moisture effect

b) Braided nylon hose

Used for medium to high pressure

Fatigue free hoses

Outer cover can be made by the Dacron

Polyurethane cover is compatible to both minerals and synthetic hydraulic

fluids

Cheaper, lighter in weight and last longer than rubber and wire tubes

Expands less and dimensionally stable when pressurised compared to other

materials

c) Polyvinyl chloride

Low pressure pneumatic applications only

More flexible than nylon and polyethylene

Can be produced in longer lengths

Poor temp. capacity (-10 to +40o c)

Poor abrasion and corrosion resistance – reduced working life

d) Thermoplasts

Working temp. range = -40 to +110 o c

Can be used with any type of liquid

e) Ethylene propylene diene (EPDM)

Excellent weathering, ozone and heat resistance

Poor compatibility with petroleum based oils and very good for phosphate

ester

It is also compatible with water glycol though its resistance to flame is poor

Its low temp. resistance is good

f) Chlorinated polyethylene:

Excellent for water glycol and water oil emulsions as well as petroleum base

oils and very good for phosphate ester

Ozone resistance, permeability are also good which makes it ideal hose

material for general purpose hydraulic system

2. Homogeneous synthetic rubber:

Various types of synthetic elastomers are used for medium to high pressure hydraulic system

as inner tube as given below.

a) Buna N

It is also known as nitrile rubber

Most commonly used within temp. range = -50 to +150 oc



It is a synthetic rubber and can be used with petroleum based oils but not with

fire resistant phosphate ester based fluids

b) Neoprene

It is also known as chloroprene

Good oil resistance but is not resistant to aromatic hydrocarbons

Very good abrasion resistance

When used with oil, moderate swelling may occur with passage of time

c) Natural rubber

Mostly used in lines only

Can be used with water glycol based fluid

It is not oil resistant and will swell to over 20% of its original size when used

with petroleum based fluids though it is very good in its abrasion resistance

property too

d) Butyl

Synthetic compound suitable for use within the temp. range of -50 to +150oc

Very poor oil resistance and deteriorates rapidly if it comes into contact with

petroleum based fluids

Extremely compatible with phosphate ester oils

4.7.3 Installation of hoses

To avoid frequent maintenance and to facilitate easy working to increase the service life of

the hoses, the following points are of most important to the machine builders and

maintenance mechanics.

1. While connecting two elements through a hose, it will be better if excessive taut in the

connected hose is a avoided

2. Some slackness is to be permitted to avoid strain to the hose as it is noted that when

pressure is applied to hose, the hose tends to bulge and decrease in length.

Elastomeric hoses fastened between two fixed points are found to have about 5%

change ion length.

3. Similarly enough slackness is also helpful to avoid kinking of the hose. It should be

noted that only the hose and not the fitting is flexible. Hence the hose run should be

long enough to bend during the machine movement.

4. Hoses should never be installed with a twisting as to much twisting may weaken the

hose structurally and may also cause to loosen the fittings.

5. Sometimes to avoid long loops, a number of fittings may have to be used. However,

one should not forget that more number of fittings may be detrimental to the system

performance as lot of heat may be generated due to excessive number of line fittings.

6. High temp. may damage the hose cover and may shorten the useful life. Hence under

hot working condition it is better if the hoses are protected through proper heat

insulation by using fire sleeves or only heat resistant hose material.

7. Design and installation of the hose run should be such that rubbing and abrasion with

the metal members of the system are avoided. Clamps are very often used to group the



hoses and keep them away from the moving parts. Hose guards are also used for this

purpose.

8. During installation of hoses in a hydraulic system it is important to take into

consideration the manufacturer’s instructions and recommendations. The hose

manufacturers guidance and specific instruction should be given maximum

importance with regard to bend radius, environment and oil compatibility, pressure

capacity etc.

4.8 HEAT EXCHANGERS (HEATING AND COOLING DEVICES)

If oil gets overheated, then its good properties like viscosity, lubricity, and corrosion

resistance oxidation stability etc. get destroyed. In-turn, the oil itself becomes

corrosive and acidic, and it starts corroding the metal components internally.

So it is very important to maintain oil temperature within limits. Several types of

circuits are used to avoid over heating of oil. And several types of heat exchangers are

used to cool the hot hydraulic oil.

4.8.1 Types of Heat Exchangers

1. Based on construction

Tubular

Double pipe heat exchangers

Shell and tube heat exchangers

Spiral heat exchangers

Plate-type

Plate and frame heat exchangers

Spiral plate heat exchangers

Extended surface

Plate-fin exchanger

Tube-fin exchanger

2. Based on direction of flow

Parallel Flow/ Co - current Flow Counter Flow Cross Flow

Figure 4.35 – Direction of flow



Shell and Tube Heat Exchanger

This is the most common type of heat exchanger used in hydraulic systems.

The shell and tube heat exchanger consists of a bundle of tubes properly secured at

either ends in tube plates. The tube plates have drilled holes into which the tubes are

fixed using different techniques to have leak proof joints.

The entire tube bundle is placed inside the closed shell, which seals around the tube

sheet periphery to form the two immiscible zones for hot and cold fluids.

4.36 - Shell and Tube Heat Exchanger

Cold water (coolant) flows inside the tubes while the hot oil flows outside the tubes in

the shell. For proper distribution of shell side fluid over the tubes, baffles are placed

normal to the tube bundle. This baffle creates turbulence in the shell side fluid and

enhances the heat transfer.

Heat is transferred from hot oil to cold water. The cold water takes away the heat of

hot oil, and hence the oil gets cooled.


5 DESIGN OF HYDRAULIC CIRCUITS

Course Contents

5.1 Control of a single-acting

hydraulic cylinder

5.2 Control of a double-acting

hydraulic cylinder

5.3 Regenerative cylinder circuit

5.4 Double-pump hydraulic system

(unloading circuit)

5.5 Counterbalance valve

application

5.6 Hydraulic cylinder sequencing

circuits

5.7 Automatic cylinder

reciprocating system

5.8 Cylinder synchronizing circuits

5.9 Speed control of a hydraulic

cylinder

5.10 Fail-Safe Circuits

Oil Hydraulics and Pneumatics (2171912) 5. Design of Hydraulic Circuits


A hydraulic circuit is a group of components such as pumps, actuators, control valves,

conductors and fittings arranged to perform useful work. There are three important

considerations in designing a hydraulic circuit:

1. Safety of machine and personnel in the event of power failures.

2. Performance of given operation with minimum losses.

3. Cost of the component used in the circuit.

5.1 CONTROL OF A SINGLE-ACTING HYDRAULIC CYLINDER

Figure 5.1 - Control of a single-acting cylinder

− Figure 5.1 shows that the control of a single-acting, spring return cylinder using a three-

way two-position manually actuated, spring offset direction-control valve (DCV). In the

spring offset mode, full pump flow goes to the tank through the pressure-relief valve (PRV).

− The spring in the rod end of the cylinder retracts the piston as the oil from the blank end

drains back into the tank. When the valve is manually actuated into its next position, pump

flow extends the cylinder.

− After full extension, pump flow goes through the relief valve. Deactivation of the DCV

allows the cylinder to retract as the DCV shifts into its spring offset mode.

5.2 CONTROL OF A DOUBLE-ACTING HYDRAULIC CYLINDER

The circuit diagram to control double-acting cylinder is shown in Fig. 5.2. The control of a

double-acting hydraulic cylinder is described as follows:

− When the 4/3 valve is in its neutral position (tandem design), the cylinder is hydraulically

locked and the pump is unloaded back to the tank.



Figure 5.2 - Control of a double-acting cylinder

− When the 4/3 valve is actuated into the flow path, the cylinder is extended against its load

as oil flows from port P through port A. Oil in the rod end of the cylinder is free to flow back

to the tank through the four-way valve from port B through port T.

− When the 4/3 valve is actuated into the right-envelope configuration, the cylinder retracts

as oil flows from port P through port B. Oil in the blank end is returned to the tank via the

flow path from port A to port T.

− At the ends of the stroke, there is no system demand for oil. Thus, the pump flow goes

through the relief valve at its pressure level setting unless the four-way valve is deactivated.

5.3 REGENERATIVE CYLINDER CIRCUIT

Figure 5.3 shows a regenerative circuit that is used to speed up the extending speed of a

double-acting cylinder. The pipelines to both ends of the hydraulic cylinder are connected in

parallel and one of the ports of the 4/3 valve is blocked by simply screwing a thread plug into

the port opening.

− During retraction stroke, the 4/3 valve is configured to the right envelope. During this

stroke, the pump flow bypasses the DCV and enters the rod end of the cylinder. Oil from the

blank end then drains back to the tank through the DCV.

− When the DCV is shifted in to its left-envelope configuration, the cylinder extends as

shown in Fig. 5.3. The speed of extension is greater than that for a regular double-acting

cylinder because the flow from the rod end regenerates with the pump flow QP to provide a

total flow rate QT.



Figure 5.3 - Regenerative circuit

5.4 DOUBLE-PUMP HYDRAULIC SYSTEM (UNLOADING CIRCUIT)

Figure 5.4 shows an application for an unloading valve. It is a circuit that uses a high-

pressure, low-flow pump in conjunction with a low-pressure, high-flow pump.

− A typical application is a sheet metal punch press in which the hydraulic cylinder must

extend rapidly over a great distance with low-pressure but high-flow requirements. This

occurs under no load.

− However during the punching operation for short motion, the pressure requirements are

high, but the cylinder travel is small and thus the flow requirements are low. The circuit in

Fig. 5.4 eliminates the necessity of having a very expensive high-pressure, high-flow pump.

− When the punching operation begins, the increased pressure opens the unloading valve to

unload the low-pressure pump. The purpose of relief valve is to protect the high-pressure

pump from over pressure at the end of cylinder stroke and when the DCV is in its spring-

centered mode.

− The check valve protects the low-pressure pump from high pressure, which occurs during

punching operation, at the ends of the cylinder stroke and when the DCV is in its spring-

centered mode.



Figure 5.4 - Double-pump circuit

5.5 COUNTERBALANCE VALVE APPLICATION

Figure 5.5 - Counterbalance valve in circuit

− A counterbalance valve (Fig. 5.5) is applied to create a back pressure or cushioning

pressure on the underside of a vertically moving piston to prevent the suspended load from

free falling because of gravity while it is still being lowered.



5.5.1 Valve Operation (Lowering)

− The pressure setting on the counterbalance valve is set slightly higher than the pressure

required to prevent the load from free falling. Due to this back pressure in line A, the actuator

piston must force down when the load is being lowered.

− This causes the pressure in line A to increase, which raises the spring-opposed spool, thus

providing a flow path to discharge the exhaust flow from line A to the DCV and then to the

tank.

− The spring-controlled discharge orifice maintains back pressure in line A during the entire

downward piston stroke.

5.5.2 Valve Operation (Lifting)

As the valve is normally closed, flow in the reverse direction (from port B to port A) cannot

occur without a reverse free-flow check valve.

− When the load is raised again, the internal check valve opens to permit flow for the

retraction of the actuator.

5.5.3 Valve Operation (Suspension)

When the valve is held in suspension, the valve remains closed. Therefore, its pressure setting

must be slightly higher than the pressure caused by the load. Spool valves tend to leak

internally under pressure. This makes it advisable to use a pilot-operated check valve in

addition to the counterbalance valve if a load must be held in suspension for a prolonged

time.

5.6 HYDRAULIC CYLINDER SEQUENCING CIRCUITS

Hydraulic cylinders can be operated sequentially using a sequence valve. Figure 5.6 shows

that two sequence valves are used to sequence the operation of two double-acting cylinders.

− When the DCV is actuated to its right-envelope mode, the bending cylinder (B) retracts

fully and then the clamp cylinder (A) retracts.

− This sequence of cylinder operation is controlled by sequence valves. This hydraulic circuit

can be used in a production operation such as drilling.

− Cylinder A is used as a clamp cylinder and cylinder B as a drill cylinder.

− Cylinder A extends and clamps a work piece. Then cylinder B extends to drive a spindle to

drill a hole. Cylinder B retracts the drill spindle and then cylinder A retracts to release the

work piece for removal.

5.7 AUTOMATIC CYLINDER RECIPROCATING SYSTEM

The hydraulic circuit shown in Fig. 5.7 produces continuous reciprocation of a double-acting

cylinder using two sequence valves. Each sequence valve senses the completion of stroke by

the corresponding build-up pressure. Each check valve and the corresponding pilot line

prevent the shifting of the four-way valve until the particular stroke of the cylinder is

completed.



− The check valves are needed to allow pilot oil to leave either end of the DCV while the pilot

pressure is applied to the opposite end. This permits the spool of the DCV to shift as required.

Figure 5.6 - Sequencing circuit Figure 5.7 - Automatic Cylinder Reciprocating

System

5.8 CYLINDER SYNCHRONIZING CIRCUITS

In industry, there are instances when a large mass must be moved, and it is not feasible to

move it with just one cylinder. In such cases we use two or more cylinders to prevent a

moment or moments that might distort and damage the load. For example, in press used for

moulding and shearing parts, the platen used is very heavy. If the platen is several meter

wide, it has to be of very heavy construction to prevent the damage when it is pressed down

by a single cylinder in the middle. It can be designed with less material if it is pressed down

with two or more cylinders. These cylinders must be synchronized. There are two ways that

can be used to synchronize cylinders: Parallel and series.

5.8.1 Cylinders in Parallel

Figure 5.8 shows a hydraulic circuit in which two cylinders are arranged in parallel. When

the two cylinders are identical, the loads on the cylinders are identical, and then extension

and retraction are synchronized. If the loads are not identical, the cylinder with smaller load

extends first. Thus, the two cylinders are not synchronized. Practically, no two cylinders are

identical, because of packing (seals) friction differences. This prevents cylinder

synchronization for this circuit.



5.8.2 Cylinders in Series

During the extending stroke of cylinders, fluid from the pump is delivered to the blank end of

cylinder 1. As cylinder 1 extends, fluid from its rod end is delivered to the blank end of

cylinder 2 causing the extension of cylinder 2. As cylinder 2 extends, fluid from its rod end

reaches the tank.

− For two cylinders to be synchronized, the piston area of cylinder 2 must be equal to the

difference between the areas of piston and rod for cylinder 1. Thus, applying the continuity

equation,

Figure 5.8 - Cylinders in parallel and series

Qout (Cylinder 1) = Qin (Cylinder 2)

We get

(Ap1 – Ar1) v1 = Ap2 v2

For synchronization, v1 = v2. Therefore,

(Ap1 – Ar1) = Ap2 (5.8.1)

− The pump must deliver a pressure equal to that required for the piston of cylinder 1 by itself

to overcome loads acting on both extending cylinders. We know that the pressure acting at

the blank end of cylinder 2 is equal to the pressure acting at the rod end of cylinder 1.



Forces acting on cylinder 1 give

p1Ap1 – p2 (Ap1 –Ar1) = F1

Forces acting on cylinder 2 give

P2Ap2 – p2 (Ap2 –Ar2) = F2

Using Eq. (5.8.1) and noting that p3 = 0 (it is connected to the tank), we have

p1Ap1 – p2Ap2 = F1

(5.8.2)

p2Ap2 – 0 = F2

(5.8.3)

Now, Eq. (5.8.2) + Eq. (5.8.3) gives

p1Ap1 = F1 + F2

(5.8.4)

If Eqs. (5.8.1) and (5.8.4) are met in a hydraulic circuit, the cylinders hooked in series operate

in synchronization.

5.9 SPEED CONTROL OF A HYDRAULIC CYLINDER

− The speed control of a hydraulic cylinder circuit can be done during the extension stroke

using a flow-control valve (FCV). This is done on a meter-in circuit and meter-out circuit

as shown in Fig. 5.9.

− Refer to Fig. 5.9 (a). When the DCV is actuated, oil flows through the FCV to extend the

cylinder. The extending speed of the cylinder depends on the FCV setting. When the DCV is

deactivated, the cylinder retracts as oil from the cylinder passes through the check valve.

Thus, the retraction speed of a cylinder is not controlled.

− Figure 5.9 (b) shows meter-out circuit; when DCV is actuated, oil flows through the rod end

to retract the cylinder.

5.9.1 Meter-In Versus Meter-Out Flow-Control Valve Systems

− In Section 5.9, the FCV is placed in the line leading to the inlet port of the cylinder. Thus, it

is called the meter-in control of speed. Meter-in flow controls the oil flow rate into the

cylinder.

− A meter-out flow control system is one in which the FCV is placed in the outlet line of the

hydraulic cylinder. Thus, a meter-out flow control system controls the oil flow rate out of the

cylinder.

− Meter-in systems are used primarily when the external load opposes the direction of motion

of the hydraulic cylinder. When a load is pulled downward due to gravity, a meter-out system

is preferred. If a meter-in system is used in this case, the load would drop by pulling the

piston rod, even if the FCV is completely closed.

− One drawback of a meter-out system is the excessive pressure build-up in the rod end of the

cylinder while it is extending. In addition, an excessive pressure in the rod end results in a



large pressure drop across the FCV. This produces an undesirable effect of a high heat

generation rate with a resulting increase in oil temperature.

Figure 5.9 - Speed control of cylinders: (a) Meter in and (b) meter out.

5.10 Fail-Safe Circuits

Fail-safe circuits are those designed to prevent injury to the operator or damage to the

equipment. In general, they prevent the system from accidentally falling on an operator and

also prevent overloading of the system. In following sections we shall discuss two fail-safe

circuits: One is protection from inadvertent cylinder extension and other is fail-safe overload

protection.

1. Protection from inadvertent cylinder extension:

− Figure 5.10 shows a fail-safe circuit that is designed to prevent the cylinder from

accidentally falling in the event when a hydraulic line ruptures or a person inadvertently

operates the manual override on the pilot-actuated DCV when the pump is not working.

− To lower the cylinder, pilot pressure from the blank end of piston must pilot open the check

valve to allow oil to return through the DCV to the tank.

− This happens when the push button is actuated to permit the pilot pressure actuation of

DCV or when the DCV is directly manually actuated when the pump operates.



− The pilot-operated DCV allows free flow in the opposite direction to retract the cylinder

when this DCV returns to its offset mode.

2. Fail-Safe System with Overload Protection:

− Figure 5.11 shows a fail-safe system that provides overload protection for system

components. The DCV V1 is controlled by the push-button three-way valve V2.

− When the overload valve V3 is in its spring offset mode, it drains the pilot line of valve V1.

If the cylinder experiences excessive resistance during the extension stroke, sequence valve

V4 pilot-actuates overload valve V3.

− This drains the pilot line of valve V1 causing it to return to its spring offset mode. If a

person then operates the push-button valve V2 nothing happens unless overload valve V3 is

manually shifted into its blocked-port configuration.

− Thus, the system components are protected against excessive pressure due to an excessive

cylinder load during its extension stroke.

Figure 5.10 - Fail-safe circuits – Inadvertent Figure 5.11 - Fail-safe circuits -

Cylinder Extension Overload Protection.


6 INTRODUCTION PNEUMATIC SYSTEM

Course Contents

6.1 Pneumatics and its meaning

6.2 Choice of working medium and

system

6.3 Applications of pneumatics

6.4 Properties of air

6.5 Gas laws

6.6 Basic components of

pneumatic system

6.7 Valves

Oil Hydraulics and Pneumatics (2171912) 6. Introduction to Pneumatic System


6.1 PNEUMATICS AND ITS MEANING

− The English word pneumatic and its associate noun pneumatics are derived from the Greek

“pneuma” meaning breath or air. Originally coined to give a name to the science of the

motions and properties of air. Compressed air is a vital utility- just like water, gas and

electricity used in countless ways to benefit everyday life.

− Pneumatics is application of compressed air (pressurized air) to power machine or control

or regulate machines. Simply put, Pneumatics may be defined as branch of engineering

science which deals with the study of the behaviour and application of compressed air.

Pneumatics can also be defined as the branch of fluid power technology that deals with

generation, transmission and control of power using pressurized air. Gas in a pneumatic

system behaves like a spring since it is compressible.

− Any gas can be used in pneumatic system but air is the most usual, for obvious reasons.

Exceptions are most likely to occur on aircraft and space vehicles where an inert gas such as

nitrogen is preferred or the gas is one which is generated on board. Pure nitrogen may be used

if there is a danger of combustion in a work environment. In Pneumatic control, compressed

air is used as the working medium, normally at a pressure from 6 bar to 8 bar. Using

Pneumatic

− Control, maximum force up to 50 kN can be developed. Actuation of the controls can be

manual, Pneumatic or Electrical actuation. Signal medium such as compressed air at pressure

of 1-2 bar can be used [Pilot operated Pneumatics] or Electrical signals [D.C or A.C source-

24V – 230V] can be used [Electro pneumatics]

6.2 CHOICE OF WORKING MEDIUM AND SYSTEM

− The choice of medium depends on the application. Some of the general, broad rules

followed in the selection of a working medium are listed below.

− When the system requirement is high speed, medium pressure (usually 6 to 8 bar) and less

accuracy of position, then pneumatic system is preferred.

− If the system requirement is high pressure and high precision, a fluid system with oil is

good.

− When the power requirement is high like in forging presses, sheet metal press, it is

impossible to use air system. Oil hydraulics is the only choice

− Air is used where quick response of actuator is required.

− If temperate variation range in the system is large, then use of air system may run into

condensation problems and oil is preferred.

− If the application requires only a medium pressure and high positional accuracy is required

then hydro –pneumatic system is preferred

− Air is non-explosive, it is preferred where fire/electric hazard are expected. Oil systems are

more prone to fire and electrical hazards and are not recommended in such applications.



− Because air contains oxygen (about 20%) and is not sufficient alone to provide adequate

lubrication of moving parts and seals, oil is usually introduced into the air stream near the

actuator to provide this lubrication preventing excessive wear and oxidation.

In a practical sense, compressed air is a medium that carries potential energy. However it can

be expensive to produce, and from a simple energy efficiency point of view compressed air

may not appear advantageous at first. Considering that it takes about 6 kW of electrical

energy to generate 0.75 kW output on an air motor, compressed air has an efficiency rating of

only 12%. In spite of that compressed air is used due to its other advantages.

Table 6.1 - The advantages and disadvantages of compressed air

Advantages of compressed air Disadvantages of compressed air

Air is available in unlimited quantities.

Compressed air is easily conveyed in

pipelines even over longer distances

Compressive air is relatively expensive

means of conveying energy

The higher costs are, however. Largely

compensated by the cheaper elements.

Simpler and more compact equipment

Compressed air can be stored

Compressed air requires good

conditioning. No dirt or moisture

residues may be contained in it. Dirt and

dust leads to wear on tools and

equipment

Compressed air need not be returned. It

can be vented to atmosphere after it has

performed work

It is not possible to achieve uniform and

constant piston speeds( air is

compressible)

Compressed air is insensitive to

temperature fluctuation. This ensures

reliable operation even in extreme

temperature conditions

Compressed air is economical ony up to

certain force expenditure. Owing to the

commonly used pressure of 7 bar and

limit is about 20 to 50 kN, depending on

the travel and the speed. If the force

which is required exceeds this level,

hydraulics is preferred

Compressed air is clean. This is

especially important in food,

pharmaceutical, textile, beverage

industries

The exhaust is loud. As the result of

intensive development work on

materials for silencing purposes, this

problems has however now largely been

solved

Operating elements for compressed air

operation are of simple and inexpensive

construction.

The oil mist mixed with the air for

lubricating the equipment escapes with

the exhaust to atmosphere.

Compressed air is fast. Thus, high

operational speed can be attained.

Air due to its low conductivity , cannot

dissipate heat as much as hydraulic fluid

Speeds and forces of the pneumatics

elements can be infinitely adjusted

Air cannot seal the fine gaps between

the moving parts unlike hydraulic

system

Tools and operating elements are

overload proof. Straight line movement

can be produced directly

Air is not a good lubricating medium

unlike hydraulic fluid.



6.3 APPLICATIONS OF PNEUMATICS

− Pneumatic systems are used in many applications. New uses for pneumatics are constantly

being discovered. In construction, it is indispensable source of power for such tools as air

drills, hammers, wrenches, and even air cushion supported structures, not to mention the

many vehicles using air suspension , braking and pneumatic tires.

− In manufacturing, air is used to power high speed clamping, drilling, grinding, and

assembly using pneumatic wrenches and riveting machines. Plant air is also used to power

hoists and cushion support to transport loads through the plant.

− Many recent advances in air – cushion support are used in the military and commercial

marine transport industry.

Some of the Industrial applications of pneumatics are listed in the Table 6.2.

Table 6.2 - Industrial applications of Pneumatics

Material

Handling Manufacturing

Other

applications

Clamping

Shifting

positioning

Orienting

Feeding

Ejection

Braking

Bonding

Locking

Packaging

Feeding

Sorting

stacking

Drilling

Turning

Milling

Sawing

Finishing

Forming

Quality Control

Stamping

Embossing

Filling

Aircraft

Cement plants

chemical plants

Coal mines

Cotton mills

Dairies

Forge shops

Machine tools

Door or chute

control

Turning and

inverting parts

6.4 PROPERTIES OF AIR

Composition:

− Air is one of the three states of matter. It has characteristics similar to those of liquids in

that it has no definite shape but conforms to the shape of its container and readily transmits

pressure. Gases differ from liquids in that they have no definite volume. That is, regardless of

the size or shape of the containing vessel, a gas will completely fill it.

− Gases are highly compressible, while liquids are only slightly so. Also, gases are lighter

than equal number of liquids, making gases less dense than liquids.

− Air is a mechanical mixture of gases containing by volume, approximately 78 % of

nitrogen and 21 % of oxygen, and about 1 % of other gases, including argon and carbon

dioxide. Water being the most important remaining ingredient as far as pneumatics is

concerned. The dilution of the oxygen by nitrogen makes air much less chemically active

than pure oxygen but it is still capable of causing spontaneous combustion or explosion,

particularly if oil vapor at an elevated temperature is present, as may occur in an air receiver.

− Air is colourless, odourless, tasteless, and compressible and has weight. Air has a great

affinity with water and unless specifically dried, contains considerable quantities of water



vapour, sometimes as much as 1% by weight. Life on earth depends on air for survival and

man harness its forces to do useful work. Table 1.3 gives the physical properties of air

Table 6.3 - Properties of air

Property Value

Molecular weight 28.96 kg/kmol

Density of air at 15 and 1

bar 1.21 kg/m3

Boiling point at 1 bar 191 to -194

Freezing point at 1 bar -212 to -216

Gas constant 286.9 J/kg K

Free air and Standard air

In pneumatics, the existence of the following two conditions of atmospheric air is well

accepted

• Free air: Air at the atmospheric condition at the point where the compressor is located is

defined as free air. Free air will vary with atmospheric conditions like altitude, pressure and

temperature.

• Standard air: It is also called normal air. It is defined as the air at sea level conditions

(1.01324 bar as per ISO –R554 and 20 and Relative humidity of 36%). The condition of

normal atmosphere is used as a basis for getting average values for compressor delivery

volumes, efficiencies and operating characteristics.

Atmospheric pressure, Gauge pressure and Absolute pressure

− Air has mass and exerts a pressure on the surface of the earth. A barometer consisting of an

inverted tube close at the top will support a column of mercury at exactly 760 mm at sea level

when measured at standard conditions.

− Pressure above one atmosphere (~ 1 bar) are positive, whereas the pressure below one

atmosphere cause a vacuum to be formed.

− Both positive pressures and vacuum pressures have useful purposes in pneumatics. Vacuum

measurement is usually given a mm of mercury and then converted into the holding force for

such devices such as suction pads and cylinders with a specified diameter.

Atmospheric pressure:

− The earth is surrounded by air. Since air has weight it can exert a pressure on the earth’s

surface. The weight of the column of air on one square meter of earth’s surface is known as

atmospheric pressure or reference pressure.

− The atmospheric pressure varies slightly from day to day. In pneumatic circuit calculations,

standard atmospheric pressure is taken as 101.325 kPa. (14.7 psia)(760 mm of Hg). The

atmospheric pressure is measured using barometer.

− Atmospheric pressure = Density x Acceleration due to gravity x height of barometer

column (usually mercury)

P atm = ρ g h



Gauge Pressure:

− In pneumatic application, pressure is measured using pressure gage and pressure gauges are

calibrated to indicate the pressure above that of the Atmospheric pressure. Gauge pressure

refers to pressure indicated by pressure gauge.

Absolute pressure:

− Refers to the true or total pressure.

Absolute pressure = Atmospheric pressure + Gauge pressure.

− Calculations involving formulae associated with the Gas laws must be made with absolute

pressure. Figure 6.1 shows the difference between the gauge and absolute pressure.

Figure 6.1 - Difference between absolute and gauge pressure

6.5 GAS LAWS

Early experiments were conducted concerning the behaviour of air and similar gases. These

experiments were conducted by scientists such as Boyle, Charles and Gay-Lussac. The results

of their experiments indicated that gases behaviours follow the law known as ideal gas laws.

6.5.1 Boyle’s Law

− Robert Boyle (1627-1691), an English scientist, was among the first to experiment with the

pressure volume relationship of gas at constant temperature.

− Statement: If a given mass of a gas is compressed or expanded at a constant temperature,

then the absolute pressure is inversely proportional to the volume.

𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝛼 1

𝑉𝑜𝑙𝑢𝑚𝑒 When temperature = constant

or pV = Constant for state 1 and 2



6.5.2 Charle’s law

− Boyle’s law assumes conditions of constant temperature. In actual situations this is rarely

the case. Temperature changes continually and affects the volume of a given mass of gas.

− Jacques Charles (1746 to 1823),a French physicist, provided much of the foundations for

modern kinetic theory of gases. Through experiments, he found that all gases expand and

contract proportionally to the change in the absolute temperature, providing the pressure

remains constant.

− Statement: If a given mass of a gas is heated or cooled at a constant pressure, then the

volume is directly proportional to the absolute temperature.

Volume α Temperature, when Pressure = Constant

Or V/T = constant, for state 1 and 2

6.5.3 Gay-Lussac’s Law

− A third gas law may be derived as a corollary to Boyle's and Charles's laws. Suppose we

double the thermodynamic temperature of a sample of gas. According to Charles’s law, the

volume should double. Now, how much pressure would be required at the higher temperature

to return the gas to its original volume? According to Boyle’s law, we would have to double

the pressure to halve the volume. Thus, if the volume of gas is to remain the same, doubling

the temperature will require doubling the pressure. This law was first stated by the

Frenchman Joseph Gay-Lussac (1778 to 1850).

− Statement: At constant pressure, the absolute pressure of an ideal gas will vary directly with

the absolute temperature.

Pressure α Temperature, When volume = constant

Or P/T = constant

6.5.4 General gas equation

− For any given mass of gas undergoing changes of pressure, temperature and volume, the

general gas equation can be used. By combining Boyle’s law and Gay-Lussac’s law we get,

6.6 BASIC COMPONENTS OF PNEUMATIC SYSTEM

Important components of a pneumatic system are shown in figure 6.2.

a) Air filters: These are used to filter out the contaminants from the air.

b) Compressor: Compressed air is generated by using air compressors. Air compressors are

either diesel or electrically operated. Based on the requirement of compressed air, suitable

capacity compressors may be used.

c) Air cooler: During compression operation, air temperature increases. Therefore coolers

are used to reduce the temperature of the compressed air.



Figure 6.2 - Components of a pneumatic system

d) Dryer: The water vapor or moisture in the air is separated from the air by using a dryer.

e) Control Valves: Control valves are used to regulate, control and monitor for control of

direction flow, pressure etc.

f) Air Actuator: Air cylinders and motors are used to obtain the required movements of

mechanical elements of pneumatic system.

g) Electric Motor: Transforms electrical energy into mechanical energy. It is used to drive

the compressor.

h) Receiver tank: The compressed air coming from the compressor is stored in the air

receiver.

Advantages of Pneumatic system

− Low inertia effect of pneumatic components due to low density of air.

− Pneumatic Systems are light in weight.

− Operating elements are cheaper and easy to operate

− Power losses are less due to low viscosity of air

− High output to weight ratio

− Pneumatic systems offers a safe power source in explosive environment

− Leakage is less and does not influence the systems. Moreover, leakage is not harmful

Disadvantages of Pneumatic systems

− Suitable only for low pressure and hence low force applications

− Compressed air actuators are economical up to 50 kN only.

− Generation of the compressed air is expensive compared to electricity



− Exhaust air noise is unpleasant and silence has to be used.

− Rigidity of the system is poor

− Weight to pressure ratio is large

− Less precise. It is not possible to achieve uniform speed due to compressibility of air

− Pneumatic systems is vulnerable to dirt and contamination

Table 6.4 - Comparison of Electrical, mechanical, pneumatic and hydraulic transmission systems

Property Electrical/Mechanical Pneumatic Hydraulic

Energy

IC engines, electrical

energy is used to drive

motors

Electrical energy is

used to drive

compressor and other

equipment

I C Engines

Electric Motor

Air Turbine are used

to drive hydraulic

pumps.

Medium

. There is no medium,

Energy is transferred

through Levers, Gears,

Shafts

Compressed air/gas

in Pipes and hoses

Pressurized liquid in

Pipes and hoses

Energy storage Batteries Reservoir, air tank Accumulators

Regulations Variable frequency

drives Pneumatic valves Hydraulic valves

Transmitters

Transmitted through

mechanical

components like levers,

gears, cams, screw etc

Transmitted through

pneumatic cylinders ,

rotary drives and

rotary actuators

Transmitted through

hydraulic cylinders,

and hydraulic rotary

actuators.

Distribution system

efficiency Good

Limited (say up to

1000m)

Good (say up to 100

m)

Operating speed Low Limited (up to 1.5

m/s)

Limited (up to 0.5

m/s)

Positioning accuracy

Precision in terms of

few micron can be

achieved

. Precision in terms of

few mm ( usually 0.1

mm) can be achieved

Precision in terms of

few micron can be

achieved

Stability

Good stability is

possible using

mechanical elements

. Low stability is due

to High

compressibility of air

Good stability is

possible due to low

compressibility

Forces

Mechanical elements

break down if

overloaded. Poor

overloading capacity

Protected against

overload with system

pressure of 6 to 8 bar,

forces up to 50 kN

can be generated.

Protected against

overload with high

system pressure of 600

bar, very large forces

can be generated.

Cost of energy Lowest Highest Medium

Linear actuators

Short stroke length

using

mechanical/electrical

transmission elements

Is possible using

pneumatic actuators

(cylinders), it can

produce medium

force.

Is possible using

hydraulic

actuators(cylinders),

and it can produce

heavy force

Rotary actuators AC , DC, Servo motors

and steeper motors can

Pneumatic rotary

actuators can be used

Hydraulic motors and

vane motors can be



be used used

Controllable force

Possible with solenoid

and DC motors, Needs

cooling and hence

complicated

medium force can be

controlled easily

High force can be

controlled

6.6.1 Receiver tank

− The air is compressed slowly in the compressor. But since the pneumatic system needs

continuous supply of air, this compressed air has to be stored. The compressed air is stored in

an air receiver as shown in Figure 6.3. The air receiver smoothens the pulsating flow from the

compressor. It also helps the air to cool and condense the moisture present.

− The air receiver should be large enough to hold all the air delivered by the compressor. The

pressure in the receiver is held higher than the system operating pressure to compensate

pressure loss in the pipes. Also the large surface area of the receiver helps in dissipating the

heat from the compressed air. Generally the size of receiver depends on,

− Delivery volume of compressor.

− Air consumption.

− Pipeline network

− Type and nature of on-off regulation

− Permissible pressure difference in the pipelines

Figure 6.3 - Air receiver

6.6.2 Compressor

− It is a mechanical device which converts mechanical energy into fluid energy. The

compressor increases the air pressure by reducing its volume which also increases the

temperature of the compressed air.

− The compressor is selected based on the pressure it needs to operate and the delivery

volume.



− The compressor can be classified into two main types

a. Positive displacement compressors and

b. Dynamic displacement compressor

− Positive displacement compressors include piston type, vane type, diaphragm type and

screw type.

Piston compressors

Figure 6.4 - Single acting piston compressor

− Piston compressors are commonly used in pneumatic systems. The simplest form is single

cylinder compressor (Figure 6.4). It produces one pulse of air per piston stroke.

− As the piston moves down during the inlet stroke the inlet valve opens and air is drawn into

the cylinder. As the piston moves up the inlet valve closes and the exhaust valve opens which

allows the air to be expelled. The valves are spring loaded.

− The single cylinder compressor gives significant amount of pressure pulses at the outlet

port. The pressure developed is about 3-40 bar.

Double acting compressor

− The pulsation of air can be reduced by using double acting compressor as shown in Figure

6.5. It has two sets of valves and a crosshead.

− As the piston moves, the air is compressed on one side whilst on the other side of the

piston, the air is sucked in. Due to the reciprocating action of the piston, the air is compressed

and delivered twice in one piston stroke. Pressure higher than 30bar can be produced.

Multistage compressor

− As the pressure of the air increases, its temperature rises. It is essential to reduce the air

temperature to avoid damage of compressor and other mechanical elements. The multistage

compressor with intercooler in-between is shown in Figure 6.6.

− It is used to reduce the temperature of compressed air during the compression stages. The

inter-cooling reduces the volume of air which used to increase due to heat.



Figure 6.5 - Double acting piston compressor

Figure 6.6 - Multi-stage compressor

− The compressed air from the first stage enters the intercooler where it is cooled. This air is

given as input to the second stage where it is compressed again.

− The multistage compressor can develop a pressure of around 50 bar.

Combined two stage compressors

− In this type, two-stage compression is carried out by using the same piston (Fig. 6.7).

Initially when the piston moves down, air is sucked in through the inlet valve.

− During the compression process, the air moves out of the exhaust valve into the intercooler.

As the piston moves further the stepped head provided on the piston moves into the cavity

thus causing the compression of air. Then, this is let out by the exhaust port.



Figure 6.7 - Combined to stage compressor

Diaphragm compressor

Figure 6.8 - Diaphragm compressor

− These are small capacity compressors. In piston compressors the lubricating oil from the

pistons walls may contaminate the compressed air.

− The contamination is undesirable in food, pharmaceutical and chemical industries. For such

applications diaphragm type compressor can be used. Figure 6.8 shows the construction of

Diaphragm compressor.

− The piston reciprocates by a motor driven crankshaft. As the piston moves down it pulls the

hydraulic fluid down causing the diaphragm to move along and the air is sucked in.



− When the piston moves up the fluid pushes the diaphragm up causing the ejection of air

from the outlet port. Since the flexible diaphragm is placed in between the piston and the air

no contamination takes place.

Screw compressor

− Piston compressors are used when high pressures and relatively low volume of air is

needed. The system is complex as it has many moving parts. For medium flow and pressure

applications, screw compressor can be used. It is simple in construction with less number of

moving parts.

− The air delivered is steady with no pressure pulsation. It has two meshing screws. The air

from the inlet is trapped between the meshing screws and is compressed. The contact

between the two meshing surface is minimum, hence no cooling is required.

− These systems are quite in operation compared to piston type. The screws are synchronized

by using external timing gears.

Rotary vane compressors

− The principle of operation of vane compressor is similar to the hydraulic vane pump. Figure

6.9 shows the working principle of Rotary vane compressor. The unbalanced vane

compressor consists of spring loaded vanes seating in the slots of the rotor.

− The pumping action occurs due to movement of the vanes along a cam ring. The rotor is

eccentric to the cam ring.

− As the rotor rotates, the vanes follow the inner surface of the cam ring. The space between

the vanes decreases near the outlet due to the eccentricity. This causes compression of the air.

These compressors are free from pulsation. If the eccentricity is zero no flow takes place.

Figure 6.9 - Screw compressor

Liquid ring vane compressor

− Liquid ring vane compressor is a variation of vane compressors. Figure 6.10 shows the

construction of Liquid ring compressor. The casing is filled with liquid up to rotor center.

The air enters the compressor through the distributor fixed to the compressor.

− During the impeller rotation, the liquid will be centrifuged along the inner ring of the casing

to form the liquid ring.



Figure 6.10 - Rotary vane compressor

− There are two suction and discharge ports provided in the distributor. During the first

quarter of cycle, the air is sucked in both suction chambers of the casing and during the

second quarter of the cycle, the air is compressed and pushed out through the two discharge

ports.

− During the third and fourth quarters of the cycle, the process is repeated. This type of

compressor has no leakage and has minimal friction. For smooth operation, the rotation speed

should be about 3000 rpm. The delivery pressure is low (about 5 bar).

Figure 6.11 - Liquid ring compressor

Lobe compressor

− The lobe compressor is used when high delivery volume but low pressure is needed. It

consists of two lobes with one being driven and the other driving.

− Figure 6.11 shows the construction and working of Lobe compressor. It is similar to the

Lobe pump used in hydraulic systems.

− The operating pressure is limited by leakage between rotors and housing. As the wear

increases during the operation, the efficiency falls rapidly.



Figure 6.12 - Lobe compressor

6.6.3 Filter, regulator and lubrication (FRL) unit

Filter

− To prevent any damage to the compressor, the contaminants present in the air need to be

filtered out. This is done by using inlet filters. These can be dry or wet filters.

− Dry filters use disposable cartridges. In the wet filter, the incoming air is passed through an

oil bath and then through a fine wire mesh filter.

− Dirt particles cling to the oil drops during bubbling and are removed by wire mesh as they

pass through it. In the dry filter the cartridges are replaced during servicing. The wet filters

are cleaned using detergent solution.

Lubricators

− The compressed air is first filtered and then passed through a lubricator in order to form a

mist of oil and air to provide lubrication to the mating components. Figure 6.13 shows the

schematic of a typical lubricator. The principle of working of venture meter is followed in the

operation of lubricator.

− The compressed air from the dryer enters in the lubricator. Its velocity increases due to a

pressure differential between the upper and lower changer (oil reservoir). Due to the low

pressure in the upper chamber the oil is pushed into the upper chamber from the oil reservoir

through a siphon tube with check valve.

− The main function of the valve is to control the amount of oil passing through it. The oil

drops inside the throttled zone where the velocity of air is much higher and this high velocity

air breaks the oil drops into tiny particles. Thus a mist of air and oil is generated.

− The pressure differential across chambers is adjusted by a needle valve. It is difficult to

hold an oil mixed air in the air receiver as oil may settle down. Thus air is lubricated during

secondary air treatment process.

− Low viscosity oil forms better mist than high viscosity oil and hence ensures that oil is

always present in the air.



Figure 6.13 - Air lubricator

Pressure regulation

In pneumatic systems, during high velocity compressed air flow, there is flow-dependent

pressure drop between the receiver and load (application). Therefore the pressure in the

receiver is always kept higher than the system pressure. At the application site, the pressure is

regulated to keep it constant. There are three ways to control the local pressure, these are

shown in Figure 6.14.

− In the first method, load X vents the air into atmosphere continuously. The pressure

regulator restricts the air flow to the load, thus controlling the air pressure. In this type of

pressure regulation, some minimum flow is required to operate the regulator. If the load is a

dead end type which draws no air, the pressure in the receiver will rise to the manifold

pressure. These type of regulators are called as ‘non-relieving regulators’, since the air must

pass through the load.

− In the second type, load Y is a dead end load. However the regulator vents the air into

atmosphere to reduce the pressure. This type of regulator is called as ‘relieving regulator’.

− The third type of regulator has a very large load Z. Therefore its requirement of air volume

is very high and can’t be fulfilled by using a simple regulator. In such cases, a control loop

comprising of pressure transducer, controller and vent valve is used. Due to large load the

system pressure may rise above its critical value. It is detected by a transducer. Then the

signal will be processed by the controller which will direct the valve to be opened to vent out

the air. This technique can be also be used when it is difficult to mount the pressure

regulating valve close to the point where pressure regulation is needed.



Figure 6.14 - Types of pressure regulation

6.7 VALVES

Valve are defined as devices to control or regulate the commencement, termination and

direction and also the pressure or rate of flow of a fluid under pressure which is delivered by

a compressor or vacuum pump or is stored in a vessel.

Values of one sort or another, perform three main function in pneumatic installation

− They control the supply of air to power units, example cylinders

− They provide signal which govern the sequence of operation

− They act as interlock and safety devices

− The type of valve used is of little importance in a pneumatic control for most part. What is

important is the function that can be initiated with the valves, its mode of actuation and line

connection size, the last named characteristics also determining the flow size of the valve.

− Valves used in pneumatics mainly have a control function that is when they act on some

process, operation or quantity to be stopped. A control function requires control energy, it

being desirable to achieve the greatest possible effect with the least effort. The form of

control energy will be dictated by the valve’s mode of actuation and may be manual,

mechanical, electrical hydraulic or pneumatic.

− Valve available for pneumatic control can be classified into four principal groups according

to their function:

1. Direction control valve (Same as used in hydraulics. Range of working pressure is less)

2. Non return valves

3. Flow control valves (Same as used in hydraulics. Range of working pressure is less)

4. Pressure control valves (Same as used in hydraulics. Range of working pressure is less)



Non return valves

− Non return valves permit flow of air in one direction only, the other direction through the

valve being at all times blocked to the air flow. Mostly the valves are designed so that the

check is additionally loaded by the downstream air pressure, thus supporting the non-return

action.

− Among the various types of non-return valves available, those preferentially employed in

pneumatic controls are as follows

A) Check valve

B) Shuttle valve

C) Restrictor check valve

D) Quick exhaust valve

E) Two pressure valve

A. Check valve

− The simplest type of non-return valve is the check valve (Figure 6.15) which completely

blocks air flow in one direction while permitting flow in the opposite direction with minimum

pressure loss across the valve.

− As soon as the inlet pressure in the direction of free flow develops a force greater than that

of the internal spring, the check is lifted clear of the valve seat. The check in such valve may

be plug, ball, plate or diaphragm.

Figure 6.15 - Check valve

B. Shuttle valve

− It is also known as a double control valve or double check valve. A shuttle valve has two

inlets and one outlet. At any one time, flow is shut off in the direction of whichever inlet is

unloaded and is open from the loaded inlet to the outlet (Figure 6.16).

− A shuttle valve may be installed, for example, when a power unit (cylinder) or control unit

(valve) is to be actuated from two points, which may be remote from one other.



Figure 6.16 - Pneumatic Shuttle valve

C. Quick Exhaust Valves

− A quick exhaust valve is a typical shuttle valve. The quick exhaust valve is used to exhaust

the cylinder air quickly to atmosphere. Schematic diagram of quick exhaust valve is shown in

Figure 6.17. In many applications especially with single acting cylinders, it is a common

practice to increase the piston speed during retraction of the cylinder to save the cycle time.

− The higher speed of the piston is possible by reducing the resistance to flow of the

exhausting air during the motion of cylinder. The resistance can be reduced by expelling the

exhausting air to the atmosphere quickly by using Quick exhaust valve.

− The construction and operation of a quick exhaust valve is shown in Figure 6.17. It consist

of a movable disc (also called flexible ring) and three ports namely, Supply port 1, which is

connected to the output of the final control element (Directional control valve). The Output

port, 2 of this valve is directly fitted on to the working port of cylinder. The exhaust port, 3 is

left open to the atmosphere

Figure 6.17 - Functional diagram of quick exhaust valve

Forward Motion: During forward movement of piston, compressed air is directly admitted

behind the piston through ports 1 and 2 Port 3 is closed due to the supply pressure acting on

the diaphragm. Port 3 is usually provided with a silencer to minimise the noise due to

exhaust.

Return Motion: During return movement of piston, exhaust air from cylinder is directly

exhausted to atmosphere through opening 3 (usually larger and fitted with silencer) .Port 2 is



sealed by the diaphragm. Thus exhaust air is not required to pass through long and narrow

passages in the working line and final control valve

D. Two Pressure Valve

− This valve is the pneumatic AND valve. It is also derivate of Non Return Valve. A two

pressure valve requires two pressurised inputs to allow an output from itself. The cross

sectional views of two pressure valve in two positions are given in Figure 6.18.

− As shown in the figure, this valve has two inputs 12 and 14 and one output 2. If the

compressed air is applied to either 12 or input 14, the spool moves to block the flow, and no

signal appears at output 2. If signals are applied to both the inputs 12 and 14, the compressed

air flows through the valve, and the signal appears at output 2.

Figure 6.18 - Two pressure valve




7 PNEUMATIC CIRCUITS

Course Contents

7.1 Introduction

7.2 Single acting cylinder control

7.3 Double acting cylinder control

7.4 Supply air throttling and

exhaust air throttling

7.5 Time dependent controls

7.6 Single actuator circuit versus

multi actuator circuits

Oil Hydraulics and Pneumatics (2171912) 7. Pneumatic Circuits


7.1 INTRODUCTION

Pneumatic control systems can be designed in the form of pneumatic circuits. A pneumatic

circuit is formed by various pneumatic components, such as cylinders, directional control

valves, flow control valves, pressure regulator, signal processing elements such as shuttle

valve, two pressure valve etc. Pneumatic circuits have the following functions

To control the entry and exit of compressed air in the cylinders.

To use one valve to control another valve

To control actuators or any other pneumatic devices

− A pneumatic circuit diagram uses pneumatic symbols to describe its design. Some basic

rules must be followed when drawing pneumatic diagrams.

− To be able to design pneumatic circuits, it is better for one to have basic knowledge on the

designing simple pneumatic circuits. With this foundation, one would be able to move on to

the designing more complicated circuits involving many more cylinders.

7.2 SINGLE ACTING CYLINDER CONTROL

7.2.1 Direct control of single acting cylinder

Figure 7.1 - Direct control of a single acting cylinder

− Pneumatic cylinders can be directly controlled by actuation of final directional control valve

(Figure 7.1). These valves can be controlled manually or electrically. This circuit can be used

for small cylinders as well as cylinders which operates at low speeds where the flow rate

requirements are less.

− When the directional control valve is actuated by push button, the valve switches over to the

open position, communicating working source to the cylinder volume. This results in the

forward motion of the piston. When the push button is released, the reset spring of the valve

restores the valve to the initial position [closed].



− The cylinder space is connected to exhaust port there by piston retracts either due to spring

or supply pressure applied from the other port.

Example 1: A small single acting cylinder is to extend and clamp a work piece when a push

button is pressed. As long as the push button is activated, the cylinder should remain in the

clamped position. If the push button is released, the clamp is to retract. Use additional start

button. Schematic diagram of the setup is shown in Figure 7.2

Figure 7.2

Solution

The control valve used for the single acting cylinder is the 3/2 way valve. In this case, since

the cylinder is of small capacity, the operation can be directly controlled by a push button 3/2

way directional control valve with spring return.

Figure 7.3

When start button and 3/2 NC valve is operated, cylinder moves forward to clamp the work

piece. When start button and 3/2 way valve is released cylinder comes back to the retracted

position as shown in Figure 7.3.



7.2.2 Indirect control of single acting cylinder

− This type of circuit (Figure 7.4) is suitable for large single cylinders as well as cylinders

operating at high speeds. The final pilot control valve is actuated by normally closed 3/2 push

button operated valve. The final control valves handle large quantity of air.

− When the push button is pressed, 3/2 normally closed valve generate a pilot signal 12 which

controls the final valve thereby connecting the working medium to piston side of the cylinder

so as to advance the cylinder.

− When the push button is released, pilot air from final valve is vented to atmosphere through

3/2 NC – DCV.

Figure 7.4 - Indirect control of a single acting cylinder

− The signal pressure required can be around 1-1.5 bar. The working pressure passing through

the final control valve depends on the force requirement which will be around 4-6 bar.

− Indirect control as permits processing of input signals. Single piloted valves are rarely used

in applications where the piston has to retract immediately on taking out the set pilot signal.

Example 2: A large single acting cylinder is to extend and clamp a work piece when a push

button is pressed. As long as the push button is activated, the cylinder should remain in the

clamped position. If the push button is released, the clamp is to retract. Use additional start

button.

The control valve used for the single acting cylinder is the 3/2 way valve. In this case, since

the cylinder is of large capacity, the operation cannot be directly controlled by a push button

3/2 way directional control valve with spring return. Indirect control is to be used as shown in

the Figure 7.5

Valve 2 is a small capacity valve which controls the large capacity valve 3. When the valve 2

is unactuated the cylinder is in the retracted condition. When the valve 2 is actuated the

cylinder is in the extended position to clamp the work piece.



Figure 7.5 - Indirect control of single acting cylinder

7.2.3 Control of single acting cylinder using “or” valve (Shuttle valve)

− Shuttle valve is also known as double control valve or double check valve. A shuttle valve

has two inlets and one outlet (Figure 7.6). At any one time, flow is shut off in the direction of

whichever inlet is unloaded and is open from the loaded inlet to the outlet. This valve is also

called an OR valve.

− A shuttle valve may be installed for example, when the cylinder or valve is to be actuated

from two points, which may be remote from one another.

Figure 7.6 - Shuttle valve (OR valve)

− The single acting cylinder in Figure 7.7 can be operated by two different circuits. Examples

include manual operation and relying on automatic circuit signals, that is, when either control



valve 1 or control valve 2 is operated, the cylinder will work. Therefore, the circuit in Figure

7.7 possesses the OR function.

Figure 7.7 - Control of a single acting cylinder using OR valve

7.2.4 Control of single acting cylinder using “and” valve (Non Return Valve)

− This valve is the pneumatic AND valve. It is also derivate of Non Return Valve. A two

pressure valve requires two pressurised inputs to allow an output from itself. The cross

sectional views of two pressure valve in two positions are given in Figure 7.8.

− As shown in the Figure 7.8, this valve has two inputs 12 and 14 and one output 2. If the

compressed air is applied to either 12 or input 14, the spool moves to block the flow, and no

signal appears at output 2. If signals are applied to both the inputs 12 and 14, the compressed

air flows through the valve, and the signal appears at output 2.

Figure 7.8 - control of a single acting cylinder using OR valve

7.2.5 Control of single acting cylinder using “Not” valve

− Another name for a NOT function is inverse control. In order to hold or lock an operating

conveyor or a similar machine, the cylinder must be locked until a signal for cancelling the

lock is received. Therefore, the signal for cancelling the lock should be operated by a

normally open type control valve.

− However, to cancel the lock, the same signal must also cancel the locks on other devices,

like the indication signal . Figure 7.9 shows how the normally closed type control valve

can be used to cut off the normally open type control valve and achieve the goal of

changing the signal.



Figure 7.9 - Control of a single acting cylinder using NOT valve

7.3 DOUBLE ACTING CYLINDER CONROL

7.3.1 Direct control of double acting cylinder

− The only difference between a single acting cylinder and a double acting cylinder is that a

double acting cylinder uses a 5/2 directional control valve instead of a 3/2 directional control

valve (Figure 7.10).

− Usually, when a double acting cylinder is not operated, outlet ‘B’ and inlet ‘P’ will be

connected. In this circuit, whenever the operation button is pushed manually, the double

acting cylinder will move back and forth once.

Figure 7.10 - Direct control of a double acting cylinder

− In order to control the speed in both directions, flow control valves are connected to the

inlets on both sides of the cylinder. The direction of the flow control valve is opposite to that

of the release of air by the flow control valve of the single acting cylinder.

− Compared to the throttle inlet, the flow control valve is tougher and more stable.

Connecting the circuit in this way allows the input of sufficient air pressure and energy to

drive the piston.



Example 3: Pneumatic system is to be designed to operate a door of public transport

vehicles. (Figure 7.11). Assuming that the opening and closing of the doors are controlled by

two button switches ON and OFF. When the button switch ON is pressed, the door will open.

When the button switch OFF is pushed, the doors will close.

Figure 7.11 - Operation of pneumatic system that controls the door of vehicle

Solution.

Solution is given in Figure 7.12, which is self-explanatory

Figure 7.12 - Pneumatic circuit to control the door of vehicle

7.3.2 Indirect control of double acting cylinder using memory valve

− When the 3/2 way valve meant for Forward motion (Figure 7.13 b) is pressed, the 5/2

memory valve switches over through the signal applied to its pilot port 14. The piston travels

out and remains in the forward end position.

− Double piloted valve is also called as the Memory valve because now even if this push

button meant Forward is released the final 5/2 control valve remains in the actuated status as



the both the pilot ports of 5/2 valves are exposed to the atmosphere pressure and the piston

remains in the forward end position.

Figure 7.13 - Indirect control of Double acting cylinder using memory valve

− When the 3/2 way valve meant for return motion (Figure 7.13 a) is pressed, the 5/2 way

valve switches back to initial position through the signal applied to its pilot port 12. The

piston then returns to its initial position and remains in the rear end position. Now even if the

Return push button is released the status of the cylinder will not change.

− The circuit is called a memory circuit because it uses a 5/2 way double pilot memory valve.

5/2 way valve can remember the last signal applied in terms of the position of the spool in the

absence of reset springs, thus memorizing or storing the pneumatic signal. Double piloted 4/2

way valve also can be used as memory valve.

7.4 SUPPLY AIR THROTTLING AND EXHAUST AIR THROTTLING

− It is always necessary to reduce the speed of cylinder from maximum speed based on

selected size of final control valve to the nominal speed depending on the application.

− Speed control of Pneumatic Cylinders can be conveniently achieved by regulating the flow

rate supply or exhaust air.

− The volume flow rate of air can be controlled by using flow control valves which can be

either two way flow control valve or one way flow control valve

− There are two types of throttling circuits for double acting cylinders:

i) Supply air throttling

ii) Exhaust air throttling

7.4.1 Supply air throttling.

− This method of speed control of double acting cylinders is also called meter –in circuit

(Figure 7.14 a).For supply air throttling, one way flow control valves are installed so that air

entering the cylinder is throttled.



− The exhaust air can escape freely through the check valve of the throttle valve on the outlet

side of the cylinder. There is no air cushion on the exhaust side of the cylinder piston with

this throttling arrangement. As a result, considerable differences in stroking velocity may be

obtained even with very small variations of load on the piston rod.

− Any load in the direction of operating motion will accelerate the piston above the set

velocity. Therefore supply air throttling can be used for single acting and small volume

cylinders.

Figure 7.14 - Throttling Circuits

7.4.2 Exhaust air throttling.

− This method of speed control of double acting cylinders is also called meter-out (Figure

7.14 b). In exhaust air throttling throttle relief valves are installed between the cylinder and

the main valve in such a way that the exhaust air leaving the cylinder is throttled in both

directions of the motion of the cylinder.

− The supply air can pass freely through the corresponding check valves in each case. In this

case, the piston is loaded between two cushions of air while the cylinder is in motion and

hence a smooth motion of the cylinder can be obtained.

− The first cushion effect is due to supply air entering the cylinder through check valve, and

second cushion effect is due to the exhaust air leaving the cylinder through the throttle valve

at a slower rate. Therefore, exhaust air throttling is practically used for the speed control of

double acting cylinders.

− Arranging throttle valves in this way contributes substantially to the improvement of feed

behaviour.



7.5 TIME DEPENDENT CONTROLS

− Pneumatic timers are used to create time delay of signals in pilot operated circuits.

Available as normally closed timers and normally open timers. Usually pneumatic timers are

on delay timers.

− Delay of signals is very commonly experienced in applications such as bonding of two

pieces. Normally open pneumatic timers are also used in signal elimination. Normally open

pneumatic timers are used as safety device in two hand blocks

− Time delay valve is a combination of a pneumatically actuated 3/2 direction control valve,

an air reservoir and a throttle relief valve.

− The time delay function is obtained by controlling the air flow rate to or from the reservoir

by using the throttle valve. Adjustment of throttle valve permits fine control of time delay

between minimum and maximum times.

− In pneumatic time delay valves, typical time delays in the range 5-30 seconds are possible.

The time delay can be extended with the addition of external reservoir.

Pneumatic timer can be classified as

1. On –delay timer

2. Off – delay timer

− In on-delay timer, the 3/2 DCV is actuated after a delay with reference to the application of

pilot signal and is rest immediately on the application of the pilot signal. In off delay timer,

the 3/2 DCV is actuated immediately on the application of the pilot signal and is reset only

after a delay with reference to the release of the pilot signal.

− Pneumatic timers can also be classified according to type of pneumatically actuated 3/2

DCV as:

1) Time delay valve, NC type

2) Time delay valve, NO type.

Time delay valve, NC type. The constructions of an on-delay timer (NC) type in the normal

and actuated are shown in Figure 7.15 It can be seen that 3/2 DCV operates in the on delay

mode permanently. But, in some designs, the valve can be operated in the off-delay mode by

connecting the check valve in reverse direction. For this purpose, the ports of the throttle

check valve should be brought out.

Time delay valve, NO type. The construction and function of an on-delay timer (NO) type is

similar to that of an on-delay timer (NC) type except for the type of 3/2 DCV valve. In the

on-delay valve (NO) type, a 3/2 DCV (NO) type is used whereas in the on-delay timer (NC)

type, a 3/2 DCV (NC) type is used.



Figure 7.15

Example 4: A double acting cylinder is used to press together glued components. Upon

operation of a press button, the clamping cylinder slowly advances. Once the fully extended

position is reached, the cylinder is to remain for a time of t = 6 seconds and then immediately

retract to the initial position. A new start cycle is only possible after the cylinder has fully

retracted and after a delay of 5 seconds. During this delay the finished part is manually

removed and replaced with new parts for gluing. The retracting speed should be fast, but

adjustable.

Figure 7.16



− If the push button S1 is actuated for a sufficiently long time period (t = 5 second) then the

air reservoir of time delay valve V1 is filled and corresponding 3/2 valve is switched,

following which a signal is applied at input 1 of the dual pressure valve V2.

− If the push button S1 is actuated, the AND condition at the dual pressure valve is met. A

signal is applied at the control port 12 of the control element V4. The valve V4 switches,

pressure is applied to the piston side of the cylinder 1A and the piston rod advances. After as

short advancing distances, the limit switch S2 is released, pressure is reduced in the air

reservoir of the time delay valve V1 via the roller lever valve S2, and the integrated 3/2 way

valve switches back to its initial position. The AND condition at the dual pressure valve is

now no longer met. Actuation of the push button S1 becomes ineffective.

− Upon reaching the advancing position, the piston rod actuates the roller lever S3. The

pressure line to the time delay valve V3 is now released and pressure in the air reservoir is

increased. The rate of pressure increase is adjustable via the integrated flow control valve.

When the switching pressure has been reached, the integrated 3/2 way valve switches and a

signal is applied at the control port 12 of the final control element V4. The valve V4 reverses

and the piston rod retracts. Upon release of the limit switch S3, the time delay valve V3

Switches to its initial position again.

− The limit switch S2 is actuated, when the piston rod reaches its initial position, the pressure

in the air reservoir of the time delay V1 starts to increase until the switching pressure has

been reached after t = 5 seconds. The integrated 3/2 way valve switches. The initial status of

the system is now reached again and a new cycle can be started. The piston rod speed is set at

the restrictors of the one way flow control valves V5 and V6.

7.6 SINGLE ACTUATOR CIRCUIT VERSUS MULTI ACTUATOR CIRCUITS

− We have learnt about the various means and ways to control a single actuator circuits, both

for single acting and double acting cylinders. Implementation of logic gates along with use of

pressure sequence valve and time delay was systematically presented.

− Most of the practical pneumatic circuits use multi cylinders. They are operated in specific

sequence to carry out the desired task. For example, to drill a wooden component first we

need to clamp and then drill. We can only unclamp the cylinder, if and only if the drill is

withdrawn away from the work piece. Here sequencing of movement of clamp cylinders and

cylinder which carries the drill is important. This sequencing is carried out by actuation of

appropriate final control valves like directional control valves. The position of the cylinders is

sensed by the sensors like limit switches, roller or cam operated valves.

− Multi cylinder pneumatics circuits can be designed in various methods. There is no

universal circuit design method that suits all types of circuits. Some methods are commonly

used for compound circuits but would be too expensive for simple circuits. There are five

common methods used by engineering and they are given below

Classic method or Intuitive method

Cascade method

Step counter method



Karnaugh–veitch method

Combinational circuit design

7.6.1 Classic method or intuitive method

In intuitive method, circuit design is done by use of general knowledge of pneumatics

following the sequence through intuitively. In general, steps involves

Write down sequence and draw motion diagrams

Draw in cylinders and control valves

Complete circuits intuitively.

Coordinated and sequential motion control.

− In majority of the pneumatic applications more than one cylinder is used. The movement of

these cylinders are coordinated as per the required sequence. Sensors are used for confirming

the cylinder position and the resultant actuation of the final control element. Normally limit

switches are used.

− The activation of limit switches of different cylinders will provide set or reset signal to the

final control valves for further controlling the movement of various cylinders. The limit

switches have to be arranged in the proper location with the help of motion diagram

Demonstration of Classic method

− In order to develop control circuitry for multi cylinder applications, it is necessary to draw

the motion diagram to understand the sequence of actuation of various signal input switches-

limit switches and sensors.

− Motion diagram represents status of cylinder position -whether extended or retracted in a

particular step.

Example 1: In a press shop, stamping operation to be performed using a stamping machine.

Before stamping, workpice has to be clamped under stamping station. Then stamping tool

comes and performs stamping operation. Work piece must be unclamped only after stamping

operation.

Step 1: Write the statement of the problem:

Let A be the clamping cylinder and B be the stamping cylinder as shown in the Figure xxx.

First cylinder A extends and brings under stamping station where cylinder B is located.

Cylinder B then extends and stamps the job. Cylinder A can return back only cylinder B has

retracted fully.

Step 2: Draw the positional layout. (Figure 7.17)

Step3: Represent the control task using notational form

Cylinder A advancing step is designated as A+

Cylinder A retracting step is designated as A-



Figure 7.17 - Positional layout

Cylinder B advancing step is designated as B+

Cylinder B retracting step is designated as B-

Therefore, given sequence for clamping and stamping is A+B+B-A-

Step 4: Draw the Displacement –step diagram (Figure 7.18)

Figure 7.18 - Displacement step diagram

Step 5: Draw the Displacement –time diagram (Figure 7.19)

Figure 7.19 - Displacement time diagram

Step 6: Analyse and Draw Pneumatic circuit.

Step 6.1 Analyse input and output signals.

Input Signals:



Cylinder A – Limit switch at home position ao

Limit switch at home position a1

Cylinder B - Limit switch at home position bo

Limit switch at home position b1

Output Signal:

Forward motion of cylinder A (A+)

Return motion of cylinder A (A-)

Forward motion of cylinder B (B+)

Return motion of cylinder B (B+)

Step 6.2 using the displacement time/step diagram link input signal and output signal. (Figure

7.20)

Usually start signal is also required along with a0 signal for obtaining A+ motion.

1. A+ action generates sensor signal a1, which is used for B+ motion

2. B+ action generates sensor signal b1, which is used for B- motion

3. B- action generates sensor signal b0, which is used for A- motion

4. A- action generates sensor signal a0, which is used for A+ motion

Above is shown below graphically.

Figure 7.20 - Input/output signal flow

Step 7: Draw the power circuit (Figure 7.21)

i) Draw the cylinders A(1.0) and B(2.0).

ii) Draw the DCVs 1.1 and 2.1 in unactuated conditions

iii) Mark the limit switch positions for cylinders A (1.0) and B (2.0).



Figure 7.21 - Power circuit diagram

Figure 7.22 - Control circuit diagram

Step 9: Analysis of pneumatic circuit

1. When the start button is pressed, the signal appears at port 14 of valve 1.1 through limit

switch signal a0.

2. Check for the presence of the signal at the other end (12) of valve 1.1. Notice that the

signal is also present at port 12 of valve 1.1. (Because bo is also pressed). This results in

signal conflict and valve 1.1 is unable to move. (Figure 7.23).

3. Let us assume for time being, bo is somehow disengaged so that valve 1.1 can switch over

and consequently cylinder A can extend. When the start button is pressed. (Figure 7.24)

4. When cylinder A fully extends, it generates a limit switch signal a1, which is applied to

port 14 of the valve 2.1.

5. Check for the presence of the signal at the other end (12) of valve 2.1. Signal is not present

at port 12 of valve 2.1 and hence there is no signal conflict



Figure 7.23 - Signal conflict at valve 1.1

Figure 7.24 - Pneumatic circuit after cylinders A and B are extended

6. Signal applied to port 14 of the valve 2.1 causes the shifting of DCV 2.1 and cylinder B

extends.

7. When cylinder B fully extends, it generates a limit switch signal b1, which is applied to

port 12 of valve 2.1

1. Check for the presence of the signal at the other end of 14 of valve 2.1. It can be seen that

signal is also present at the port 14 of valve 2.1(because a1 is also pressed). This results in

signal conflict and valve 2.1 is unable to move (Figure 7.25)



Figure 7.25 - Signal conflict at valve 2.1

9. Let us assume for time being, b1 is somehow disengaged so that valve 2.1 can switch over

and consequently cylinder B can retract. (Figure 7.26)

Figure 7.26 - Position when cylinder B is reversing (B -)

10. When the cylinder B is fully retracted, it generates a limit switch signal b0, which is

applied to port 12 of the valve 1.1. (Figure 7.27)

11. Check for the signal at the other end 14 of the valve 1.1 Notice that signal is not present at

port 14 of the valve 1.1 and hence there is no signal conflict. So valve 1.1 can switch over

and Cylinder A can retract.



Figure 7.27 - Position when cylinder A has retracted fully (A-)

7.6.2 Cascade method

− A Bi-stable memory valve or reversing valve can be used to eliminate signal conflicts.

Signal conflict is avoided by allowing the signal to be effective only at times when they are

needed. Two of the possible designs are possible.

i) Cascade method

ii) Shift register method

Demonstration of Cascade method

− In order to develop control circuitry for multi cylinder applications, as done before in

classic method, it is necessary to draw the motion diagram to understand the sequence of

actuation of various signal input switches-limit switches and sensors.

− Motion diagram represents status of cylinder position -whether extended or retracted in a

particular step.

Step 1: Write the statement of the problem:

First cylinder A extends and brings under stamping station where cylinder B is located.

Cylinder B then extends and stamps the job. Cylinder A can return back only cylinder B has

retracted fully.

Step 2: Draw the positional layout. (Figure 7.28)

Step3: Represent the control task using notational form

Cylinder A advancing step is designated as A+

Cylinder A retracting step is designated as A-

Cylinder B advancing step is designated as B+

Cylinder B retracting step is designated as B-

Given sequence for clamping and stamping is A+B+B-A-



Figure 7.28 - Positional diagram

Step 4: Draw the Displacement –step diagram (Figure 7.29)

Figure 7.29 - Displacement step diagram

Step 5: Draw the Displacement –time diagram (Figure 7.30)


Step 6: Analyse and Draw Pneumatic circuit.

Step 6.1 Analyse input and output signals.

Input Signals

Cylinder A – Limit switch at home position ao

Limit switch at home position a1



Cylinder B - Limit switch at home position bo

Limit switch at home position b1

Output Signal

Forward motion of cylinder A (A+)

Return motion of cylinder A (A-)

Forward motion of cylinder B (B+)

Return motion of cylinder B (B+)

Step 6.2: Using the displacement time/step diagram link input signal and output signal.

(Figure 7.31)

Usually start signal is also required along with a0 signal for obtaining A+ motion.

1. A+ action generates sensor signal a1, which is used for B+ motion

2. B+ action generates sensor signal b1, which is used group changing.

3. B- action generates sensor signal b0, which is used for A- motion

4. A- action generates sensor signal a0, which is used for group changing

Above information is shown below graphically.


Step 7: Draw the power circuit (Figure 7.32)

i) Divide the given circuits into groups. Grouping should be done such that there is no

signal conflict. Do not put A+ and A- in the same group. Similarly B+ and B- should

not be put in the same group. In other word A+ and A- should belong to different

group to avoid signal conflict.

In our example of A+ B+ B- A- we can group as



A+ B+ B- A-

Group 1 Group 2

ii) Choose the number of group changing valve = no of groups -1

In our example, we have 2 groups so we need one group changing valve

Connect the group changing valve as follows. From the figure it is clear that when the control

signals I and II are applied to group changing valve, the air (power) supply changes from

Group 1(G1) to Group 2 (G2).

Figure 7.32 - Power circuit

iii) Arrange the limit switch and start button as given below (Figure 7.33)

Figure 7.33 – Arrangement of limit switches

iv) Draw the power circuit (Figure 7.34)

Figure 7.34 - Power circuit



Step 8 Draw the control circuit (Figure 7.35)

Figure 7.35 - Pneumatic circuits for A+ B+ B- A-

Step 9: Analysis of pneumatic circuit

1. Assume that air is available in the line G2 to start with. (Say from last operation)

2. When the start button is pressed, Air supply from Group G2 is directed to line 2 through

actuated limit switch a0. Now the air available in line 2, actuates the Group changing valve

(GCV) to switch over to position I. This switching of the GCV causes air supply to change

from G2 to G1.

3. Now the air is available in line G1. The air supply from group G1 is directed to port 14 of

the valve 1.1. As there is no possibility of signal conflict here, valve 1.1 switches over

causing the A+ action.

4. Sensor a1 is actuated as the result of A+ action, allowing the air supply from the Group G1

to reach to line 1 through a1. Now the air available reaches port 14 of valve 2.1. As there is

no possibility of signal conflict here, valve 2.1 switches over, causing B+ action

automatically.

5. Sensor b1 is actuated as result of B+ action, allowing the air supply in line 3. Air from line

3 allows the air to reach port 12 of Group changing valve (also called reversing valve). As a



result, the Group changing valve switches over, causing the group supply to change from G1

to G2.

6. Now the air is available in G2. Air from G2 acts on port 12 of the Valve 2.1. As there is no

possibility of signal conflict here, valve 2.1 switches over, causing B- action automatically.

7. Sensor is actuated as the result of B- action. Now the air is available in line 4.Air from line

4 reach port 12 of the valve 1.1, as there is no possibility of signal conflict here, valve 2.1

switches over, causing A- action automatically.

The cascade system provides a straightforward method of designing any sequential circuit.

Following are the important points to note:

a) Present – the system must be set to the last group for start-up

b) Pressure drop- Because the air supply is cascaded, a large circuit can suffer from

more pressure drop.

c) Cost – Costly due to additional reversing valves and other hardware


8 INTRODUCTION TO AUTOMATION IN HYDRAULIC AND PNEUMATIC SYSTEM

Course Contents

8.1 Introduction

8.2 Types of automation in the

industry

8.3 Advantages and disadvantages

of automation control in industry

Oil Hydraulics and Pneumatics (2171912) 8. Introduction to Pneumatic System Automation in Hydraulic and Pneumatic System


8.1 INTRODUCTION

- In today’s fast-moving, highly competitive industrial world, a company must be flexible,

cost -effective and efficient if it wishes to survive. In the process and manufacturing

industries, this has resulted in a great demand for industrial control systems/ automation in

order to stream-line operations in terms of speed, reliability and product output. Automation

plays an increasingly important role in the world economy and in daily experience.

- Automation is the use of control systems and information technologies to reduce the need

for human work in the production of goods and services. In the scope of industrialization,

automation is a step beyond mechanization. Whereas mechanization provided human

operators with machinery to assist them with the muscular requirements of work, automation

greatly decreases the need for human sensory and mental requirements as well.

8.1.1 What is automation control system?

- Automation Control System - system that is able to control a process with minimal human

assistance or without manual and have the ability to initiate, adjust, action show or measures

the variables in the process and stop the process in order to obtain the desired output.

- The main objective of Automation Control System used in the industry are:

1. To increase productivity

2. To improve quality of the product

3. Control production cost

8.2 TYPES OF AUTOMATION IN THE INDUSTRY

Classification of automation

a) Permanent/Fixed Automation

- This control system is designed to perform a specific task

- Functions of control circuit is fixed and permanent.

- It will be complicated if we want to do other task apart from the existing task

b) Programmable /Flexible Automation

- Programmable automation or flexible automation is a complex control

system that can perform several tasks

- Functions of control circuit programmed by the user and can be modified.

- When the task to be performed by machines changed, changes only need to

be done by making modifications to the machine control program.



8.2.1 Comparison between fixed and flexible automation system

FIXED

AUTOMATION

FLEXIBLE

AUTOMATION

Purpose Specific Variety

Ease of making changes / upgrade Difficult Easy

Maintenance Hard Easy

Capability

Depends on

manufacturing and

design

Very high

Speed Slow Fast

Economy efficiency

Suitable for small

system

Suitable for

all types of systems

Example:

Fixed automation

Figure 8.1 – Fixed automation

Programmed automation

- There are three types of the control system based on supply

a) Pneumatic control systems

b) Hydraulic control system

c) Electrical control system

a) Pneumatic control system

- Pneumatic control system is a system that uses compressed air to produce power / energy to

perform any task

- Pneumatic systems found in many industrial systems such as food industry, petrochemical

and industrial involves robotics.



Figure 8.2 – Programmed automation

- Pneumatic systems requires:

i. Compressed air supply

ii. Control valve

iii. Connecting tube

iv. Transducer

- Pneumatic control system can be controlled manually and automatically.

Figure 8.3 - Basic Block Diagram of Pneumatic Control System using manual/PLC

b) Hydraulic control system

- Hydraulic control system is a system that uses fluid to generate power/energy.

- The hydraulic system used in the automobile industry such as power systems, braking

systems, cranes, car jack, satellite and others.

- The fluid used is oil.

- The hydraulic system requires:



a) Hydraulic fluid supply

b) Control Valve

c) Cylinder

- Hydraulic control system can be controlled manually and automatically

Figure 8.4 - Basic block diagram of an automatic hydraulic control system by Manual /PLC

c) Electrical control system

- A control system that uses an electric current; either direct current (DC) or current shuttle

(AC) as a source of supply.

- Electrical control systems generally requires:

a) Electricity (DC) or (AC)

b) Input elements (switches, sensors, transducer, valves, electronic components, etc.)

c) Output elements (motor, lights, etc.)

d) Extension cable

Figure 8.5 - Basic block diagrams of electrical control system using PLC

8.2.2 Comparison between pneumatic control systems, hydraulic control system and

electric control system

i. Pneumatic control system

a) Easy installation

b) Simple design

c) Use compressed air as a supply source to perform task.



ii. Hydraulic control system

a) Complex to assemble

b) Use fluid like oil as a supply source to perform task.

c) Potential leakage will lead to pollution.

iii. Electric control system

a) Simple system

b) Use electricity as a supply source to perform task.

c) Widely use either for home user or in industrial.

8.3 ADVANTAGES AND DISADVANTAGES OF AUTOMATION CONTROL IN

INDUSTRY

The main advantages of automation are:

- Replacing human operators in tasks that involve hard physical work.

- Replacing humans in tasks done in dangerous environments (i.e. fire, space, volcanoes,

nuclear facilities, underwater, etc.)

- Performing tasks that are beyond human capabilities of size, weight, speed, endurance, etc.

- Economy improvement: Automation may improve in economy of enterprises, society or

most of humanity. For example, when an enterprise invests in automation, technology

recovers its investment; or when a state or country increases its income due to automation

like Germany or Japan in the 20th Century.

- Reduces operation time and work handling time significantly.

The main disadvantages of automation are:

- Unemployment rate increases due to machines replacing humans and putting those humans

out of their jobs.

- Technical Limitation: Current technology is unable to automate all the desired tasks.

- Security Threats/Vulnerability: An automated system may have limited level of intelligence,

hence it is most likely susceptible to commit error.

- Unpredictable development costs: The research and development cost of automating a

process may exceed the cost saved by the automation itself.

- High initial cost: The automation of a new product or plant requires a huge initial

investment in comparison with the unit cost of the product, although the cost of automation is

spread in many product batches of things

Documents

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