Basic Hydraulics by Teh

8/10/2019 Basic Hydraulics by Teh

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Definition of hydraulics

Generation of forces and motion using

hydraulic fluids

Hydraulic fluid represents the medium of

power transmission

(note pg: 7)


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Hydro-mechanics

1. Hydrostatics 2. Hydrodynamics

(note pg: 13)


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APPLICATIONS

Production and assembly

machines of all types

Transfer lines

Lifting and conveying devices

Presses

Injection moulding machines

Rolling lines

Lifts

Stationary hydraulics

(note pg: 8-10)


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APPLICATIONS

Construction machinery

Tippers, excavators, elevating

platforms

Lifting and conveying devices

Agricultural machinery

Mobile hydraulics

(note pg: 8-9)


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Advantages of hydraulics Transmission of large forces using small

components, i.e. great power intensity Precise positioning

Start-up under heavy load

Even movements independent of load, sinceliquids are scarcely compressible and flowcontrol valves can be used

Smooth operation and reversal

Good control and regulation

Favourable heat dissipation

(note pg:10)


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Disadvantages of hydraulics

Pollution of the environment by waste oil

(danger of fire or accidents) Sensitivity to dirt

Danger resulting from excessive pressures

(severed lines)

Temperature dependence (change in viscosity)

Unfavourable efficiency factor

(note pg:10)


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Comparisions

Protected against overload, forces

limited by pneumatic pressure and

cylinder diameter F < 30 kN at 6

bar.

Protected against overload, with high

system pressure of up to 600 bar, very

large forces can be generated F < 3000

kN.

Forces

Low, air is compressible.High, since oil is almost incompressible, in

addition, the pressure level is considerably

higher than for pneumatics.

Stability

Without load change precision of

1/10 mm possible.

Precision of up to 1 m can be achieved

depending on expenditure.

Positioning accuracy

Simple, inefficient, high speed.Simple, high turning moment, low speed.Rotary motion

Simple using cylinders, limited

forces, speed extremely, load-

dependent.

Simple using cylinders, good speed

control, very large forces.

Linear motion

Very high (2.5)High (1)Power supply costs

v = 1.5 m/sv = 0.5 m/sOperating speed

Up to 1000 m, flow rate v = 20 40

m/s, signal speed 20 40 m/s.

Up to 100 m, flow rate v = 2 6 m/s, signal

speed up to 1000 m/s.

Energy transmissionEasyLimited, with the help of gases.Energy storage

Explosion-proof, insensitive to

temperature.

Sensitive in case of temperature

fluctuation, risk of fire in case of leakage.

Environmental

influences

No disadvantages apart from

energy loss

ContaminationLeakage

PneumaticsHydraulics


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BASIC PHYSICAL PRINCIPLES

Hydrostatic pressure Open vessel

ps = hydrostatic pressure (gravitational pressure) [Pa]

h = level of the column of liquid [m]

= density of the liquid [kg/m3]

g = acceleration due to gravity [m/s2]

ps = h..g

(note pg:14)


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Examples:

Column Reservoir Elevated tank

(note pg:15)


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Pascals law: pressure exists when a force F is imposed on

an enclosed fluid with a surface A, The pressure exerts anequal effect on all points of the surfaces.

Hydrostatic pressure closed vessel

FP

A

P =

F

A N/m2

(note pg:17)


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Example:A cylinder is supplied with 100 bar pressure, its effective piston surface is

equal to 7.85 cm2. Find the maximum force which can be attained.

Given that: p = 100 bar = 1000 N/cm2A = 7.85 cm2

F

P


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Example:

4

2D

A

=

P

F=15000N

cmD

D

05.5)20(4

)20(42

==

=


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Power transmission: The same pressure applies at every point in a closed

system

(note pg:22)


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Example:

(note pg:23)


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Displacement transmission:

(note pg:25)


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Example:

Calculate S2Calculate S2

Given:Given:

A1 = 40 cmA1 = 40 cm22

A2 = 1200 cmA2 = 1200 cm22S1 = 15 cmS1 = 15 cm

(note pg:26)


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FLOW RATE:

Flow rate is the term used to describe the volume of liquidflowing through a pipe in a specific period of time. For

example, approximately one minute is required to fill a 10litre bucket from a tap. Thus, the flow rate amounts to 10l/min.

(note pg:29)


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Flow measuring instruments:

(note pg:277)


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CONTINIUTY EQUATION

If the time t is replaced by s/v (v = s/t) in the formula for

the flow rate (Q = V/t) and it is taken into account that the

volume V can be replaced by As, the following equation

is produced:

Q = A v

Q = Flow rate [m3/s]

v = Flow velocity [m/s]

A = Pipe cross-section [m2]

(note pg:31)


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Example:

Calculate the oil flow velocity in a pipeline

Given that:Q = 4.2 l/min = = 0.0710-3m3/s

A = 0.28 cm2

= 0.28 10-4

m2

Q v

60s

4.2dm3


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Example:

Calculate the flow rate needed for

the following movement

Given that: A = 8 cm2

s = 10 cm

t = 1 min

Q

(note pg:32)


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CONTINIUTY EQUATION

The flow rate of a liquid in terms of volume per unit of time which flowsthrough a pipe with several changes in cross-section is the same at allpoints in the pipe (see diagram). This means that the liquid flows

through small cross-sections faster than through large cross-sections.The following equation applies:

Q1 = A1v1 Q2 = A2v2 Q3 = A3v3 etc.

As within one line the value for Q is always the same, the followingequation of continuity applies:

Q1 = Q2 = Q3

A1

v1 = A2

v2 = A3

v3 = etc...

(note pg:34)


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PRESSURE MEASUREMENT

(note pg:37)


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TYPE OF FLOW

Two types of flow

Laminar, Re < 2300

Turbulent, Re > 2300

Re = v x d / v

v is flow velocity in m/s

D is pipe diameter in m

v is kinetic viscocity in m2/s

(note pg:39)


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Energy Loss By Turbulent Flow


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Hydraulic fluid

Types:

Mineral based

For low risk of fire

Phosphate-ester based (Synthetic oil)

For high risk of fire

(note pg:70)


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Hydraulic fluid

Tasks for hydraulic fluids

pressure transfer, lubrication of the moving parts of devices,

cooling, i.e. diversion of the heat produced byenergy conversion (pressure losses),

cushioning of oscillations caused by pressurejerks,

corrosion protection,

scuff removal, signal transmission.

(note pg:70)


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Hydraulic fluid

Hydraulic fluids with low inflammability (HF liquids):

(note pg:72)


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Hydraulic fluidViscosity:

The word viscosity can be defined asresistance to flow. The viscosity of a

liquid indicates its internal friction,

The international system of standardsdefines viscosity as kinematic viscosity

(unit: mm2/s or Cst).


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Hydraulic fluidISO standard for Viscosity Grade:


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Hydraulic fluid

VG selection:

If viscosity is too low (very fluid), more leakages occur.The lubricating film is thin and, thus, able to break away

more easily resulting in reduced protection against wear. High viscosity results in increased friction leading to

excessive pressure losses and heating particularly atthrottle points. This makes cold start and the separation

of air bubbles more difficult and, thus, leads to cavitation.


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Hydraulic fluidVG selection:


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Hydraulic system


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Hydraulic system


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Power supply section

The power supply section provides the energy required bythe hydraulic system. The most important components in

this section are: drive

pump

pressure relief valve

coupling

reservoir

filter

cooler heater


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Power supply unit (Power Pack)

Example:


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Hydraulic Pump

The pump converts the mechanical energy in a drive unit into hydraulicenergy (pressure energy).

Types:


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Gear pumpGear pumps are fixed displacement pumps since the displaced volume

which is determined by the tooth gap is not adjustable.


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Axial Piston Pump


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Characteristic values for the most common constant pumps

Practical:


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Practical:


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Pump characteristic


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Reservoir / Tank

The tank in a hydraulic system fulfils several tasks.It:

acts as intake and storage reservoir for the

hydraulic fluid required for operation of thesystem;

dissipates heat;

separates air, water and solid materials; supports a built-in or built-on pump and drive

motor and other hydraulic components, such as

valves, accumulators, etc.


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Reservoir / Tank

Filt


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Filters

Filters are of great significance in hydraulic systems for the reliable

functioning and long service life of the components.

The effects of polluted oil:

Filt t


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Filter arrangement


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Filter Grades


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Filter Grades


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Filter designs

Valve Symbols


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Valve Symbols

Flow path

Switching position

Flow path blocked

Connection ports

Directional Control Valves

(note pg:92)


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- Way valve

Number of ports

Number of switching positions

22

- Way valve3 2


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Connection portsP

T

A , B

L

; Pressure supply port

; Return port (Tank)

; Power/Output/working ports

; Leakage port

- Way valve4 2

P T

A B

Methods of actuation:


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(note pg:93)


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Hydraulic actuatorsLinear actuators:

single-acting and

double-acting cylinders.

Rotary actuators:

Hydraulic motors

(note pg:228)

Single acting cylinder


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g g y

In single-acting cylinders, only the piston side is suppliedwith hydraulic fluid. Consequently, the cylinder is only able

to carry out work in one direction.

(note pg:228)



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Types:

(note pg:230)


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Double-acting cylinder


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Double acting cylinder

In the case of double-acting cylinders, both piston surfaces can be

pressurized. Therefore, it is possible to perform a working movement in both

directions.

(note pg:231)


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Double-acting cylinder



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Types:

(note pg:233)



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End position cushioning

(note pg:235)


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Hydraulic motors


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They convert hydraulic energy into mechanical energy andgenerate rotary movements (rotary actuator). If the rotary

movement only covers a certain angular range, the actuator is

referred to as a swivel drive.

(note pg:250)

Hydraulic motors


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Hydraulic motorsTypes:

(note pg:253)


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Valves


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Design: Poppet valves

slide valves

(note pg:151)

ValvesPoppet valves:


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Poppet valves:

(note pg:152)

Valvesslide valves


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slide valves

(note pg:154)

Valves


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Comparison of valve constructions:

(note pg:155)

Valves


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Control edges:

(note pg:160)

Valves


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Annular grooves:

With the grooves, the piston of valve spool is

supported on a film of oil. On actuation, only the

fluid friction needs to be overcome.

(note pg:161)


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Directional control valves


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3/2-way valve

(note pg:188)



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4/2-way valve

(note pg:190)



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4/3-way valve with pump by-pass (re-circulating)

(note pg:195)

4/3-way valve with pump by-pass (re-circulating)


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(note pg:191)



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4/3-way valve, mid position closed

(note pg:197)


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Pressure valves


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Pressure relief valves

Pressure regulator

2-way pressure

regulator

3-way pressure

regulator

(note pg:164)

Pressure valves


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Pressure relief valves

(note pg:166)

Pressure valves


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Pressure relief valve, internally controlled, cushioned: Cushioning pistons and throttles are often installed in

pressure relief valves to eliminate fluctuations in

pressure. The cushioning device shown here causes:

fast opening slow closing of the valve.

(note pg:168)

Pressure valves

P li f l t ll t ll d


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Pressure relief valve, externally controlled

(note pg:170)


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(note pg:169/171)

Pressure valvesPressure regulators:

Pressure regulators reduce the input pressure to a specified output


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Pressure regulators reduce the input pressure to a specified outputpressure. They are only used to good effect in systems where a number

of different pressures are required.

2-way pressure regulator

(note pg:172/3)

Pressure valves


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3-way pressure regulator

(note pg:176)


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Non-return valves / Check valves

Non-return valves block the flow in one direction and


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Non-return valves block the flow in one direction andpermit free flow in the other.

(note pg:201)

Pump protection


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(note pg:203)


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Non-return valves / Check valves

Piloted non return valve


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Piloted non-return valve

Flow blocked from B to A Flow from A to B Flow from B to A with X signal


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Piloted non-return valve:


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Flow control valves Flow control valves are used to reduce the speed of a cylinder or

the r.p.m. of a motor.


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Flow control valves are classified as either:

flow control valves or

flow regulating valves.

Flow control valves

One-way flow control valve


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Flow control valves

Two-way flow control valve

To maintain a constant speed in the case of a changing load. thepress re drop p ia the throttle point can be kept constant


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pressure drop p via the throttle point can be kept constant.

Flow control valvesTwo-way flow control valve


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Accumulator


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Accumulators perform special functions inhydraulic systems:

To act as an emergency power source, e.g. tocomplete a working stroke in case of drive orpump failure.

To compensate for leakage losses. To compensate for variations in fluid volume due

to changes in temperature.

Absorption of shock waves and pressure peaksdue to switching actions and applications.

Accumulator

Design:


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Bladder accumulator


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Bladder accumulator

Operation:


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Accumulator applications

Reduce vibration and shock:


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Accumulator applicationsInstallation for emergency power source:


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Thank you

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Basic Hydraulics by Teh