Hydraulics Accessories

7/31/2019 Hydraulics Accessories

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ACCESSORIES

RESERVOIRS:

A hydraulic system is closed, and the oil used isstored in a tank or reservoir to which it is returned after

use. Although probably the most mundane part of the

system, the design and maintenance of the reservoir is

importance for reliable operation.

Figure shows details of a typical reservoir. Thevolume of fluid in a tank varies according to temperature

and the state of the actuators in the system, being

minimum at low temperature with all cylinders


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extended, and maximum at high temperature with all

cylinders retracted.

Normally the tank volume is set at the larger of fourtimes the pump draw per minute or twice the external

system volume. A substantial space must be provided

above the fluid surface to allow for expansion and to

prevent any froth on the surface from spilling out.

The tank also serves as a heat exchanger, allowing

fluid heat to be removed. To obtain maximum cooling,fluid is forced to follow the walls of the tank, from the

return line to pump suction inlet, by a baffle plate down

the tank centre line.

This plate also encourages any contamination to fall

to the tank bottom before reaching the pump inlet, and

allows any entrapped air to escape to the surface.

Low pressure returns (such as drains from motors or

valves) must be returned above fluid level to prevent

back pressure and formation of hydraulic locks.

Fluid level is critical. If it is too low, a whirlpool

forms above the pump inlet, resulting in air being drawn

into the pump. This air results in maloperation, and will

probably result in pump damage.

A level sight glass is essential to allow maintenance

checks to be carried out. The only route for oil to leave a


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hydraulic system is, of course, by leaks so the cause of

any gross loss of fluid needs investigation. In all bar the

smallest and simplest systems, two electrical float

switches are generally included giving a remote (low

level) warning indication and a last ditch (very low level)

signal which leads to automatic shutdown of the pump

before damage can occur.

The temperature of fluid in the tank also needs

monitoring and as an absolute minimum a simple visualthermometer should be included. The ideal temperature

range is around 45 to 50 and, usually, the problem is

keeping the temperature down to this level.

Ideally an electrical over-temperature switch is used

to warn the user when oil temperature is too high.

Oil is added through a filler cap in the tank top. This

doubles as a breather allowing air into and out of the

tank as the volume of fluid changes. A coarse filter below

the breather prevents contamination entering the tank as

fluid is added.

Tank air filters are commonly forgotten in routinemaintenance. The oil in a typical tank changes

considerably during operation as temperatures change

and actuators operate. This change in volume is directly

reflected in air changes both in and out of the tank.


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The only route for this air flow is through the filters.

If these become blocked the tank may become

pressurized and fail disastrously. Reservoirs are

generally constructed from welded steel plate with thin

side walls to encourage heat loss.

The inside of the tank is shot blasted then treated

with protective paint to prevent formation of rust

particles. At some time in the life of a hydraulic system

there will eventually be oil spillage around the tank,whether from leakage, overenthusiastic filling or careless

maintenance.

It is therefore good practice to put substantial drip

trays under reservoir pumps and associated valves to

limit oil spread when the inevitable mishaps occur.


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ACCUMULATORS

A hydraulic accumulator is a device that stores the

potential energy of an incompressible fluid held underpressure by an external source against some dynamic

force. This dynamic force can be from three different

sources

1.Gravity2.Mechanical3.Compressed gasesA hydraulic accumulator is a device in which potential

energy is stored in the form of a compressed gas orspring, or by a raised weight to be used to exert a forceagainst a relatively incompressible fluid.

The stored potential energy in the accumulator is aquick secondary source of fluid power capable of doing

useful work as required by the system.

They are used in fluid power systems to accumulateenergy and to smooth out pulsations. A hydraulic systemutilizing an accumulator can use a smaller fluid pump

since the accumulator stores energy from the pumpduring low demand periods. This energy is available forinstantaneous use, released upon demand at a rate manytimes greater than could be supplied by the pump alone.

Accumulators can also act as surge or pulsationabsorbers, much as an air dome is used on pulsating


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piston or rotary pumps. They will cushion hydraulichammer, reducing shocks caused by rapid operation orsudden starting and stopping of power cylinders in a

hydraulic circuit.

There are three types of accumulators used in hydraulic

system:

1.Weight loaded or gravity type2.Spring loaded type3.

Gas loaded type.

The weight loaded type was the first used but is muchlarger and heavier for its capacity than modern pistonand bladder types. Both the weighted type andmechanical spring type are very seldom used today.

The hydro-pneumatic types use a gas as a spring

cushion in conjunction with a hydraulic fluid, the gas andfluid being separated by a thin diaphragm or a piston.Tobul accumulators, having an aluminum piston of lowinertia as standard equipment, are superior to othermakes in absorbing either high or low frequencypulsations.


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Weight loaded Hydraulic Accumulator:

This consists of a vertical cylinder filled with fluidand a piston within, with heavy weight that gives out adownward force, which in turn activates the fluid insidethe cylinder. An advantage of using this kind ofhydraulic accumulator is that there is full pressure, evenif there is a decrease in the amount of fluid content.


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Spring Loaded Hydraulic Accumulator:

In the spring loaded hydraulic accumulator, there is

a spring which along with the container and a movablepiston replaces the weight used in the raised weighthydraulic accumulator. Either one or more springs maybe used as per the requirement. These springs create therequired pressure on the hydraulic piston to pump outfluid.

The amount of pumping depends upon the

compression rate of the spring. The main advantage ofthis type of hydraulic accumulators is that they arecomparatively small in size, have a low weight and areeasy to be assembled in the hydraulic system and are atthe same time cost effective. They can be used as mobilehydraulic accumulators due to their lightweight.

Compressed Gas Accumulator:

A compressed gas accumulator is also called gascharged accumulator or hydro-pneumaticaccumulator. The compressed gas accumulator is madeup of a container with twin chambers. While onechamber is filled with hydraulic fluid, the other is filled

with inert gases that create sufficient pressure. Mostlydry nitrogen gas is used, as it is quite suitable forexerting the required pressure and is relative safe. Thesetwo chambers are separated by a diaphragm or a bladder.


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Metal Bellows Hydraulic Accumulators:

The metal bellows hydraulic accumulators are

similar to the compressed gas accumulators except thatthe diaphragm is replaced by metal bellows. Theadvantages with metal bellows hydraulic accumulatorsare that they support varied types of fluids, providesmaximum storage capacity, low spring force, and have ahigh and efficient performance

Piston Type Accumulator:

It consists of a cylinder body and a movable piston

with proper seals. The piston serves as the barrier

between the gas and oil. The gas is confined at the

volume above the piston and the oil at the volume

below the piston. The gas is compressed when thecharged oil pushed the piston against it. This gas

pressure is used as the potential energy to force the oil

out when it is required in the circuit.


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Diaphragm type Accumulator:

The spherical vessel is separated into twocompartments by a flexible diaphragm. One

compartment is connected to the hydraulic system and

the other to the high pressure gas system. Thus the

diaphragm serves as an elastic barrier between the oil

and the gas.

In most designs, a spring loaded, normally open

check valve or a screen is provided at the liquid

connection to prevent extrusion of the diaphragm into

the oil line when the fluid is discharged.

When the oil is delivered into the accumulator, it

deforms the diaphragm. The gas is compressed when

the charged oil pushes the diaphragm against it. This

gas pressure is used as the potential energy to force

the oil out when it is required in the circuit.


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Bladder type Accumulator:

It consists of a bag or bladder of synthetic materialwhich is precharged with gas to a determined pressure.

This bladder is placed within the accumulator shell

and the balance of the space filled with oil. Thus the

bladder serves as an elastic barrier between the gas

and oil.

When the oil under pressure enters the accumulator,

the gas in the bladder is compressive, thus forming a

reservoir of pressure. When oil called for from the

accumulator, the bladder expands and forces the oil out

into the circuit.


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INTENSIFIERS

Pressures intensifiers also known as pressure

boosters are used to compress the liquid in a hydraulicsystem to a valve above the pump discharge pressure.

(Or)

A hydraulic intensifier is a device which converts a

large-volume, low-pressure fluid supply into a

proportionately small-volume, high pressure fluid

outlet.

The intensifier is usually located in between the

pump and the machine (e.g., press,crane,lift) that needs

high pressure liquid for its operation.

The intensifier unit consists of two pistons-lowpressure and high pressure-having a common pistonrod. The larger, piston is exposed to pressure from alow pressure pump. The low pressure fluid (oil) isintroduced to the larger piston side and thus it forcesthe piston to move.


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Intensifier ratio:

It may be noted that the increase in pressure is directlyproportional to the difference in areas of the two sizes ofpistons.

PoOperating end of the piston

PiIntensifying end of the piston

AoArea of the larger

AiArea of the smaller

Since force load acting on the two pistons is same we get

Po x Ao = Pi x Ai

Intensifier ratio, Pi Ao

---- = ----

Po AiThe volume output is inversely proportional to the aboveratio.

High outlet pressure (Pi) Area of the large piston (Ao)

------------------------------- = -----------------------------

Low inlet pressure (Po ) Area of the small piston (Ai)High inlet flow rate (Qin)

= ----------------------------------

Low outlet flow rate (Qout)


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ACCUMULATOR CIRCUITS

Accumulators are device used to

1.Auxiliary power sourcesstore energy and supplywhen needed.

2.Serve as Leakage compensators.3.Serve as Emergency power sources4.Serve as Shock absorbers.1.Auxiliary power sources:One of the most common uses of accumulators is to act

as an auxiliary source of power.

In this application, the accumulator stores the oil

during one portion of the work cycle and releases the oil

during remaining cycle. Thus the accumulator serves asa secondary power source.


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First operator places work piece on slide table and

shifts handle of 4/2 DC valve. Now oil flows from the

accumulators to blind end of slide cylinder. This extends

the piston until slide table reaches end of stroke.

When the cylinder is in the fully extended position,

the accumulators are charged with the oil by the pump.

Then the operators shifts the handle of 4/2 DC valve for

the retraction of the cylinder. Now the oil flows from the

pump as well as from the accumulators to retract thecylinder quickly.

2. Accumulators as Leakage compensators:

A basic hydraulic circuit employing a bladder type

accumulators as a leakage compensator. This circuit

consists of a power unit (pump), a check valve,

4/2DCV, pressure switch, power cylinder and

accumulator.


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First operator place work piece on the press and

shifts handle of the 4/2 DCV. Now the oil flows to rod

end of the cylinder and piston extends contacting the

work. The pressure builds up and oil fills the

accumulators. When maximum pressure is reached, the

pressure switch stops the pump motor.

In these applications, the cylinder and piston

arrangement is required to press the work piece for a

longer period of time.

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Hydraulics Accessories