Hydraulic Valves Sureclean

7/30/2019 Hydraulic Valves Sureclean

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HYDRAULIC SYSTEMS

PRESSURE CONTROL VALVES

Function

Pressure control valves perform a number of functions within a hydraulic system. Most

importantly they limit the maximum system pressure. In doing so, they protect individual

components from the risk of overload and subsequent damage. In addition, they can

control the pressure in a particular part of a system, unload a pump or determine the

sequence in which actuators or valves perform their functions.

Pressure Relief Valves

Situated on the outlet line from the pump the pressure relief valve (PRV) is designed to

limit the maximum system pressure. Normally closed, this valve opens when system

pressure is reached diverting surplus pump delivery back to tank. They may also be found

in a particular branch of a circuit where they are termed circuit relief valves. Closed loop

arrangements deploy them on pump / motor circuits where they are connected across the

two main lines between the pump and motor; thus the term cross-line relief valves.

Simple / Direct Acting Relief Valves

Inlet

CLOSED OPEN

Contained within a body, this component comprises a ball or poppet held firmly on its

seat by the action of a strong spring. Input pressure oil acts against the exposed area of

the valve and applies a force to the strong spring. When the force of the pressure oil is

Outlet


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greater than the force of the spring the valve opens allowing input oil to flow via an

output connection back to tank.

Many valves of this type are adjustable either by the use of shims or an adjusting screw.

This type of valve is very fast acting making it ideal for relieving shock loads. However,

it is only suited to low volume applications.

Pressure Override

Cracking Pressure is the term used to describe the pressure at which the valve initially

moves off its seat. As inlet flow increases valve movement causes the spring to be further

compressed. This leads to a consequent rise in pressure called pressure override; the main

disadvantage of direct acting relief valves.

Compound / Pilot Operated Relief Valve

This valve is utilised where large volumes of oil require to be dumped with a small

pressure differential. Comprising two stages, the pilot stage is a simple direct acting valve

which controls the second stage.

Inlet pressure oil is applied to the underside of the main valve where it passes through a

small orifice to the top side of the valve. As well as applying pressure oil to the pilot

stage the main valve is now in a balanced condition with equal pressure acting on equal

areas. Consequently, only a relatively weak spring is required to keep the main valve

seated. This situation remains as long as the oil is at a lower pressure than that of the pilot

valve setting.

When oil pressure reaches the setting of the pilot valve it cracks off with a resultant drop

in pressure at the main valve orifice. A pressure differential now exists between the top

side and underside of the main valve. This causes the main valve to lift off its seat

allowing oil to be dumped through the outlet connection.

When inlet pressure drops below the valve setting the pilot stage will initially close.

Equal pressure thus develops at both sides of the main valve and the spring causes it to

close.


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VOLUME CONTROL VALVES

It is necessary on many hydraulic systems to control the speed of a motor or ram.

Systems using a fixed displacement hydraulic pump employ a volume control valve for

this purpose. It is possible to achieve the same result by using a variable displacement

pump to regulate oil flow.

Flow Control Methods

Three methods are used to deploy volume control valves for controlling actuator speeds:

Meter-In

A volume control valve is placed in series in the line between the DCV and actuator. This

controls the quantity of oil entering the actuator. Where speed control is required in one

direction only a by-pass valve (check valve) is fitted within the flow control valve. This

system is ideal where the load always resists actuator movement.


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Meter-Out

Oil exhausting out of the cylinder passes through a flow control valve which is fitted in

the line between actuator and DCV. Again, a by-pass valve is fitted if speed control is

required in one direction only. A common application of this system is when the load hasa tendency to run away.

Bleed-Off

A tee piece in the line between actuator and DCV allows a portion of the pump oil

delivery to bleed off through the flow control valve back to reservoir. This is the least

accurate of the three methods.


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Needle and Globe Flow Control Valves

This very simple valve places a restriction in the line. With the needle screwed down

fully, flow is completely shut off. Unscrewing the valve increases the size of orifice

allowing more oil to flow through. Globe valves operate on the same principle as needle

valves except that they have a rounded metering tip.

Proving the load remains constant, speed control is fairly accurate. However, a change in

load results in a variation in pressure drop across the valve which alters oil flow.

Pressure Compensated Flow Control Valve

Functions which require flow rates to be consistent, irrespective of pressure drop, use

pressure compensated flow control valves. Regardless of upstream or downstream

pressure changes, this valve maintains a precise flowrate.

Fluid enters the valve at Port P before passing through the pressure control orifice. It then

passes through the control orifice before leaving at Port A. Having areas which are

exactly the same, the compensator spool is forced to the right by spring action during no

flow conditions. The compensator orifice is fully open at this point.

When fluid flows through the valve the compensator orifice will remain fully open when

the pressure drop is less than the spring force. A pressure drop which is greater than the

spring force will reduce the size of the compensator orifice. Where pressure on one side

of the compensator spool changes, without the same change on the other side, movement

of the compensator spool will result. Thereby, a fixed pressure drop across the controlorifice is always maintained. A constant flow rate is therefore maintained.


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Outlet Port

Inlet Port

Pilot Operated Check Valves

Valve Closed

Valve Opened by Pilot Signal

Under normal circumstances this valve operates like a normal check valve allowing flow

in one direction only. However, applying pressurised fluid to the pilot port opens the

valve and allows reverse flow. This type of check valve is commonly used in applications

where a cylinder has to be locked in position. It prevents leakage from the cylinder if ahose burst. A crane boom cylinder is a typical example.

Spool Valve

Valves of this type are fitted to most hydraulic circuits where positional control of an

actuator (cylinder or motor) is required. A cylindrical valve comprising a number of

spools is moved within a machined bore inside the valve body. The spools isolate each

part of the circuit directing oil to and from the valve passages.

B

PT T

A

Pilot Port


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The valve shown above is a closed centre design. Connections to the valve are labelled A,

B, P and T.

A Actuator Port A

B Actuator Port B

P Pressure SupplyT - Tank

Fluid entering at port P whilst in the neutral position (shown) is closed off and pressure

builds up causing the pump to off-load. Moving the spool within its bore to the right

connects port P to B. Port A connects to T. Conversely, sliding the spool to the left

connects P to A and B to T.

Hydraulic Valves Sureclean

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