hydraulic valves sureclean
Post on 14-Apr-2018
215 Views
Preview:
TRANSCRIPT
-
7/30/2019 Hydraulic Valves Sureclean
1/8
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
-
7/30/2019 Hydraulic Valves Sureclean
2/8
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.
-
7/30/2019 Hydraulic Valves Sureclean
3/8
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.
-
7/30/2019 Hydraulic Valves Sureclean
4/8
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.
-
7/30/2019 Hydraulic Valves Sureclean
5/8
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.
-
7/30/2019 Hydraulic Valves Sureclean
6/8
-
7/30/2019 Hydraulic Valves Sureclean
7/8
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
-
7/30/2019 Hydraulic Valves Sureclean
8/8
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.
top related