Hydraulic Valves 1

Hydraulic Control Valves

Valves can be classified according to their functions to three main types 1- Pressure Control Valves 2- Flow Control Valves 3- Directional Control Valves

Pressure Control Valves It can be used to limit the maximum pressure (a relief valve), to set a back pressure (a counterbalance valve)or to pass a signal when a certain pressure has been reached (sequence valves)

Relief Valves

Setting of the main relief valve pressure It is to be se at 10-20% above the maximum required working pressure. Dual relief valves: Frequently, RVs are required in pairs to relieve the pressure on either side of an actuator.

Hydraulic Valves 2

More explanations of the direct acting and indirect acting (pilot operated) relief valve:

Two stage relief vale or indirect acting relief valve or pilot operated relief valve symbol

Unloader Valves A relief valve can be unloaded in two ways: 1- By pressure signal 2- By venting

Hydraulic Valves 3

Unloading Relief Valve Symbols

Counterbalance Valves (also called load holding valves)

♦ It is basically a relief valve but used in a particular application to set up a back pressure in a circuit

♦ It is used in load lowering applications such as lift trucks, winched and presses. ♦ The usual load pressure setting is 1.3 times the load-induced pressure, i.e., Ps=1.3 Pl

Over –center Valve (Brake Valve) A disadvantage of a counter balance valve is that it reduces the available force. This disadvantage of the counterbalance balance can be overcome by using remote pilot counterbalance valve or brake valve.

Hydraulic Valves 4

Over –center Valve (Brake Valve) and its symbol So the function of the over-center valve is:

1- Hold the load in the neutral position 2- Prevent over-run during lowering 3- Gently brake the actuator to a halt on switching

Pressure Sequence Valve ♦ It is a pressure relief valve used in a circuit to make sure that a hydraulic line does not open

until a certain pressure requirement at another location meets a certain requirement.

♦ An important feature of all seq. valves is a separate drain connection from the spring chamber. This is because, unlike a conventional R.V., a high pressure can occur in the output port during the normal course of operation.

Hydraulic Valves 5

Pressure Reducing Valve ♦ It maintains an output pressure which is lower than the input pressure by venting the

excess flow to tank or limiting the orifice area based on desired and actual output pressure sensing.

♦ Pressure-Reducing Valves may be:

(a) Non-reliving type: They do not limit any pressure increase downstream of the valve set up by external force

(b) Relieving type: This limits the pressure downstream of the valve even which it is increased by an external force.

Hydraulic Valves 6

Flow Control Valves (FCV)

♦ FCV are used to regulate the fluid rate to actuators and so give speed control. ♦ Flow through the control orifice is usually considered to be turbulent and the quantity of a

fluid flowing can be given by:

PxAKQ δ)(= Where Q is flow, A is orifice area, dP is the pressure drop over the orifice and K is a constant which may include the function of the orifice characteristics, velocity of the fluid and the Rynolds Number.

Three specialist forms of flow-control valves are now considered: 1-Decelaration valves 2- Viscosity or temperature –compensated valves 3- Pressure-compensated valves

Hydraulic Valves 7

Deceleration Valve ♦ It is used to control deceleration or acceleration in which the throttle opening is controlled

by a roller or roller lever. ♦ It combines two throttles (main and secondary) and one check valve for reverse flow. The

secondary throttle valve is to provide an adjustable minimum flow when the main throttle is closed.

Viscosity or temperature –compensated valves The simplest way to eliminate the effect of viscosity is to use a sharp-edged orifice, the flow through which is independent of viscosity. Pressure-compensated valves Pressure compensated flow controls must be used when accurate speed control at varying supply or load pressure is required. Compensator valves are used in:

1- Circuit where load is variable and the flow rate should not vary as a function of the load as long as the valve is not saturated.

2- Multiple parallel hydraulic circuits where all circuit share the same pump supply pressure, but the load pressure in each cylinder may be different.

Hydraulic Valves 8

Pressure-compensated valve

1 an adjustable metering orifice, 2 the compensating spool, 3 the compensating spring, 4 the compensating orifice, 5 the damping orifice

Hydraulic Valves 9

Speed control of a cylinder ♦ Meter in: The quantity of oil entering the cylinder is controlled ♦ Meter out: The flow valve is installed in the return line metering the fluid discharge. ♦ Bleed off: The flow control valve is arranged to bypass part of the pump output directly to

tank. Bleed off speed control is best employed when the vast majority of the pump flow is utilized by the cylinder and only a small percentage is by passed.

Bridge Network

It is used for accurate flow control in either direction

Modified Bridge Network to work as a lock valve Applying a pilot signal at point C through a check valve, PC is slightly higher than PA and PB. This will shut off the valves VA and VB

Hydraulic Valves 10

Multiple –speed systems using flow control valves

Hydraulic Valves 11

Directional Control Valves

Check Valves

Types of moving elements of the Directional Control Valves

a) Poppet b) Spool

Poppet DCV

Hydraulic Valves 12

Sliding spool-type DCV

Hydraulic Valves 13

Selection of center position of a DCV

Hydraulic Valves 14

Valve Operation

Types of solenoids ♦ Air gab AC solenoid: Very fast switching time(30 ms) ♦ Air gab DC solenoid: Switching time approximately 60 ms ♦ Oil immersed or wet pin solenoid: Two Stage DCVs

Hydraulic Valves 15

Cartridge Valves These consist of a valve shell which can be mounted in a standard recess in a valve block or manifold. Poppet type cartridge valve The opening and closing movements of the poppet is a cartridge valve are pressure dependent and a function of the forces on these areas. AX=AA+AB

Hydraulic Valves 16

Restrictor cartridge poppets