ValvesValvesValvesValves

This Chapter is devoted to each of the following categories of control valves

1. Direction Control valves

2. Pressure control valves

3. Flow control valves

1. Direction control valves (DCV)

The primary function of a DC valve is to control the direction of a fluid flow in a

hydraulic circuit. There are different types of DC valves & are explained one by one as

follows.

i) One-way valve or Check Valve

The simplest directional valve is the one-way valve or the check valve. It allows

flow only in one direction and prevents a flow in the reverse direction. A light spring

holds the poppet in the closed position in the free flow direction. The fluid pressure

overcomes the spring force and allows the flow through the valve. If a flow is attempted

in the opposite direction, the fluid pressure pushes the poppet in the closed position. Thus

no flow is permitted.

Check valves are available with different spring rates to give particular cracking

pressure. The cracking pressure is that at which the check valve just opens. Ball type

check valves have the least expensive form of construction, but as the ball is not guided

there is a tendency for leakage to occur.

Fig. SN Page no 42

ii) Pilot operated Check Valve

The pilot operated valve is a design of a two-way valve. This type of check valve

allows free flow in one direction, but prevents reverse flow until the pilot pressure is

applied at the pressure port of the valve. When the system pressure through the free flow

port overcomes the spring force of the poppet, the poppet is pushed off & it allows one

way free flow.

The reverse flow is prevented because the fluid pressure pushes the poppet into

the closed position. When desired to permit free reverse flow, a pilot pressure is made

available at the pilot port. The pilot pressure pushes the pilot piston, which in turn pushes

the poppet from its seat. Now port B is connected to port A & the free flow is obtained

in the reverse direction also.

iii) Shuttle Valves

Fig 6.4 33 JJ page 192

Shuttle valves allow two alternate flow sources to be connected to one branch

circuit. They have two inlets (P1& P2) & one outlet (A). Outlet A receives flow from

whichever inlet is at higher pressure. Fig. illustrates the operation of shuttle valve. If the

pressure at P1 is greater than at P2, the ball slides to the right & allows P1 to send flow to

outlet A. If the pressure at P2 is greater than P1, the ball slides to the left & P2 supplies

flow to outlet A.

iv) Sliding spool-type DC valves

a) Two-way DC valve:

The number of ways refers to number of ports in the valve. A two-way spool type DC

valve is shown in figure. A spool valve consists of a cylindrical spool that slides back &

forth inside the valve body either to connect or block flow between the ports. Two ports

are labeled as P & A. P is connected to the pump & A is the out let to the system.

Pag 194-195 JJ

Fig. 6-5 illustrate JJ page 194, the operation of a 2 way normally closed DC valve.

Fig. 6-6 illustrate JJ page 195, the operation of a 2 way normally open DC valve.

b) Three-way Dc valves:

Two-way valves are used to start & stop fluid flow in a particular line. Three-way

valves also either block or allow flow from an inlet to outlet. They also allow the outlet to

flow back to the tank, while a two-way valve dose not. Three-way valve has three ports,

Pressure inlet P, outlet to the system A & return to the tank T.

Fig. 6-11 illustrate JJ page 199, the operation of a 3 way normally closed DC valve.

Fig. 6.12 page 201 JJ illustrates the operation of a 3 way normally open DC valve.

c) Four-way DC valve:

Four way valves are the most commonly used DC valves in hydraulic circuits,

because they are capable of controlling double acting cylinders & bi-directional motors.

The four ports are labeled as P. T, A, B, where P-Pressure port, T- Tank port, A & B are

outlet to the system. In the normal position, pump flow is sent to outlet B and outlet A is

connected to the tank. In the actuated position, pump flow is sent to port A and port B is

connected to the tank.

Fig. 6.17 page 206 JJ illustrates the operation of a 4-way two-position DC valve.

d) Five-way DC valve:

Fig 3.54 JA page 100 illustrates the operation of a 5-way two-position valve.

The vast majority of directional control valves fall into this category. The

construction consists of a valve body with a sliding spool. Reduced sections in the spool

interconnect ports and passages in the valve body as the spool is moved axially. Fig 3.54

shows how the passageways in a five-port valve are connected when the spool is in the

extreme positions. The valve ports are normally designated

P-Pressure port, T- Tank port, A & B are outlet to the system.

v) Solenoid operated DC Valve

Fig 3.60 JA page 105

When the coil is energized, the armature is pulled into the coil by electromagnetic

forces. The armature moves the push rod and hence the spool. With the movement of

spool, switching the flow paths through the valve can be achieved. The coil may be AC or

DC, air gap or oil immersed.

Control circuits for AC solenoid systems are generally less sophisticated and

consequently less expensive than those for DC systems. However great care must be

taken to ensure that two opposing AC solenoids cannot be energized at the same time.

So far DC valves described earlier based on the number of ports / ways. (2-way

DC valve, 3-way DC valve etc.) The DC valves again further classified based on the

number of positions. Valve position does not refer to the location of the valve but rather

to the position of the internal movable parts of the valve & resultant flow conditions

determined by that position. Normally DC valves have 2 or 3 positions. Fig 6.17 page 206

JJ is an example of 4way-2position valves. In this fig.6.17 there are two positions of

spool. It has seen that in a two position valve the normal connection made by the spool

are P to A, B to T in one position & P to B, A to T in other position.

If a valve offers two usable flow combinations or positions it is called a two-

position valve. Some valves have three positions that position is called neutral position. A

three-position valve is basically the same valve but it is possible to select the mid state as

a definite operating position. Many cylinders requires a third DCV position (neutral) in

which the actuator is not subjected to pump pressure. In DC valve with spring return, the

neutral position is defined to be the position to which valve return after actuating force

has been removed.

This valve position (neutral) is used where it is necessary to stop or hold the

hydraulic actuator at some intermediate point. Varieties of center positions (neutral) are

possible in DC valve by suitably designing the spool.

According to the center positions the valves could be termed

i) Closed neutral

ii) Tandem neutral

iii) Float neutral

iv) Open neutral

v) Regenerative neutral

i) Closed neutral

All the ports P, A, B & T are blocked to each other, in the center position. In such

condition the motion of an actuator could be stopped as well as a number of individual

cylinders can operate independently from a single power source. But the major

disadvantage is that pump flow cannot be unloaded to the tank through the valve. When it

is used in a hydraulic circuit, pump flow must go over to the pressure relief valve when it

is not being used to move the cylinder.

ii) Tandem neutral

Here during the center position the ports P & T get connected & A & B ports get

blocked. Therefore, while tandem center valve stops the motion of the cylinder, the pump

flow is unloaded to the tank through T, without passing through pressure relief valve &

thus generating less heat. The application of such design is to permit the pump flow to be

connected to a series of valves for multiple circuitry.

iii) Float neutral

The use of such port condition will allow the same power source & at the same

time will facilitate free motion of each cylinder at start. No pressure will build up in the

cylinder lines when P remains closed. In this position the pressure port is blocked &

outlets are connected to the tank, the motor will float or spin freely when DC valve is in

neutral. This type of valve is used in motor circuits because it allows motor to spin to

stop.

iv) Open neutral

In this valve in the center position all ports P, A, B & T are open to each other.

This condition will allow free movement of an actuator when flow from pump is returned

to the tank at a low pressure. This type of valve prevents heat build up in the system.

v) Regenerative neutral

For all these Fig. AE page 316

Valve Actuation

Dc valves may be actuated by variety of method. Valve actuator is the method of

moving the valve element from one position to another.

Fig 3.59 JA page 105

Notes

DC Valve Symbols Notes- page No. 80,81,82,83

Pilot valve Notes

2. Pressure Control Valves

The PCV performs the following functions.

1) Limiting maximum system pressure as a safety measure pressure relief valve.

2) Regulating/ reducing pressure in certain positions of the circuit- pressure-reducing

valve.

3) Assisting sequential operation of actuators in a circuit with pressure control

sequence valve.

4) Any other pressure related functions by virtue of pressure control

counterbalance.

A pressure control valve can be used to limit the maximum pressure (to set a

backpressure or to pass a signal when certain pressure has been reached). The principal

feature of the most pressure controls is that the hydraulic forces are restricted by a spring.

Direct operated pressure relief valve: Notes-ADK

The function of a relief valve is to set the maximum pressure in a hydraulic

system. Many simple direct acting relief valves employ either a conical poppet or a ball to

seal against a mating valve seat. Being a contact type seal this is more effective at high

pressure. In this poppet type relief valve pressure at port P acts on the exposed surface of

the poopet to apply a force, which is resisted by the spring force. When the pressure at

port P has risen sufficiently to overcome the spring force, the poppet is lifted off its seat

permitting fluid to flow to the tank port T relieving pressure in the system.

JA 59 Fig 3.3

Pilot operated pressure relief valve (Compound pressure relief valve):

Direct type pressure control valve is suitable for pressure up to 300 bar. For higher

pressure & higher flow rate applications this pilot operated pressure relief valve is used.

This valve is also called compound relief valve.

The pilot operated pressure relief valve is a two-stage valve, which gives good

regulation of pressure over a wide range of flow.

Fig. JJ page 246

Fig.7.2 illustrate the working principle of pilot operated pressure relief valve.

If direct operated pressure relief valve is used for controlling high-pressure rate,

bigger spring with high stiffness is to be used. But bigger spring will have spring rate

with its attendant problems & its cross section will be also large requiring more space.

This type of pressure relief valve is not suitable for compact hydraulic system.

During normal operation, the piston is in hydraulic balance. Pressure at the inlet

port which acts under the piston, is the same as the pressure on its top because orifice is

drilled in the piston. So the piston is held to its seat by the light spring (bias spring).

Moreover, the fluid enters through the pilot line & is forced against the poppet

valve, which is pressed to its seat by the strong pilot spring. When the system pressure

exceeds setting of the poppet valve by adjustable screw, the poppet is pushed from its

seat. This permits the escape of the pressurized fluid from the topside to the reservoir

(tank). This reduces the pressure in the upper chamber of the piston. Due to restricted

flow through the orifice, the fluid cannot enter into the upper chamber as rapidly as the

fluid leaves through the drain hole. This causes an increase in pressure in the lower

chamber of the piston. Thus, there is imbalance in the hydraulic forces, which tends to

raise the piston from its seat to permit the flow directly to the tank.

When the pressure falls below the setting pressure the poppet returns to it seat.

Since the pressure in the upper & lower chamber become equal, the piston is returned

back to its seat.

Sequence Valves:

Sequence valves extensively used in hydraulic systems are also pressure control

valves. But unlike a pressure relief valve a pressure sequence valve is used in a hydraulic

system to cause various operations in a sequential order i.e. one after another. For

example a pressure sequence valve used in a clamping & machining circuit may permit

the clamping operation to take place first & when the clamping cylinder is fully extended,

the machining cylinder is actuated.

The sequence valve is to direct flow in a predetermined sequence. The sequence

valve operates on the principle that when system pressure overcomes the spring setting,

the valve spool moves up allowing flow to the secondary port that is connected with the

second operating hydraulic cylinder. It has a built in check valve to allow reverse flow.

Fig. SN page 49

Pressure Reducing Valve:

This type of valve is used to maintain reduced pressure in a specific location of

hydraulic system. It is normally an open valve. It is actuated by down stream pressure &

tends to close as this pressure reaches the valve setting.

A pressure-reducing valve is one, which uses a spring-loaded spool to control the

downstream pressure. If the downstream pressure is below the valve pressure, fluid will

flow freely from the inlet to the outlet. When the outlet pressure increases the valve

setting, the spool moves to partially block the outlet port. If the valve is closed completely

by the spool, it could cause the downstream pressure to buildup above the valve setting.

To avoid this, a drain line is provided to drain the fluid to the tank.

Fig. SN page 53

Counter Balance Valve

The purpose of a counter balance valve is to maintain control of vertical cylinder

to prevent it from descending due to gravity. The primary port of this valve is connected

to the bottom of the cylinder & secondary port is connected to the direction control valve.

The pressure setting of the counterbalance valve is somewhat higher than is necessary to

prevent the cylinder load from falling.

When the pump flow is directed via direction control valve to the top of the

cylinder, the cylinder piston is pushed downward. This causes pressure at the primary

port to increase to raise the spool. This opens a flow path for discharge through the

secondary port to the direction control valve & back to the tank. When raising the

cylinder, an integral check valve opens to allow free flow for retracting the cylinder.

Fig. SN page 51.

Unloading Valve

In case of pressure relief valve, the pump delivers full pump flow at the pressure

relief valve setting & thus operates at maximum horsepower condition. The unloading

valve is used to permit a pump to operate at minimum load & is therefore at minimum

horsepower.

Fig. SN page 48

The In port of the unloading is connected to the line which is to be unloaded. The

pilot port is connected to the line, which is supposed to send the pressure impulse for

unloading the flow. As soon as system pressure reaches the setting pressure, which is

available at the pilot port, it lifts the spool against the spring force. The valve is held open

by pilot pressure & delivery from the pump start going into the reservoir. When the pilot

pressure is released, the spool is moved down by the spring & flow is directed through the

valve into the circuit.

The loading valve is useful to control the amount of flow at any given time in

systems having more than one fixed delivery pump.

3. Flow Control Valves:

Flow control valves are used to regulate the flow of fluid in hydraulic circuits.

Needle valve: Needle valves have a pointed stem that can be adjusted normally to control

accurately the rate of fluid flow through the valve. Needle valves are often made from

steel bar stock and are among the most common hydraulic flow control devices. The

needle valve is also used as a stop valve in hydraulic circuits to shut off the flow of fluid

from one part of a circuit to another part. Needle valve throttles the flow through them.

Globe valve: Globe valves have a round disk to control or stop the fluid flow. The flow

area through a globe valve is larger than through a needle valve. Hence a globe valve will

have larger capacity at a lower pressure drop than a needle valve of the same size. Globe

valves are not suitable for throttling service.

Gate valve: Gate valves are not normally used as flow control valves. Most gate valves

are used as stops, to shut off fluid flow or to open the line to full flow.

Pressure Compensated Flow Control Valve

The pressure compensated flow control valves automatically adjust to pressure

changes & maintains a constant pressure drops from inlet to outlet, thus providing

constant flow.

Fig. 283 JJ

It consists primarily of a main spool & a compensator spool. The adjacent knob

controls the main spools position, which controls the orifice size at the outlet & in turn

flow rate. The pressure upstream of (before) the main spool is ported to left side of the

compensator spool, through the pilot line (A). Pressure down stream of (after) the main

spool is ported to the right side of the compensator spool through pilot line B. The

compensator spring biases the compensator spool to the fully open (left) position. If the

pressure upstream if the main spool increases too much relative to the downstream

pressure (pressure drop becomes too high) the compensator spring will move right against

the force of spring. This acts to keep the pressure drop across the main spool and

consequently nearly constant.