HYDRAULIC DIRECTIONAL CONTROL VALVE

General directional control valve terminology

Directional control valves are specified generally by the number of ports or ways (lines attached to thesymbol’s box) and the number of positions (boxes or envelopes in the symbol) they have. Other

information about them includes whether they are normally closed (not passing fluid), normally open

(passing fluid), how they are operated (solenoid, manual, or spring) and other features such as manualoverrides, drain ports, pilot ports, etc.

Some general rules for drawing symbols are:only draw flow lines to one box of the symbol

always see that flow paths and direction of flow in each box is compatible

on 4-way hydraulic valves, pipe the A port to the cap end of the cylinder and the B port to the

rod end draw all symbols in their at-rest position. Show valves that are held actuated by a machine

member in their shifted condition, and provide information such as pressure settings, flow rates, orifice sizes, horsepower and rpm where applicable.

(According to this method of specifying, check valves and pre-fill valves would be 2-way valves because they have two ports. However, because these valves are basically single function and have infinitely variable flow paths, their symbols and terminology do not follow general directional control valve rules.) Figure shows the symbol for a 2-way directional control valve and how it could function in a circuit. Notice the symbol has two boxes (or envelopes) to indicate two positions. Each position is a flow path. The box with flow lines coming to it is the normal or at-rest position of the valve. The normal or at-rest position is usually at the spring end of a spring-return valve as seen in the figure.

Circuits in which 2-position, 2-way valves
operate cylinders

The circuit at rest in Figure 10-10 illustrates how a schematic drawing shows the component symbols

for the system builder or troubleshooter. Valves, actuators, flow paths and line connections are all

shown according to the ANSI or ISO graphic symbols that were explained in Chapter 4. To understand how the circuit operates, a person must be able to read the symbols and know how they represent a piece of hardware. The valve in this circuit is 2-way, 2-position, direct solenoid-operated, spring return, normally closed. The diagrams to the right of the circuit at rest show how the directional control valve shifts to its second position and ports fluid to the cylinder. In the real world, this is done in a person’s imagination . . . and can be confusing when several valves are working simultaneously. In the diagram it is easy to see that with the solenoid energized, the normally open box moves in line with the input flow and sends fluid to the cylinder. The arrow in the normally open box shows flow from inlet to cylinder port, causing the piston to extend. If the solenoid is de-energized, the spring returns the valve to the circuit at rest condition and the cylinder stops in its last position.

Two-way valves cannot have more than two positions because they can only stop or allow fluid flow. It is easy to see that a 2-way directional control valve will not operate a single-acting cylinder. These valves are only good for operations that require an on-off supply. As shown in the bottom half of Figure 10-10, two 2-way valves are needed to control a single-acting cylinder. A double-acting cylinder needs four 2-way valves to control it. There are both normally closed and normally open valves in these circuits.

Figure 10-11 shows how 3-way valves can replace 2-way valves and make a machine simpler. This

circuit at rest has a cylinder powered by a 3-way, 2-position, solenoid pilot-operated, spring-return,

normally closed directional control valve. Because this valve has a flow path from the pressure port to the cylinder port and from the cylinder port to atmosphere, it can control a single-acting cylinder. The diagrams to the right show that when the solenoid is energized, the cylinder extends under power. The next schematic diagram shows the cylinder retracting from external forces with the solenoid de-energized

Circuits in which 2- and 3-position,
3-way valves operate cylinders

Two 3-way valves are needed to power a double-acting cylinder as shown in The double-acting

palm button activates this circuit. The valve on the cap end is normally closed and the valve on

the head end is normally open. This is a simple anti-tie down circuit, but is not OSHA safe because one palm button can be depressed before the second one and the cylinder will move. OSHA requires that both buttons be operated concurrently to make the cylinder extend. It does meet the anti-tie down requirement because the cylinder will not retract until both palm buttons are released.

The double-acting inching circuit in Figure 10-11 uses two 3-way, 3-position, spring-centered valves to make it possible to stop the cylinder at any point in its stroke. A 3-way valve can have a third position to perform another function. The pictured center condition has all ports blocked, which stops flow at all ports. This is the center condition normally found on a 3-way valve.Note: pneumatic inching circuits cannot stop and hold a load consistently. Any change in speed, load,

or pressure can produce a different stopping position. About plus or minus one inch would be the best position accuracy an air cylinder would achieve, unless it is moving very slowly. Air leaks in the

plumbing or valves also interfere with trying to stop and hold an intermediate position. Leaks may let

one end of the cylinder bleed off and allow air from the opposite end to expand and move the cylinder out of position.

Using two 3-way valves attached directly to each cylinder port will save air. By eliminating all piping between the valve and the actuator, less air is consumed during each cycle. The air savings per cycle may not be great, but it can add up on fast-cycling equipment with multiple cylinders.

A 3-way valve can be used as a 2-way function when an on-off condition is needed.

The 4-way valve in Figure 10-12 makes it possible to operate a double-acting cylinder with a single

valve. The four ports on hydraulic valves are marked P for pump, T for tank, and A and B for cylinder or outlet ports. Most valve manufacturers follow this universal marking system. Most air valves are configured as 5-way functions with two exhaust ports. This works well for air valves because atmosphere is the tank. Return piping is not required.

Circuits in which 2- and 3-position, 4-way
valves operate cylinders

The circuit at rest in Figure 10-12 shows a 4-way, 2-position, direct solenoid-operated, spring-return

directional control valve. In at-rest condition, pump flow holds the cylinder in the retracted position

while the cap end is ported to tank.In the solenoid-energized, cylinder-extending condition, pump flow connects to the cylinder cap end while the head end is connected to tank. The cylinder is extending under power at this time. In the solenoid-de-energized, cylinder-retracting condition, the valve returns to normal and the cylinder retracts under power.

A single 4-way directional control valve can power an actuator in both directions. At the bottom of

Figure 10-12, a 4-way, 3-position, double direct solenoid-operated, spring-centered, tandem-center

directional control valve powers a double-acting cylinder in the vertical position with its rod up. As

shown in the at-rest condition, pump flow goes to tank and the cylinder ports are blocked. Energizing

the extend solenoid sends pump flow to the cylinder cap end to make it extend. Energizing the retract

solenoid sends pump flow to the cylinder head end, making it retract. With both solenoids

de-energized, the cylinder stops and holds position for some time. Because most directional control

valves use a metal-to-metal fit spool, there is some bypass, so the cylinder might drift when it has

external forces acting on it. Note: this double-acting inching circuit may need a counterbalance valve

to stop it from running away as it retracts.Some manufacturers offer 4-way valves in special 4-position configurations. The fourth position is often a regeneration path to move the cylinder more rapidly at reduced force.

The 5-way valve in Figure 10-13 is found most often in pneumatic circuits. Although most hydraulic

valve designs are 5-ported, the tank ports are connected internally by cored passages so only one

external tank connection is needed. Air valves exhaust to atmosphere so having two exhaust ports is not a problem.

Circuits in which 2- and 3-position,
5-way valves operate cylinders

Notice the speed-control mufflers in these circuits. They reduce exhaust noise and act as meter-out

flow controls. A 5-ported valve, especially if it’s spool type, can offer advantages when piping certainpneumatic circuits.

When this circuit is at rest, air pressure ported to the cylinder’s head end holds the cylinder in its

retracted position. Meanwhile the cap end is exhausted to atmosphere. With the solenoid energized,

cylinder-extending air is ported to the cylinder cap end while the head end exhausts. With the solenoid de-energized, the return spring shifts the valve back to normal and the cylinder retracts under power.

A 5-way spool-type valve also can be piped with dual inlets at different pressures -- to conserve

energy, to smooth stroke times and speed, or to cause a cylinder to stroke at high speed. (See Chapter

13 on Flow Controls and Chapter 17 on Quick-Exhaust Valves for circuits to do these.)

At the bottom of Figure 10-13, the double-acting inching circuit uses a 3-position, 5-way valve with all ports blocked in center condition to cycle a cylinder. Within reasonable limits, the cylinder can be

stopped and held for short periods. (See the note on 3-way valves from Figure 10-11 on the reasons

for poor results in pneumatic inching circuits.) Another center condition for a 5-way air valve is pressure blocked and cylinder ports open to atmosphere. This center condition can be used for mid-stroke stopping of a horizontally mounted cylinder.

Both 4- and 5-way valves can replace 2- and 3-way valves by plugging or not using certain ports to

produce the desired function. This can save money in inventory and time when troubleshooting. Only

one spare valve of a given size takes care of many problems on the floor.
See also
working-of-hydraulc-press
hydraulic-press-pre-fill-valves
hydraulics