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Directional control valves are the most widely used . . . and the least understood . . . valves in fluid power circuits. Many people are confused by the schematic symbol representations, and have difficulty understanding the terms ways, positions, and operators. Learning to read schematic drawings is similar to learning a foreign language. To the trained eye, a symbol speaks volumes even when no words are present. This chapter attempts to take away some of the confusion and apparent magic of fluid power schematic drawings, and thus help make designing and maintaining fluid power systems easier.

Directional control valves can only perform three functions:

stop or block fluid flow allow fluid flow, and

change direction of fluid flow.

This seems to simplify a seemingly complex subject, but remember, many valves may combine these functions. This makes them a little more complicated, but still not rocket-science material.

Check valves

At first glance, the valve type shown in Figure 10-1 does not appear to be a directional control valve. However, check valves do allow flow in one direction and block flow the opposite direction. Use a check valve in any line where back flow cannot be tolerated. Also

pilot-operated check valves (discussed in the next section) can be shifted by an external source to allow reverse flow or stop free flow.

Fig. 10-1. Cross-sectional views and symbols for two types of check valves

The cross-sectional views show the standard poppet design used in most check valves. As in most early designs, the symbol still pictures a ball on a seat. Ball check valves work well until they are disassembled for repair or when troubleshooting. As these valves operate, they wear a groove where the ball contacts the seat. If this wear groove is not reinstalled exactly where it was, the valve is no longer leak free.

On the other hand, a guided poppet always goes back in the same relationship to the seat and seals easily after reassembly.

It is easy to understand the function of a check valve. Fluid entering opposite a spring pushes against the poppet and spring to move it out of the way. The inline valve has holes around the angled seat face above the body seat to allow flow to pass. The right-angle design pushes the poppet out of the way and fluid flows by with little restriction.

Check valves are almost trouble-free devices. Seldom is one the cause of a problem. Potential problems can be minimized further if the check valves are: right-angle types, screw-in cartridges, or subplate mounted. Note that an inline check valve’s plumbing must be disassembled before the valve can be checked.

Check valves also can control pressure. Almost all check valves use a spring to return the poppet. In most valves, this spring has very light force, because any spring force results in an energy loss and heat. The light springs from most suppliers require about 5 psi to move the poppet against them (some go as low as 1 psi). Some large check valves, when they are mounted vertically, may require no spring because the weight of the poppet causes it to fall onto its seat.

Strong springs give extra resistance to flow so a check valve could replace a relief valve when low-pressure bypass is required. Many manufacturers have check valves with springs that require as much as 125 psi to push their poppets back. These valves work for low-pressure circuits such as a bypass around a low-low-pressure filter or heat exchanger, or to maintain minimum pilot low-pressure for pilot-operated directional control valves. When the spring functions as a backpressure or relief valve, the symbol usually shows the spring as part of the symbol.

Fig. 10-2. In-line check valve with orifice drilled through

poppet

Another lesser-known use for check valves is as a fixed-orifice flow-control function. Figure 10-2 shows an inline check valve with an orifice drilled through the poppet. The orifice allows free flow in one direction and measured flow the opposite way. The orifice is non-adjustable, so this component is tamper proof. The only way to change actuator speed is to physically change the orifice size. This orificed

check valve could protect an actuator that might run away if a line broke or a valve malfunctioned. It will not affect speed in the opposite direction. For this application it should be flange fitted or hard piped directly to the actuator port.

Pilot-operated check valves

The check valves in Figure 10-3 operate like standard check valves, but can permit reverse flow when required. They are called pilot-to-open check valves because they are normally closed but can be pilot-to-opened for reverse flow by a signal from an external pilot supply.

Fig. 10-3. Three types of pilot-to-open check valves with symbols

The first cutaway view of a pilot-to-open check valve in Figure 10-3 is a standard design using a pilot piston with a stem to unseat the check valve poppet for reverse flow. The pilot piston has an area three to four times that of the poppet seat. This produces enough force to open the poppet against backpressure. Some pilot-operated check valves have area ratios up to 100:1, allowing a very low pilot pressure to open the valve against high backpressure.

The second valve in Figure 10-3 shows a pilot-to-open with decompression function. It has a small, inner decompression poppet that allows low pilot pressure to open a small flow passage to reduce backpressure. After releasing high backpressure, the pilot piston can easily open the main poppet for full flow to tank. (This arrangement does not work when the high backpressure is load-induced or generated by other continuous forces.)

The third valve, pilot-operated with external drain, isolates the stem side of the pilot piston from the in free-flow port backpressure that would resist pilot pressure trying to open the poppet. Notice that in the other two cutaway views, any pressure in the in free-flow port pushes against the pilot piston stem side and resists pilot pressure’s attempt to open the poppet. Backpressure could be from a downstream flow control or counterbalance valve in some circuits.

The external-drain port also can be used to make the pilot piston return when using the valve for a pilot-operated 2-way function.

Fig. 10-4. Typical circuit incorporating pilot-operated check valve

The circuit in Figure 10-4 shows a typical application for pilot-operated check valves. Spool-type directional control valves cannot keep a cylinder from moving from a mid-stroke position for any length of time. All spool valves allow some bypass, so a cylinder with an outside force working against it slowly moves out of position when stopped. Installing pilot-operated check valves in the cylinder lines and

connecting the directional valve’s A and B ports to tank in center position assures that the cylinder will stay where it stops (unless the piston seals leak).

Fig. 10-5. Circuit with pilot-operated check valve that prevents load from running away if pilot pressure is lost

The circuit in Figure 10-5 shows a pilot-operated check valve holding a load on the rod end of a vertically mounted cylinder. Pilot-operated check valves can hold potential runaway loads in place without creep, but this circuit usually has problems on the extend stroke.

This is because a pilot-operated check valve opens the rod end of the cylinder to tank, letting it run away. When the cylinder moves faster than the pump can fill it, pressure in the cap end and pilot pressure to the pilot-operated check valve’s pilot port drops and the valve closes quickly. This can generate high-pressure spikes that may cause pipe and part damage. Almost immediately, pressure to the pilot-operated check valve’s pilot port builds again and the runaway/stop scenario repeats until the cylinder meets resistance or something fails.

The best valve to control runaway loads is the counterbalance valve explained in Chapter 14.

Fig. 10-6. Circuit with vertically mounted cylinder that is unable to extend

Figure 10-6 illustrates another problem with using a pilot-operated check valve to hold back a runaway load: a pilot-operated check valve may not open when signaled to let a cylinder with an oversize rod and heavy load extend. When the directional valve shifts to extend the cylinder, load-induced pressure can hold the pilot-operated check valve poppet closed. It may take 300 to 400 psi to force the poppet open, even with its 3:1 or 4:1 area difference. Pressure builds at the pilot port, but at the same time it increases in the cylinder cap end.

With a 2:1 rod-differential cylinder, it can add 600 to 800 psi to the load-induced pressure. The additional downward force causes pilot pressure to increase, which causes more downward force, which causes more pilot pressure -- until the circuit reaches maximum pressure.

At that point, the relief valve bypasses or the pump compensator kicks in to stop flow. The cylinder simply cannot start to extend . . . and

even if it could, the action would be erratic, as in Figure 10-5.

Pilot-to-close check valves

There is also a pilot-to-close check valve, but it is seldom used. It is rarely necessary to have a valve that always stops flow in one direction and also is capable of stopping it the opposite direction.

Fig. 10-7. Pilot-to-close check valve and symbol

Notice in the cutaway view in Figure 10-7 that the spring-loaded poppet does not have communicating holes through it to the spring chamber. Flow passes freely from inlet to outlet until a pilot signal is fed to the pilot port. Because the pilot port side of the main-flow poppet has more area than the inlet side, this valve can be closed against free flow.

Poppet-type pre-fill valves

Pre-fill valves operate similarly to pilot-operated check valves, but they are usually much larger. Some pre-fill valves can handle flows in excess of 6000 gpm at pressure drops of less than 4 to 8 psi. Their normal function is to fill and exhaust a large bore cylinder as it travels to and from contact with the work piece. Large, high-tonnage presses -- both vertical and horizontal -- use pre-fill valves to reduce pump size while maintaining cycle time.

The cutaway view and symbol in Figure 10-8 show the construction of a typical poppet-type pre-fill valve. A large main-flow poppet seals the path between the tank and the cylinder ports. As the piston advances, vacuum in the void behind it allows atmospheric pressure to push the main-flow poppet open so fluid from the tank can fill this void. On the retraction stroke, a signal to the pilot piston pushes the main-flow poppet open so fluid can return to tank. While a pilot-operated check valve’s pilot piston is larger than the poppet it opens, the main-flow poppet in a pre-fill is much larger in diameter than the pilot piston. Thus it is impossible to open the main-flow poppet against high backpressure. This keeps decompression shock from damaging pipes and components.

Fig. 10-8. Two models of pre-fill valves with symbols

Decompression shock occurs when large volumes of fluid at high pressure are released suddenly. Because all hydraulic oil has some entrained air (bubbles so small they cannot be seen without magnification), there is a 0.5 to 1% compressibility that must be dealt with when using large-bore cylinders. On top of fluid compressibility, the cylinder tube may stretch diametrically and longitudinally. In addition, the framework that is resisting the tonnage produced also can stretch. Summing all these factors, a 50-in. bore cylinder with a 72-in. stroke can contain more than 25 gal of extra fluid at 3000 psi. If this trapped fluid suddenly has a large open path to atmosphere, its velocity at first release is such that it can break fittings, blow hoses, straighten tubes or pipe bends with relative ease. Releasing this

same trapped fluid in a controlled manner over a few seconds dissipates the excess energy and no damage is seen.

The plain pre-fill valve might be used on smaller cylinders or circuits that have other means for decompressing. The pre-fill valve with decompression has a small poppet in the large poppet that is easy to open at high pressure but will not allow the high flow that causes decompression shock. This decompression poppet usually has a means to adjust how fast the cylinder decompresses.

Another pre-fill valve design is the sleeve type that must be externally shifted open and closed. Both designs give the same results even though their operation is different. (See Chapter 4 for a cutaway view and symbol of a sleeve type pre-fill valve.)

Typical decompression circuit

The circuit in Figure 10-9 operates a vertical single-acting hydraulic ram press with pullback cylinders for the retraction stroke. The press has a poppet-type pre-fill and gets a fast stroke from only filling the pullback cylinders during the approach stroke. A sequence valve keeps pump flow from going to the ram until pressure reaches a preset level.

Fig. 10-9. Typical vertical ram circuit with pre-fill valve

During the approach part of the stroke, atmospheric pressure pushes fluid into the large-bore ram through the pre-fill valve because there is vacuum behind the extending ram. When it contacts the work, the ram stops and the pre-fill valve closes. Pressure starts to rise and when it is high enough to open the sequence valve, pump flow goes to the pullback cylinders and the ram. Extension speed slows and tonnage increases to do the work required.

A signal that the work is complete shifts the directional control valve to send pump flow to the rod ends of the pullback cylinders and to the pilot signal of the pre-fill valve. The pre-fill valve’s pilot piston moves forward and contacts the decompression poppet. This lets trapped fluid flow out at a controlled rate. Pressure in the ram drops quickly and smoothly. When pressure is low enough, the pilot piston opens the main poppet to let fluid from the ram return to tank. When the ram loses pressure, the pullback cylinders can raise the platen and push fluid from the ram back to tank.

General directional control valve terminology

Directional control valves are specified generally by the number of ports or ways (lines attached to the symbol’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 manual overrides, 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 10-10 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.

Fig. 10-10. 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.

Fig. 10-11. 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 Figure 10-11. 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

Two 3-way valves are needed to power a double-acting cylinder as shown in Figure 10-11. 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