Hydraulic 4-way spool valves
Most manufacturers of hydraulic valves only build a basic 4-way function. When they offer a 2-way function, it is usually a 4-way valve with a different spool and the unused ports plugged or piped to tank. Any 4-way valve can perform 2- or 3-way functions in a normally closed or normally open configuration by using the right ports and plugging or draining unused ones.
Hydraulic 4-way valves usually come in 2- or 3-position configurations. They may be 2-position, single-solenoid, spring-return; double-solenoid, detented; or 3-position, double-solenoid, spring-centered. Some manufacturers offer 4-position valves with a float or regeneration center position for special circuits, but they are rare.
The majority of hydraulic circuits use a 4-way, 3-position directional valve even though it complicates the electrical circuit. One reason may be to provide the ability to stop an actuator in mid cycle -- either for manufacturing or setup functions. Other reasons are to port pump flow to tank while the machine is idling or to let external forces move an actuator.
Figures 10-18 through 10-21 show typical circuits in schematic form with valve cutaways for the four commonly used center conditions in hydraulic 4-way directional control valves. (Symbols for other spool center conditions are shown in Chapter 4.) Each center condition offers a flow path to meet a specific circuit need that should be obvious when reading a schematic. Note that these typical circuits are not the only way to apply these valves.
The circuit in Figure 10-18 uses an all-ports-open center-condition valve that allows flow to and from all ports. Notice how the spool lands are too narrow to block the fluid ports. This means fluid is free to go to other ports while the spool is centered. The symbol for the valve plainly shows this open-center condition. A circuit with this type of valve normally has a fixed-volume pump. The open center lets all pump flow return to tank at little or no backpressure. This saves energy and reduces heat to the point that a heat exchanger is not necessary on most circuits.
The cylinder in Figure 10-18 is free to float when acted on by outside forces. Otherwise it sits still. Normally this circuit only has one valve and actuator. Other valves and actuators trying to use this pump would not receive fluid due to the free path to tank.
The valve in Figure 10-19a has an all-ports-closed center-condition that blocks pump flow. This valve appears to be able to stop and hold a cylinder in place. Notice how the spool lands are wide enough to completely cover the A and B ports. This blocks flow to and from them, and also stops flow at the P port. This circuit normally has a pressure-compensated pump. System pressure is at the pump compensator setting until all pump flow is going to the actuators at their working pressures. The pump in a closed-center circuit can supply other circuits one at a time or simultaneously with low to medium energy loss -- even when operating at less than maximum flow.
Because all metal-to-metal fit valves have some spool bypass, a closed-center valve will not stop and hold a single-rod cylinder except for a short period. Figure 10-19b shows how bypass fluid at the spool lands leaks directly into the A and B ports and pressurizes both ends of the cylinder at roughly half system pressure. Equal pressure at both ends of a single-rod cylinder always causes it to extend due to unequal forces on unequal areas. The cylinder will not move rapidly because some fluid must go to tank across another leak path. This cylinder action is called regeneration, and will be explained under cylinders in Chapter 15.
In a new circuit, bypassing fluid may not affect the cycle, but it can cause problems later on. Also, cylinders with small rods and/or heavy loads may not have enough force to move -- especially when machine fits are new and tight. The actual force in this regeneration mode is calculated by multiplying the rod area by pressure at the cylinder cap end.
The circuit in Figure 10-20 shows a float-center valve. The P port to the pump is blocked, and ports A, B, and T are interconnected so that both cylinder ports are open to tank. Notice that the spool lands are wide enough to block the P port but still allow flow to or from A and B ports flow to or from each other or tank. A pressure-compensated pump normally supplies a circuit with this valve. System pressure is the pump compensator setting until all pump flow is going to the actuators at their working pressures. The pump in a float-center circuit can supply other circuits one at a time or simultaneously with low to medium energy loss, even when operating at less than maximum flow.
When a cylinder in a multi-actuator circuit must be positively locked in place, select a valve with a float-center condition, and add pilot-operated check valves or a counterbalance valve. (Adding these blocking valves to an all-ports-closed-center directional control valve often does not prevent cylinder movement.)
With a float-center valve a single-rod cylinder may extend at any speed when the circuit has high tank backpressure. High tank backpressure goes to both ends of the cylinder and causes it to extend if the load is low and/or the cylinder has an oversize rod. To overcome this situation, install a check valve at the T port to stop back flow from the tank line.
On horizontally mounted cylinders, use pilot-operated check valves in the cylinder lines to positively lock the cylinder from moving due to external forces. On vertically mounted cylinders, use a counterbalance valve to hold the load and keep it from running away while cycling. (Counterbalance valves are explained in Chapter 14.)
The circuit in Figure 10-21 uses a valve center condition with the A and B ports blocked, and the P connected to T. This valve lets pump flow go to tank and blocks both cylinder ports. This configuration is often referred to as a tandem-center valve. Notice that the spool lands are wide enough to block the A, B, and P ports – the same as an all-ports-closed valve. However, this valve has a hollow spool and cross-drilled ports at P and both ends at T. The drilled passages provide a path for all pump flow to go directly to tank in the center position. Because the drilled passages also introduce extra backpressure, most suppliers’ catalogs show a lower nominal flow or higher pressure drop for tandem-center valves.
Circuits with tandem-center valves normally have fixed-volume pumps. The pump-to-tank path lets all flow return to tank at little or no backpressure. This saves energy and reduces heat to the point that a heat exchanger is unnecessary in most circuits. Be aware of the reduced flow or higher backpressure when specifying or using tandem-center valves. A circuit may look good on paper, but can run hot because of wasted energy. This is especially true when using tandem-center valves in series. The backpressure for each valve is additive. A three-valve circuit can easily require more than 300 psi to unload the pump.
Figure 10-22 presents symbols and cutaway views for standard 2-position valves. Notice that the double-solenoid detented valve has the same outward appearance as a 3-position valve. The only way to tell the difference in these two configurations is to know the part number designation or to probe the manual overrides. A 3-position valve shifts easily and follows the probe as it is withdrawn. A detented valve shifts hard when breaking out of the detent notches but stays shifted when the probe is withdrawn.
Most 2-position valves are found in circuits with pressure-compensated pumps because they block flow when an actuator stalls. When actuators need pressure continuously, a 2-position valve simplifies the electrical control circuit. A momentary signal shifts a double-solenoid detented valve to its other position and the valve stays there until it receives the opposite signal. A single-solenoid spring-return valve must have a maintained electrical signal to stay shifted. This solenoid setup causes all actuators to return to home position at the loss of control circuit power or after an emergency-stop signal.
Single-solenoid spring-return or double-solenoid detented valves are commonly used in air circuits because pressure is always available and blocking flow does not cause overheating.