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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.
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.
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 certain pneumatic 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.
Types of directional control valves
Directional control valve designs generally fall into three categories:
- sliding-plate valves
- poppet valves, and
- spool valves.
Sliding-plate valves use linear or rotary action to open and close ports to change flow paths. Figure 10-14 has a cutaway representation of each type. A linear sliding-plate valve usually is pilot- or solenoid pilot-operated to generate enough force to reliably move the lap-fitted linear sliding plate. As the hollow linear sliding plate passes over openings in the body, fluid is channeled to a working port or to exhaust through the hollowed out cavity or through the body. (Linear sliding-plate valves are used only in pneumatic circuits.)
Rotary sliding-plate valves are often manually operated. Some manufacturers offer a pneumatic or electrically powered rotary actuator for automatic operation as well. These valves are used in pneumatic and hydraulic circuits as control and/or isolation valves -- when high shifting speed is not needed. The seal between the rotary sliding-plate and the body can be lap-fitted or may have spring- and pressure-loaded seals to eliminate leakage.