Other solenoid pilot-operated valve options
The solenoid pilot-operated valve in Figure 10-26 shows some other options that manufacturers offer for special needs. All of these options may not be available from all suppliers, so check with the distributor before specifying a brand.
The spool-position indicator option helps to troubleshoot a circuit. When the pilot-control valve shifts electrically or manually but the actuator will not move, one reason could be that the working spool did not shift. On a valve without the indicator, the only positive way to know if this spool can move is to take the valve apart and check it.
The spool stroke limiter option imitates a simple flow control for different actuator speeds in both directions. This option limits spool travel, which restricts flow to or from the actuator -- similar to a flow control. This option should only be used where speed can fluctuate as pressure and force change.
The pilot-choke option installs a modular meter-out flow control between the pilot-control valve and the working spool to slow spool movement. Slowing the spool can give an actuator smoother acceleration and deceleration, thus reducing shock. The idea is great, but note that slower spool movement may increase cycle times beyond limits.
One thing that causes cycle time to increase is the fact that all solenoid-operated spool valve lands overlap the body lands. This overlap means the spool has to move some distance before fluid flow starts. When the spool moves slowly enough to give good control, the shift time out of overlap can be 0.5 to 1.0 second or greater. After the spool clears overlap, the actuator can accelerate very smoothly, but the extra time often cannot be tolerated.
Another addition to cycle time comes when the spool shifts to the end of its stroke. A spool can continue to move to the end of its stroke even though a partial stroke is passing all available flow. When reversing actuator motion or decelerating before the end of stroke, the spool may be shifted 1/16 in. or more past available flow. When the spool starts slowly moving to center, the actuator continues at full speed until the spool moves far enough to start restricting flow. From this point on, deceleration is very smooth, but time has been lost. Also remember: the speed at which the spool goes to center is the rate for accelerating the actuator in the opposite direction. This means that adjusting for acceleration both ways also affects deceleration in both ways. The spool stroke-limiter option can eliminate the time loss here, but will not help return shifting speed.
Another possible problem with the pilot-choke module is that it often has a flow rating of only 4 to 8 gpm. Adjusting that small amount of flow is very difficult, if not impossible. This problem limits the usefulness of the pilot-choke option. At present, the need for acceleration, deceleration, and flow variation can be handled better by proportional valves, which will be discussed in Chapter 12.
The integral backpressure check valve option was discussed as Option 4 on Figure 10-12.
Hydraulically centered valves
The cutaway view in Figure 10-27 represents a solenoid pilot-operated directional control valve that is hydraulically centered. A few designers prefer hydraulic centering to spring centering. The reasons given are: spring force changes over time, springs may break, response is slower with springs, and springs are relatively weak. Hydraulic centering has none of these faults, but is still specified on less than 2% of all hydraulic circuits. Part of the reason is lack of knowledge of many designers and users.
Notice that in the complete symbol, the pilot-control valve has port T to tank blocked in the center condition, with ports P, A, and B connected. With the pump running, the pilot-control valve sends pilot oil to both ends of the working spool, centering it. The working spool can center because the differential-area sleeves with centering washers can only move until they contact the valve body. With pressure at both ends, these items give a difference in area that causes the working spool to move until it centers. Other than the way the working spool centers, this design valve works the same as other solenoid pilot-operated directional valves.
Figure 10-28 shows all the operators for directional control valves. Prior to 1966, the operator box on the symbol had letter abbreviations for the method of operating the valve written in them. (See Chapter 4, where old operator symbols are shown across from present day symbols.).
When ISO standards were adopted, all writing was eliminated from the picture-like symbols. The abbreviation MAN, for a manual operator, changed to extended lines at the operator box outer end or to a stick drawing of a palm button, hand lever, foot pedal or treadle.
With picture-like drawings there was no language barrier when schematic diagrams went from one country to another. ANSI adopted the new standards (with a few exceptions) and the fluid power industry changed soon thereafter. Old machines with pre-1966 schematics can confuse newcomers, but the drawing usually can be deciphered with a little effort.