Figures 22-15 through 18 illustrate a common way of synchronizing cylinders. Many designers use this circuit and consider it to be one of the best ways to synchronize cylinders. It is reasonably accurate, but may allow the cylinders to get out of phase in certain conditions.

Figure 22-15
Figure 22-15. Double-pump-and-valve synchronizing circuit -- at rest with pump running.

The two pumps in Figure 22-15 have identical flow. They are attached to two double-solenoid, spring-centered valves that are piped to two matching cylinders. Both pumps have a relief valve set at the same maximum pressure. Because both pumps have the same flow and both cylinders use the same volume, the cylinders will stroke at approximately the same rate.

The cylinders are shown extending in Figure 22-16. Energizing solenoids A1 and A2 on the directional valves simultaneously causes the cylinders to extend at the same rate. If one cylinder's load needs more pressure, the pump for that side continues to feed nearly the same flow until the relief valve dumps.

Figure 22-16

Figure 22-16. Double-pump-and-valve synchronizing circuit. Solenoids A1 and A2 energized, cylinders extending.

To retract the cylinders, energize solenoids B1 and B2 on both directional valves simultaneously, as in Figure 22-17. The cylinders retract at the same rate.

Figure 22-17
Figure 22-17. Double-pump-and-valve synchronizing circuit. Solenoids B1 and B2 energized, cylinders retracting.

Should the cylinders get out of phase, Figure 22-18 shows how they re-synchronize. Because a separate pump and valve control each cylinder, separate limit switches drop out the retract solenoids after the cylinders reach home. This leveling happens automatically during each cycle, so position errors do not accumulate.

Figure 22-18

Figure 22-18. Double-pump-and-valve synchronizing circuit. Solenoid B1 energized, cylinder (A) leveling.

A major problem with this synchronizing circuit is the difficulty of finding two identical pumps. Even pumps manufactured at the same time often have slightly different flows. Any flow variation of the pumps lets the cylinders get out of phase. Another problem is efficiency. As pressure climbs, pump efficiency allows more slip oil, valves leak more, and some cylinder seals bypass more. All of these losses add up to poor performance especially if the cylinders have long strokes.

On top of that, what happens if one solenoid is sluggish or fails to operate? This makes one cylinder start late or not start at all. Starting late causes the cylinders to be out of phase; not starting at all may damage the machine.

This circuit has the same force problem as a flow-control synchronizing circuit. Each cylinder has to be able to lift the entire load. If the load on this circuit gets too heavy for one cylinder, its pump dumps across the relief valve and the cylinder stops. Again the other cylinder continues extending until it damages itself or the machine.

Double-pump-and-valve synchronizing circuit improvement
The circuit changes shown in Figure 22-19 overcome most of the problems mentioned about Figures 22.-5 through 18. Instead of two cylinders as before, use two or more pairs of cylinders. Connect half of the cylinders to each pump/valve combination. Pipe port A of directional valve (E) to the caps of cylinders (A) and (C). Hook port B of directional valve (E) to the rod ports of cylinders (B) and (D). Pipe port A of directional valve (F) to the cap end of cylinders (B) and (D) with its B port hooked to the rod ports of cylinders (A) and (C). Piping the circuit this way uses one pump and valve to extend two cylinders, while this same valve retracts the cylinders extended by the other pump and valve.

Figure 22-19

Figure 22-19. Modified double-pump-and-valve synchronizing circuit. Solenoid A1 on left-hand valve shifted to show condition if solenoid A2 is sluggish or fails to shift.

Should a solenoid fail, as in Figure 22-19, the platen will not move because, while cylinders (A) and (C) may be trying to extend, oil from their rod end ports cannot get back to tank through valve (F). Also, blocked inlet flow to cylinders (B) and (D) at valve (F) prevents them from stroking -- although leakage past the spool in valve (F) may allow minor movement.

After both directional valves shift and the cylinders are stroking as in Figure 22-20, the pairs of cylinders try to stay level. If pump (G) produces higher flow, cylinders (A) and (C) try to run ahead. Because cylinder (B) is between them, it will either hold the other cylinders back or be dragged along by them. The platen must be strong enough to transmit this differential cylinder loading without flexing.

Figure 22-20

Figure 22-20. Modified double-pump-and-valve synchronizing circuit. Solenoids A1 and A2 energized, cylinders extending.

This circuit is less load-sensitive because the load is always over a pair of cylinders operated by different pumps. Both pumps will relieve to tank before the load stops moving. However, the lightly loaded cylinders can move ahead in relation to the stiffness of the platen and the distance between cylinders.

Use only one limit switch for this cylinder arrangement. To re-phase the cylinders, shift both directional valves to send the cylinders to home position. One relief valve bypasses fluid until the lagging cylinders reach the positive stop.