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Tandem-cylinder synchronizing circuit
Figure 22.-3 shows another very accurate way to synchronize cylinders. The tandem cylinders in this circuit must meet in center even when they run into unequal forces.
Tandem cylinders consist of two cylinders in one housing. They have four ports and the back cylinder is single-rod end while the front cylinder is double-rod end. Because the front cylinder is double-rod end, it has equal areas and volumes on both sides of the piston.
Notice the 4-way directional valve supplies the single-rod cylinders in a conventional manner. The double-rod cylinders have the front port of the left cylinder connected to the back port of the right cylinder and the front port of the right cylinder connected to the back port of the left cylinder.
The tandem cylinders move in unison and transfer energy because hydraulic flow ties them together. If either cylinder stalls, both cylinders stop. Before the cylinders stop, energy transfers through the tandem cylinders and tries to force the lagging cylinder to do its work. The lagging cylinder may see as much as double force before stalling.
The two check valves (C), fed from a 75-psi backpressure check valve in the tank line, allow makeup oil into the trapped volume of the tandem cylinders. The pump makes up for leakage in the trapped volume through check valves (C). Makeup pressure is equal on both sides of both cylinders so the 75 psi has no effect on them. Always furnish bleed ports at both ends of the tandem cylinders to purge any trapped air.
The 2-way, normally closed directional valve (D) between the tandem cylinder connecting lines opens to level the cylinders at one end of the stroke. Leakage at the cylinder piston seals may allow the cylinders to get out of phase. Valve (D) opens when limit switches (E) and(F) do not make simultaneously as the cylinders retract. When one limit makes first, valve (D) opens and allows fluid transfer from one end of the double-rod end cylinder to the other, until both limits make.
In Figure 22-34, solenoid A1 of the 4-way directional valve is energized and the cylinders extend. As they extend, oil transfer in the tandem cylinders maintains near perfect synchronization. If either cylinder tries to lag, power transfers hydraulically through the tandem cylinder lines to keep them in unison. When the load is too great for both cylinders, they stall.
Figure 22-35 shows the cylinders retracting. Energizing solenoid B1 of the 4-way directional valve sends fluid to the rod ends of the single-rod cylinders. As the cylinders retract, the double-rod end cylinders cross piping keeps the machine in synchronization, the same as when they extend.
When the cylinders get close to home, they level or re-phase when necessary, as pictured in Figure 22-36. Limit switches (E) and (F) both have to make to center the 4-way directional valve. If one limit switch makes early, solenoid C1 of 2-way directional valve (D)energizes, allowing the lagging cylinder to transfer oil until it makes its limit switch.
This synchronizing circuit works equally well with air as the power source to the single-rod end cylinders. Use oil in the tandem cylinders because it does not compress. Oversize the oil flow lines for a velocity of 2 to 4 fps to maintain a reasonable speed. Install a makeup oil tank with check valves to feed the tandem cylinders when necessary.