Synchronizing Cylinder Circuits
Some machines with multiple cylinders require that the cylinder strokes be perfectly synchronized for the machine to operate properly. If all the loads, line sizes and lengths, and friction of the cylinders and machine members are identical, they may stroke at the same time and rate. While line sizes and lengths, and machine loading can be controlled to some extent, friction changes constantly. Thus, when cylinders have to stroke together, use some method to synchronize them.
One way of synchronizing cylinders is with external mechanical hardware. Some common mechanisms are racks and pinions, crankshafts, cables and pulleys, and chains and sprockets. The accuracy of these methods depends on the strength of the hardware and the position of the load. Mechanical methods are the most common way to accurately synchronize air cylinders. One advantage of mechanical synchronization is that the cylinders can operate anyplace in the stroke without getting out of phase. The accuracy of mechanical synchronization is about ±0.005 to 0.010 in. -- depending on load variation and strength of the mechanism used.
The most accurate way to synchronize hydraulic cylinders is with servovalves. Servovalves independently control each cylinder with electronic position feedback, and compare each actuator's position with all others. This is the most expensive way to synchronize cylinders but the most accurate. Actuator position within ±0.001 to 0.002 in. of each other is attainable using good servo practices. (This type of synchronizing also works well with cylinders that never go to a home position.)
This chapter deals with ways to synchronize cylinders by using other fluid power components. These circuits show how to arrange the components to hold multiple cylinder positions in close proximity to each other. The simplest circuit uses only flow controls to build resistance to hold the fast cylinder back. The accuracy of flow-control synchronizing is only fair to poor. Some of the more complex ways -- such as using tandem cylinders or a master-slave cylinder arrangement -- hold relative position as low as ±0.010 to 06 in.
To use fluid-power components to synchronize cylinders, all cylinders must come to a positive dead stop at the end of each cycle. Leakage in cylinder seals or valving causes minor position differences after each stroke. When the cylinders all bottom out or meet a positive, level stop, the error of each cycle cannot accumulate. This is the main reason not to use fluid-power synchronizing with cylinders that operate only in mid-stroke.
When testing cylinder synchronization on a machine, always start the circuit with the cylinders detached from the machine. Cycle the cylinders without any load attached. This allows a safe time for air purging and valve adjustment. Any sudden or out-of-control moves will not affect machine members.
Synchronizing with flow controls
The circuit in Figure 22-1 has no controls except the directional valve. If the pipes are all the same relative size and all the same length; if the load is centered; and if friction of all parts is identical, the cylinders might travel exactly together. Some of these variables are controllable, but things like friction may change even during a single cycle. With the setup in Figure 22.1, the cylinders actually move one at a time until they hit end of stroke or bind up mechanically.
With the off-center load shown in Figure 22-2, the cylinder farthest from the load would extend until it stroked out or locked up -- before the opposite cylinder starts to stroke.
Adding meter-out flow controls to each cylinder port, as in Figure 22-3, adds variable resistance for each cylinder. The added resistance may need to be changed throughout the day because of many factors that affect cylinder movement.
Flow-control synchronizing circuits work with air or hydraulic cylinders. For air cylinders, the problem of compressibility increases potential instability. However, without going to a mechanical or hydraulic option like the tandem-cylinder circuit described in Chapter 3, it is the only way to synchronize air cylinders using fluid power alone.
With flow controls, the cylinders stay reasonably synchronized only if load position does not change. If the load moves, cylinder force must change to maintain synchronization. If load position change is infrequent, resetting flow controls is an option.
Even with hydraulics, another problem with uneven loads is what happens if the cylinder does not stroke all the way. If the cylinder stops in mid stroke, as in Figure 22-4, oil from the loaded cylinder can transfer to the opposite cylinder and throw the platen out of synchronization. Figure 22-5 shows pilot-operated check valves added to the cap-end lines to overcome oil transfer during mid-stroke stopping. With these check valves in place, oil cannot transfer when the cylinders stop in mid-stroke, so the cylinders maintain their positions.
Another problem with flow-control synchronization is the maximum lifting force. With two identical cylinders positioned parallel to each other, the platen should be able to lift twice each cylinder's force. However, this is only true if the load is centered. With a double load positioned over one cylinder, that cylinder would stall while the opposite cylinder tries to extend. When using flow-control synchronization, size each cylinder to carry the whole load if the load might get off center.
When controlling hydraulic cylinders, it is best to use pressure-compensated flow controls. Pressure-compensated flow controls maintain a constant flow when load differences cause a change in pressure drop.