Figure 3-3 shows a tandem cylinder piped for speed control and mid-stroke stopping. A tandem cylinder unit provides the same results as an air-over-oil tank system. An air valve piped to the single-rod cylinder gives directional control. Install meter-out flow controls and a shut-off valve in the line between ports on the double-rod cylinder. The double-rod end cylinder is completely full of oil and sealed against leakage. A small makeup tank attached to the transfer line, with a check valve between it and the cylinder port, allows oil to enter the cylinder when needed.
This tandem cylinder arrangement operates smoothly and gives accurate mid-stroke stopping control. A tandem cylinder is about the same price as a dual tank system. However, space requirement at the workstation more than doubles. Also, long-stroke tandems can be hard to mount and maintain. Normal piston seal leakage is no problem, but rod seal leaks can let oil out and/or air in, causing a house keeping problem and spongy cylinder movement.
Figure 3-4 shows a large-bore, high-force air cylinder with a small-diameter hydraulic cylinder in tandem with it. An unmatched tandem cylinder has ample oil for smooth control and/or precise positioning. Unmatched tandems offer two big advantages: less cost and lower oil flow.
When specifying unmatched tandem cylinders, make sure the piston rod sizes are compatible between the air and oil cylinders. Rod size usually dictates the smallest bore hydraulic cylinder possible. Also, check for excessive pressure in the oil cylinder that could damage it and flow-control or stop valves. The higher oil pressure comes from intensification due to different bore sizes. Also, a vertically mounted cylinder with a heavy load adds load-induced pressure to intensification, possibly damaging components rated for air or low-pressure hydraulic service.
Figure 3-5 shows an accurate way of synchronizing two or more air cylinders with fluid power. Four tandem cylinders are hooked to one platen, which will stay level throughout the stroke. Force from all four cylinders is available to move the load, regardless of its position.
Notice the transfer valves between the tandem cylinder’s cross port lines. Always return a fluid-type synchronizing circuit to a positive home position often, preferably after every cycle. When the air cylinders return the platen to home position, the transfer valves open to allow the cylinders to resynchronize. Doing this after each cycle eliminates any cumulative position error from seal leakage.
When using unmatched tandem cylinders in a synchronizing circuit, check for overpressure from intensification and load-induced pressure as mentioned above. Placing a heavy load over one cylinder provides pressure intensification from the other actuators. All other cylinders will transfer their excess energy through the oil lines to the loaded cylinder. The loaded cylinder would then be capable of pushing with four times its normal force. One-fourth of this force would be from its own air cylinder, while three times more force comes from energy transfer. This could overpressure the cylinders or valves, causing failure.
Figure 3-6 shows opposing tandem cylinders. These cylinders will meet exactly in center. Drilling a center hole in odd sized parts is one possible use for this circuit.