Tandem cylinders are another approach to using oil for control and air for power. In Figure 17-3, the single-rod cylinder of the tandem runs on air, while the double-rod cylinder is filled with oil. Because volume is equal in both ends of the double-rod cylinder, oil flows from end to end through a flow control and/or shut-off or skip valves for accurate control of speed and stopping.

Two flow controls in opposite directions provide variable speed in both directions. A bypass flow control around the stop valve would allow for two-speed operation in one direction. (The second speed must be the slower of the two.)

The skip valve option allows a fast approach with deceleration before work contact. The deceleration signal would come from a limit switch or limit valve.

The schematic drawing in Figure 17-4 shows tandem cylinders in a synchronizing circuit. This is a practical way to make two or more air-powered cylinders move in unison. (Using flow controls to do this produces inaccurate results.) When the air valve shifts to extend the cylinders they must move at the same time. This is because the trapped hydraulic oil in the hydraulic cylinders must transfer from the top side of one cylinder to the bottom side of the other one. If one cylinder stops they both must stop at the same time.

Note that the maximum load capability is equal to the capacity of both cylinders’ thrust. With the load placed as shown, the left cylinder transfers energy to the right cylinder through the oil. This gives the right cylinder up to twice as much thrust.

A small make-up tank and check valves replenish any leakage in the plumbing or at the rod seal. If the unit is subject to heating, a small relief valve may be required to keep thermal expansion from over-pressuring the oil-filled chambers. A shut-off valve connecting the transfer lines can re-synchronize the cylinders if the piston seals allow fluid to bypass and the platen gets out of level. Re-synchronization can be handled automatically with a normally closed, 2-way spool valve and limit switches.

(For other air-oil circuits, see the author’s upcoming e-book, "Fluid Power Circuits Explained.")

Some precautions with air-oil circuits

Most air-oil circuits operate at 100 psi or less, so any pressure drop in the circuit can cut force drastically. If oil lines are undersized, cylinder movement will be very slow. Size most air-oil circuit oil lines for a velocity of about 2 to 4 fps. This low speed requires large lines and valves, but is necessary if average travel speed with maximum force is important.

Another common problem with air-oil circuits is that any air trapped in the oil makes the cylinder performance spongy. The air’s compressibility makes accurate mid-stroke stopping and smooth speed control hard to attain. Some arrangement should be provided to bleed any trapped air from the oil chambers. When using an air-oil tank system, it is best to mount the tanks higher than the cylinder they feed. All lines between the cylinder and the tanks should slope up to the tanks. Also, if possible, let the cylinders make full strokes to purge any air. With dual oil-tank systems, incorporate a means for equalizing tank levels into the design.

The cylinder seals must be as leak free and low friction as possible. Any leakage past the seals can cause tank overflow, oil misting, and loss of control.