Air-oil systems

Compressed air is suitable for many low-power systems, but air’s compressibility makes it difficult to control actuators smoothly and accurately. Some low-power systems need the smooth control, rigidity, or synchronization capabilities normally associated with oil hydraulics. All of these features are available to low-power circuits by using compressed air for power and oil for control. Purchased or specially built air-oil circuits give smooth control when the power requirement is low.

Attached oil-control cylinders

Some manufacturers offer attached oil-filled cylinders to control speed and/or position, Figure 17-1. These units usually work in one direction of travel in a meter-out circuit. They operate such things as drill feeds or other actions that may try to pull the cylinder out. (They also can be used with hydraulic cylinders at higher forces.)

Most manufacturers offer units with valves in the oil line that can stop flow and/or bypass the speed control. The stop control allows an air cylinder to be stopped reasonably accurately with very good repeatability. The bypass control makes it possible to have fast and controlled speeds as the cylinder advances.

The cross-sectional view in Figure 17-1 shows an air cylinder that advances rapidly with airflow controls until its attaching bracket comes in contact with the fast-advance stroke-length adjustment. At this point, air cylinder movement is retarded and controlled by the oil speed-control cylinder as oil flows through a flow control. The air cylinder cannot move any faster than the oil flow allows during this part of the cycle. A spring-loaded oil balance cylinder furnishes oil to make up for the differential loss from rod to cap ends. The air cylinder is controlled by oil flow for the remainder of the cycle.

As the air cylinder retracts and the attaching bracket contacts the rod nut, it pushes the oil speed-control cylinder back to the start position. A flapper-type 1-way check valve on the piston with through holes allows fluid to transfer back to the rod end. Excess cap end fluid is stored in the spring-loaded oil balance cylinder during this part of the cycle.

Some manufacturers offer attached units that are capable of control in both directions of travel. There also are self-contained air powered cylinders with built in oil cylinders and reservoirs. Air produces thrust while oil controls speed and/or mid-stroke stop-and-hold. Some units also have two-speed capabilities. These units look like a standard cylinder with an oversize rod.

Air-oil tank systems

Another common air-oil system uses low-pressure hydraulic cylinders coupled with air-oil tanks, Figure 17-2. These tanks hold more than enough oil to stroke the cylinder one way. An air valve piped to the air-oil tanks introduces compressed air to force oil from the tanks into the cylinder. Add flow controls and shut-off valves to the oil lines to give smooth, accurate cylinder control.

When control is only necessary in one direction, the tank on the uncontrolled side can be omitted. This type of circuit requires very good cylinder seals to prevent air or oil transfer.

Air-over-oil tanks do not intensify the oil pressure, regardless of the tank’s diameter or length. The highest possible oil pressure available simply equals the air pressure supplied.

Several cylinder suppliers offer air-oil tanks that consist of a cylinder tube with two cylinder end caps held on the tube with tie-rods. A sight glass can be a length of plastic tubing with air-line fittings attached opposite the air ports. A baffle at the air port keeps oil from being aerated when air blasts in from the valve. A baffle at the oil port keeps any vortex formed from sending air to the cylinder. This baffle also keeps returning fluid from blowing into the air port.