Practical Automation: Using air-over-oil circuits for more controllable pneumatic power

The air-over-oil system offers some unique and economical benefits, but it can exhaust oil vapour in the workplace. There are a few tricks which can be used to make things work better.

The past few Practical Automation columns have covered various aspects of pneumatic power circuitry used to control the position and speed of cylinder moment. The common element which created problems was the compressibility or sponginess of the compressed air. One method of overcoming these problems is to use an air-over-oil system.

If compressed air, at 100 psi, is applied to the top of an enclosed tank containing oil (or any other liquid), the same pressure (100 psi), will be transferred to any oil line leading from the bottom of the tank. When this oil line is used to power a cylinder (Fig. 1), a number of advantages can be realized.

1. A smooth non-jerky movement of the cylinder is possible using a simple flow control, especially at low speeds.

2. The cylinder can be stopped anywhere in mid-stroke with a fair degree of precision.

3. Lunging or erratic movement of the cylinder can be minimized (for example, at the breakthrough point in a drilling operation).

The reason for these benefits, of course, is that oil is relatively non-compressible. However, they do not come without paying a price. Oil, while being non-compressible, is also far more viscous or thicker than air. It takes a lot more pressure to push it through the lines. It is not uncommon in some high pressure hydraulic circuits to see a pressure drop of 100 psi across the valve alone.

It is very apparent, then, that air-over-oil systems, which normally operate at a total pressure of 100 psi, should operate at a relatively slow speed. As well, it is necessary to use lines and valves sized one or two sizes over what is normally used in an air circuit.

Another problem which must be mentioned, although I can hear the screams of protest from the dyed-in-the-wool hydraulics people as I write this, is that oil systems are messy. This is especially true of air-over-oil systems, which tend to transfer a certain amount of oil to the exhausting air as a mist or oil vapour.

If you can live with this problem, however, the air-over-oil system does offer some unique and economical benefits. There are also a few tricks which can be used to make things work better.

Single tank

The single-tank system (Fig. 2) uses oil on one side of the cylinder piston only. The directional control valve controls the air flow into the tank and the flow control valve is put in the oil line leading to the cylinder.

It is important that the oil tank be mounted at a higher level than the cylinder and flow control so that any bubbles or air inclusions will rise to the top of the oil tank and the system will not require bleeding. The piston seals in this system must be as leak-free as possible to ensure positive separation between the air and oil sides of the piston.

Speed control is generally in one direction only. The big advantage of this system is that the frictional losses due to the oil viscosity are half that of the double-tank system.

Double tank

The double-tank system (Fig. 3) uses oil on both sides of the cylinder piston and provides speed control in both directions. Piston seal leakage is not as critical as with the single-tank system, but as mentioned, the frictional losses are a lot higher.

It is a good idea to install a balancing line with a shut-off valve between the two tanks. This will allow oil to be transferred from one tank to the other to make up for leakage.

The double-tank system can also be used to accurately stop the cylinder travel in mid-stroke by using a two-way shut-off or blocking valve in both of the oil lines leading from the cylinder. Both of these valves are closed at the same time to stop the cylinder.

Turbulence in the tanks and the resulting frothing of the oil are major contributors to oil carryover into the exhausting air. Several modifications can be made to the air/oil tank to reduce this problem. A baffle arrangement, such as a slotted type canister muffler, can be mounted inside the tank in the top air-entry port to reduce the chance of oil carryover into the air system (Fig. 4). A similar device should also be mounted in the bottom oil port to disperse the oil stream and reduce turbulence in the tank.

Oil tanks should be about 50 per cent larger than the displaced volume of the cylinder they are controlling. They are easily made from a length of large-diameter pipe threaded on both ends to accept standard end caps. A method of checking the fluid level should also be included in the construction, such as a sight gauge or a threaded and plugged dipstick hole.

Use thin oil

It is important that the oil used be very thin (SAE 10 or less) and that it have anti-foaming and anti-oxidation additives.

Regardless of the precautions taken, there will always be an excess of oil or oil vapours exhausting from the air valves. This oil should be removed from the exhausting air for health reasons as well as for cleanliness. Piping both exhausts through an oil reclaimer or standard air filter fitted with a three-micron or finer filter element should do the trick.

The slowest possible operating speed will always improve the cleanliness and the general performance of any air-over-oil system.

Ted Grove is a widely experienced fluid power trainer and is corporate training manager of Wainbee Limited of Mississauga, Ont. He can be reached at tgrove@wainbee.com. Previous columns can be viewed at www.mro-esource.com.