Air Over Oil – or, get control of an air cylinder

Air over oil compressed air systems were developed to generate smooth, hesitation free and consistent movement of your air cylinder rods or carriages despite the fact that compressed air is difficult to control when driving the actuator.

In normal use, typically we plumb that compressed air into air lines, through compressed air valves, and ultimately into an air cylinder or air actuator to do the work we want. Air flowing to the air actuator drives action within the cylinder.

The air actuator, be it a rodded, rodless or rotary actuator, converts the energy stored in the compressed air in the compressed air into linear or rotary movement. This is dependent on the type of actuator being driven by the air.

Air moves from high pressure to low pressure incredibly quickly, some say near the speed of sound.

That speed-of-air movement translates into very high speed operation of your air actuators. The piston inside the air cylinder barrel reacts almost instantly to the inrush of compressed air, and since the cylinder rod is attached to the piston, it reacts immediately too. And, then so does the tooling on the end of the piston rod.

Why is air cylinder rod speed a problem?

Where the end-of-rod tooling is designed to impart blunt force, piercing or cutting type of action for example, then high speed impact on the work piece is desired.

However, many air-using applications for actuators require slowing the piston rod and tooling considerably so as not to damage sensitive tooling or the surface of the work piece. Slowing and controlling the speed of the piston rod and tooling of an air cylinder can be accomplished by using flow controls.

Smooth rod movement not always possible with air alone!

Changing conditions in your cylinder load (friction or sideloading for example), perhaps advancing seal wear inside the cylinder, periodic pulsation from the compressor or other variations in the supply air pressure and other factors will contribute to varying cylinder rod speed and smoothness.

It is the variability of compressed air (that air can be compressed at all, and the energy stored in tanks) that prevents an air cylinder from operating with consistent speed and smoothness using only a flow control.

As an air cylinder piston moves, each time anything inhibits the piston, rod or tooling travel – even by a little bit – there will be a momentary hesitation in that travel as the air pressure inside the cylinder fights to overcome that inhibition. Cylinder rod speed and rod-travel timing will, as a result, change continuously, and what is worse, inconsistently.

The use of pneumatic flow controls can do much to reduce the the speed and stroke time variations in the travel of your cylinder rod. Due to the nature of compressed air, flow controls alone cannot ensure that your cylinder stroke and timing will be consistent all the time, stroke after stroke, particularly at slower rod-travel speeds. If you need to have exact rod travel speed, and through that, exact and repetitive tooling speed, just flow controls cannot do that.

Air over oil can provide smooth actuation.

If you need consistent and smooth operation of a linear actuator, then you have a number of alternatives.

You can move from using compressed air entirely and obtain an electric linear actuator to create smooth rod movement and exact positioning. This option has significant cost ramifications as well as requiring operators to have a whole new set of skills.

An all-hydraulic system can impart consistent and smooth movement to the hydraulic cylinder rod movement. This solution, too, has cost issues; the need to acquire a hydraulic power pack, among other accessories being part of those considerations.

A hybrid solution – airoveroil!

This brings us to an excellent, cost competitive solution for imparting consistent, smooth operation to the cylinder stroke; a hybrid solution… an air over oil system.

In the graphic, at the top of the drawing, is depicted 5-ported, 2 position valve schematic. Each one of its two cylinder port lines are shown plumbed into the top of separate oil tanks, marked as A & B on the drawing.

Air Over Oil Tank
Air Over Oil Tank

The air over oil system uses compressed air introduced into the air / oil tanks via a 5/2 air valve. The air from the valve is used to pressurize and drive the oil from each tank into the cylinder.

Flow controls in the lines to the cylinder from the oil tanks (not shown on drawing) will meter the oil as it exits each cylinder port. Since the oil is in-compressible (as far as we are concerned) even though the air that is pushing on the oil may vary in strength and speed of flow, the oil, being metered through the flow control, moves through the flow control consistently. This ensures a smooth, regular, same-speed stroke of the rod and the end-of-rod tooling every cycle, regardless of the fluctuations that occur normally in the compressed air supply.

Air Over Oil Operation

When the air valve is shifted, air flows down one air line to the air/oil tank that line is connected to.

The compressed air first fills the empty space in the top of the air / oil pressure tank, and then, as air pressure builds, exerts force on the oil in that tank.

That oil then flows through the line to the cylinder port, causing the cylinder rod or carriage to extend or retract depending on which port it flows to.

The flow controls, one installed on each of the cylinder lines, will operate by restricting the flow of oil out of the cylinder, thereby dampening the flow of the oil resulting in a smooth, consistent stroking of the cylinder rod.

When the air valve is shifted in the other direction, the air flows down the other valve line to the other air / oil tank, and the cycle repeats.

Each time the valve shifts, the oil being driven into the cylinder from one tank pushes the cylinder piston towards the other end of the cylinder, and that piston drives the oil on the other side back up the line to the other air / oil tank.

A properly installed air over oil system as described will provide the cylinder stroke speed and consistency that you desire for your application.

Depending on the cylinder cycle speed, each time the valve shifts, a minute amount of oil may exhaust with the air. A re-classifier – a device to capture oil mist from the air – should be plumbed to the valve’s exhaust port(s) to strip the exhaust air of oil for re-use or disposal.

Air over oil – cycle it slowly!

Trying to drive an air – oil system at too high a speed could cause the oil to boil in the tank generating a significant amount of air bubbles in the oil, which will affect the systems control of cylinder speed and stroke time. It’s best to run the circuit at the slowest effective and acceptable speed for the application.

Air Over Oil Components

A typical air oil system will consist of these components:

  • an air / hydraulic cylinder of choice
  • two air – oil tanks sized to suit
  • two hydraulic flow controls
  • a two position four or five ported air valve or, two 2 position 3-way valves, one sending air to each air/oil tank
  • necessary lines and fittings to connect the oil tanks to cylinder and valve to the oil tanks
  • sufficient hydraulic oil

Check with the air cylinder vendor to ensure that their cylinder can be used in an air-oil application. Most can. The pressures generated in an air/oil system are usually well within the safety factors for typical pneumatic cylinders.

You can use a hydraulic cylinder of course, but if the pneumatic cylinder works, it will be less costly than an equivalent hydraulic cylinder.

Each of the oil tanks must contain enough oil to fill the cylinder during a complete stroke (an extension or retraction) without completely emptying the tank.

Plumbing An Air Oil System

Each of the cylinder ports is connected to a fitting at the bottom of its own oil tank.

The air valve is connected to the air/oil tanks, one working port to each tank.

The air lines are connected from the valve port to the fitting on the top of each air / oil tank.

The oil tanks must always be installed at a higher level than the cylinders they are supplying.