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Air over oil compressed air systems was 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. This article will provide you with all the relevant information on hydraulic cylinder

Table of Contents

Air Cylinders

In normal use, typically we plumb that compressed air into airlines, 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 a 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.

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 workpiece 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 workpiece. Slowing and controlling the speed of the piston rod and tooling of an air cylinder can be accomplished by using flow controls.

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 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.

The Air-Over-Oil Solution

If you need the consistent and smooth operation of a linear actuator, then you have a number of alternatives. Because air cylinders produce fast, repeatable motion, achieve high cycling rates, and don’t generate any substantial amount of heat, they are well suited to factory automation systems.

However, the air is compressible – which is an advantage in many applications, but it complicates closed-loop control. A solution that is very often overlooked is the incorporating of a closed hydraulic circuit to help regulate speed and provide holding for the otherwise all-pneumatic system.

Pneumatic technology has many advantages for factory automation. However, the compressibility of the air can make variable-speed control quite difficult, unless you have a very sophisticated closed-loop valve system. Air-over-oil systems can, however, allow you to achieve precise control at a fraction of the cost of their alternatives.

Air-over-oil cylinders, or otherwise known as hydra-pneumatic or hydro-pneumatic cylinders, offer a cost-effective alternative for many closed-loop speed control applications as they combine the convenience of readily available compressed air for power with the hydraulics which controls motion. Unconventional systems yes, but, they are well suited for applications that require low-power systems and offer smooth speed control, rigidity, or synchronized motion where needed.

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 requires 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.

In the graphic below, at the top of the drawing, is depicted a 5-ported, 2 position valve schematic. Each one of its two-cylinder port lines is 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

In contrast to strictly pneumatic or strictly hydraulic cylinders, air-over-oil cylinders rely on the surface area differential of an internal pneumatic piston-rod assembly to significantly increase the pressure of trapped oil above the working position, which will provide an intensified hydraulic cylinder output force.

When the air valve is shifted, air flows down one airline to the air-over-oil tank that the 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 a 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 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-over-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 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.

Initially, they do function quite similarly to double-acting cylinders, extending and retracting with output force typical of pneumatic cylinders. The difference being the second pneumatically controlled cylinder section drives a rod into the oil section, sealing it off and intensifying the internal pressure. The intensified pressure pushes against the working piston and produces an increased output thrust.

Similar to other double-acting pneumatic cylinders, air-over-oil cylinders utilize valves to control their motion. They have one four-way valve which allows the control approach and retraction motion, and another four-way valve that controls oil pressure intensification. These designs simply combine the advantages of pneumatic and hydraulic cylinders without having any disadvantages.

Air-Over-Oil System Construction & Components

Coupling low-pressure hydraulic cylinders with air-over-oil tanks is a very common way to create an air-over-oil system with ease. These tanks are able to hold more than enough oil to stroke a cylinder one way. Oil is forced into the cylinder by having an air valve connected to the air-over-oil tanks. To get a smoother and accurate cylinder control, flow controls and stop valves are added to the oil lines.

A pneumatic pressure booster, or intensifier, can be installed upstream of the compressed air inlet if greater force is needed. Air-over-oil tanks do not intensify oil, no matter what their tank length or diameter may be. The amount of air pressure provided is at the highest possible pressure available.

Another construction is a tandem cylinder system which is used to control oil and air power. Here, the single-rod cylinder of the tandem runs on air and the double-rod cylinder is full of oil. Because volume is equal in both ends of the double-rod cylinder, oil flows from end to end through the flow controls or shut-off valves, allowing accurate speed and stopping control.

Air-Over-Oil Components

Most components required for air-over-

A typical air-over-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.

Air-Over-Oil Circuit Construction Considerations

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 system’s control of cylinder speed and stroke time. It’s best to run the circuit at the slowest effective and acceptable speed for the application.

When designing with air-over-oil systems, you should take care sizing the oil lines. Most air-oil circuits operate at 100psi or less and so any pressure drop in the circuit can substantially reduce the force applied by the cylinder. If you were to undersize the oil lines, the cylinder movement will become very slow. You should size most air-over-oil circuit oil lines for a velocity of about 2 to 4 ft/sec. This low speed will require large lines and valves if average travel speed with maximum force is important!

When plumbing an air-over-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, with one working port to each tank. The airlines 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.

Bleeding air from the oil chambers can also present another common problem with air-over-oil circuits and so consideration must be taken here. Any trapped air in the oil will make the operation of the cylinder become “spongy”. Accurate mid-stroke stopping and smooth speed control then become quite difficult to achieve with this compressibility.

It is typically a good idea to mount the tanks higher than the cylinder they feed when using an air-over-oil system, and all lines between the cylinder and the tanks should slope up toward the tanks. If it’s possible, letting the cylinders make full strokes to purge the air is very beneficial as well as incorporating a means for equalizing the tank level in design when a dual-tank system is designed.

There are so many design variations of air-over-oil circuits that I will leave you with these pointers for the various designs:

  • Single tank air-over-oil systems will retract at full speed, shifting the valve routes hydraulic fluid through the check valve and orifice to provide speed control during its extension
  • The double-tank air-over-oil system provides speed control for both extension and retraction
  • Using tandem cylinders provides a simpler means of speed control in both directions but requires more space for the actuator because the tandem is larger than a standard double-acting cylinder
  • An unmatched tandem cylinder can be smaller and less expensive than a standard tandem cylinder but may undergo pressure flow intensification due to the different cylinder volumes
  • Two or more tandem cylinders can be a convenient way of achieving the synchronized motion. It’s important to fully extend or retract the cylinders after each use to prevent an accumulated positioning error
  • Opposed-mounted tandem cylinders extend equally and their piston rods meet midway which will provide a simple solution for centering different sized workpieces

Note: You should never fill a tank with oil to capacity when the cylinder is stroked away from it. Why? The oil will overflow in the tank from excess fluid in the cylinder returning to the tank and cause you some hefty problems.

FAQs (Frequently Asked Questions)

How does an air-over-oil cylinder work?

In air-over-oil cylinders, when the air valve is shifted, the air is allowed to flow down one airline to the air-over-oil tank. The compressed air first fills the empty space in the top of the air/oil pressure tank, and then, as air pressure builds, exerts a force on the oil in that tank. That oil then starts to flow through the lines to the cylinder port, causing the cylinder rod or carriage to extend or retract.

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

What is the purpose of air-over-oil intensifier circuit?

An air-over-oil intensifier circuit can help convert low-pressure into high pressure in instances where a hydraulic pressure system is not available or pressure are low. Intensifiers allow you to gain more precise speed control and positioning of the cylinders while remaining compact, reliable, and certainly cost-effective.

Will a hydraulic cylinder work with air?

You can, but it really is not advised. A hydraulic cylinder with air won’t work nearly as well as a pneumatic cylinder for a number of reasons. First of all, hydraulic cylinders are closed systems and have seals designed for high pressure and as close to zero leakage as possible. Therefore, the seals will undergo extra friction which may even lead to a far shorter lifespan. Because you cannot lubricate the hydraulic cylinders properly, the cylinder is extremely likely to jam and the sealing will become damaged.

What is air circulation in oil tank is provided?

Air valves are piped to the air-oil tanks to introduce compressed air which forces oil from the tanks and into the cylinders. This air circulation along with flow controls and shut off valves allow for the oil lines to provide a smooth and accurate control of the cylinder.

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