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Air over oil cylinders are pneumatic cylinders designed to produce greater force than standard pneumatic cylinders of the same bore size. They incorporate both oil and air within the mechanism, creating a hybrid pneumatic-hydraulic cylinder that intensifies the amount of force generated by compressed air input.Purchased or special built air-over-oil circuits provide smooth control when power requirement is low. Some manufacturers make self-contained air-powered cylinders with built-in oil cylinders and reservoirs. Air provides thrust while oil controls speed and/or mid stroke stopping.A pneumatic cylinder is a mechanical device that converts compressed air energy into a reciprocating linear motion. A double-acting cylinder uses compressed air to move a piston in and out, while a single-acting cylinder uses compressed air for one-way movement and a return spring for the other.
Contents
What is the purpose of air over oil intensifier circuit?
Purchased or special built air-over-oil circuits provide smooth control when power requirement is low. Some manufacturers make self-contained air-powered cylinders with built-in oil cylinders and reservoirs. Air provides thrust while oil controls speed and/or mid stroke stopping.
How does air cylinder work?
A pneumatic cylinder is a mechanical device that converts compressed air energy into a reciprocating linear motion. A double-acting cylinder uses compressed air to move a piston in and out, while a single-acting cylinder uses compressed air for one-way movement and a return spring for the other.
What is provided for air circulation in oil tank?
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.
How does air over hydraulics work?
Air over hydraulic brakes work by using air compression to provide power to the braking system, but has a hydraulically controlled system with a compressor, air dryer, and reserve tanks lines.
Will a hydraulic cylinder work with air?
And when dieseling occurs in a hydraulic cylinder it can damage the cylinder and destroy of its seals. When a mixture of air and oil is compressed in a hydraulic cylinder it can ignite and burn, or even explode!
Do air cylinders need lubrication?
The tight internal clearances within pneumatic cylinders require lubricants that spread in thin, even layers.
Which causes the air enter into cylinder?
Ambient atmospheric pressure forces the air-fuel mixture through the open intake valve into the cylinder to fill the low pressure area created by the piston movement.
What happens if you get air in your master cylinder?
This master cylinder is mounted at a relatively sharp angle. If air enters the left front or right rear wheel circuits it can migrate to the high point. If the vehicle is experiencing a low and/ or spongy brake pedal and the master cylinder is mounted at an angle, trapped air might be the cause.
Why is it important to bleed the air from a hydraulic system?
When air contaminates a hydraulic fluid, usually via the pump’s inlet, aeration, cavitation, or foaming can occur. Aeration is bad news, as it degrades the hydraulic fluid causing damage to the components of the system due to loss of lubrication, resulting in overheating and burning of the seals.
What is an air over hydraulic intensifier?
Get the force and stability you need by converting air to hydraulic pressure without the need of a hydraulic system. Models ranging from 5:1 to 36:1. Air/Oil Intensifiers convert typical shop air, 80-100 psi, into hydraulic pressure.
Which factor decides the working pressure of a hydraulic cylinder?
The force produced by a cylinder is equal to the cross-sectional area of the cylinder multiplied by the working pressure. Therefore, the working pressure of a hydraulic cylinder depends on the diameter of cylinder.
Why are some gas turbine engine oil reservoirs tanks pressurized?
Some incorporate a pressurized oil tank. This ensures a constant head pressure to the pressure-lubrication pump to prevent pump cavitation at high altitude. Oil consumption in a gas turbine engine is low compared to that in a reciprocating engine of equal power.
What is an air intensifier?
An air/oil intensifier can convert low-pressure into high pressure in instances where a hydraulic pressure system is not available or where hydraulic pressure is too low. That means you can spec hydraulic caliper disc brakes even for applications where there will be no hydraulic system pressure source.
What is an air over hydraulic intensifier?
Get the force and stability you need by converting air to hydraulic pressure without the need of a hydraulic system. Models ranging from 5:1 to 36:1. Air/Oil Intensifiers convert typical shop air, 80-100 psi, into hydraulic pressure.
What is the advantage of using single acting cylinder?
Advantages of Single Acting Cylinder:
Simple design; easy to install. Low cost of initial purchase. Single port and small housing. Reduction in valve and piping cost compared to double acting cylinders.
What is a weight loaded accumulator?
Working of Weight loaded Accumulator
The dead weight loaded on top of the piston develops a gravitational force which provides the potential energy in the cylinder. This accumulator, applies a constant pressure on the fluid throughout its range of motion irrespective of the rate and quantity of output fluid.
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QuickJack Hack to Bleed New Hydraulic Cylinders – YouTube
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Air Over Oil – Air Over Hydraulic Cylinder Benefits & Uses – About Air Compressors.com
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- Summary of article content: Articles about Air Over Oil – Air Over Hydraulic Cylinder Benefits & Uses – About Air Compressors.com 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 … …
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Table of Contents
Air Cylinders
The Air-Over-Oil Solution
Air-Over-Oil Operation
Air-Over-Oil System Construction & Components
Air-Over-Oil Circuit Construction Considerations
FAQs (Frequently Asked Questions)
Practical Automation: Using air-over-oil circuits for more controllable pneumatic power – MRO MagazineMRO Magazine
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- Summary of article content: Articles about Practical Automation: Using air-over-oil circuits for more controllable pneumatic power – MRO MagazineMRO Magazine The single-tank system (Fig. 2) uses oil on one se of the cylinder piston only. The directional control valve controls the air flow into the tank and the flow … …
- Most searched keywords: Whether you are looking for Practical Automation: Using air-over-oil circuits for more controllable pneumatic power – MRO MagazineMRO Magazine The single-tank system (Fig. 2) uses oil on one se of the cylinder piston only. The directional control valve controls the air flow into the tank and the flow … 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 Pra…
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how does air over oil cylinder work
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Why Use Air Over Oil Cylinders?
Air over oil cylinders are pneumatic cylinders designed to produce greater force than standard pneumatic cylinders of the same bore size. They incorporate both oil and air within the mechanism, creating a hybrid pneumatic-hydraulic cylinder that intensifies the amount of force generated by compressed air input.
HyperCyl takes the innovative design of hybrid air over oil cylinders to the next level with our patented rod and piston mechanism. Unlike other designs, our cylinders do not use an external oil reservoir. Instead, the oil reservoir is completely sealed and incorporated into the cylinder itself. Our air over oil cylinders uses compressed air on the piston to exert force within a sealed hydraulic chamber by pneumatically driving a rod into the oil reservoir, which is situated behind the work piston.
Why Use Air Over Oil Cylinders?
Air over oil cylinders offer a host of unique advantages over more traditional pneumatic systems. Since compressed air is ubiquitous in virtually every industrial setting, it offers a convenient source of clean power that can easily be intensified using HyperCyl’s air over oil cylinders.
Our state-of-the-art air over oil cylinder design offers a low-cost, highly efficient means of converting compressed air to smooth fluid force. HyperCyl’s air over oil cylinders only use 25% – 30% of compressed air when compared with multi-piston pneumatic cylinders for the same operation.
While many companies are phasing out hydraulic power units and cylinders, the HyperCyl air over oil cylinders offer a cost-effective solution for facilities that want the power of a hydraulic system in a smaller package. Our air over oil intensifier units can provide up to 200 tons of force while using significantly less space than traditional hydraulic systems. The smaller size of our air over oil cylinders allows for reduced bore size compared to standard multi-stage pneumatic cylinders.
Our air over oil cylinders make no more sound than a standard pneumatic cylinder. Since there is no power unit like hydraulics, you enjoy less noise, less heat generation, and no continuously running motor. Energy is needed only while HyperCyl is moving; at rest zero noise and zero energy is realized. To suit your specific needs, we offer six models of air over oil intensifier cylinders suitable for a broad variety of industries and applications. In addition, we are pleased to work with you to create custom stroke cylinders quickly and efficiently, within your budget.
Partnering With HyperCyl for Air Over Oil Cylinders
For more than 25 years, HyperCyl has been a leading expert in the design and manufacture of air over oil, or hydra-pneumatic, cylinders. Our extensive expertise extends to servo-controlled actuators, assembly cells, and presses using our cutting-edge cylinder technology. As a privately-owned company based out of Dundee, Michigan, we are proud to produce our products in America.
We are dedicated to helping our customers find the perfect motion solutions for their particular needs. Our seasoned experts will work closely with you to determine the appropriate force and stroke requirements for optimal power and efficiency. Each of our air-over-oil cylinders offers unique features to enhance your operations, soft initial part contact and enhanced energy efficiency. The pressure stroke of the cylinder can be regulated separately from the approach stroke, allowing you to fully tailor your force and energy usage for optimal cost savings.
At HyperCyl, we offer a variety of triggering mechanisms for high-pressure actuation, without the need for cylinder alterations. This mechanism can be triggered based on a pressure threshold (ELT sensor), specified stroke distance (LVDT), or a simple proximity sensor. Other standard features of our air over oil cylinders include:
Complete air/oil separation
Wear bands on internal pistons for increased life
Variable operating positions or attitudes
No external oil reservoir
Up to 200 tons linear output force
Three primary moving components
NFPA medium-duty mounting
Tie rods with 100,000 psi tensile-strength
Operation and maintenance manual
Heavy-duty construction for long service life
4.5 million MTBF (mean time before failure)
20 million cycle service life
Accessories include gauge kits, sensors, total stroke limiters, alignment couplers, rod locks, and more
HyperCyl’s “IntelliCyl” package offers in-process validation of each part thereby dramatically reducing part-nonconformance, scrap rates, and check QTY of fixtures involved
Limited Lifetime warranty for cylinder materials and workmanship
Superior Quality and Exceptional Workmanship by HyperCyl
Since 1994, HyperCyl has been providing innovative solutions to some of the most challenging motion problems. Our years of experience and commitment to quality have made us a leading provider of actuators and press systems for customers in almost any industry. We hold our products to the highest standard, consistent with the Buy American Act and Made in the USA requirements.
To learn more about our extensive selection of air over oil cylinders and other actuation solutions, contact us today or request a quote.
Air Over Hydraulic Cylinder Benefits & Uses
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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
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
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
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
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
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
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.
Additional oil reading:
If you have any questions regarding air over hydraulic cylinders, please leave a comment below, with a photo if applicable, so that someone can help you!
Practical Automation: Using air-over-oil circuits for more controllable pneumatic power – MRO Magazine
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 Pra…
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.
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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 [email protected]. Previous columns can be viewed at www.mro-esource.com.
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