How Airbearings Work

What is an Air Bearing?

Airbearings or air casters support loads on a cushion of air. It is a unique air support device, but may be compared with two other forms of air supported devices: the classical air bearing, and air cushion or “Hovercraft”.

Airfloat uses a flexible diaphragm beneath the load support surface. Air is pumped into the diaphragm and passes freely through the diaphragm holes and into the plenum beneath, raising the platform off the ground.

The air that is forced out between the diaphragm and the ground forms a thin lubricating air film. Since the diaphragm is flexible, it can deflect as it encounters obstacles, or fill out as it passes over depressions in the surface.

How it Works

In an operating bearing, air flows into the space above the diaphragm and flows freely through the communicating holes. Air under the diaphragm tries to escape outward, under the footprint area where the clearance gap is small.

If the operating surface has undulations, the diaphragm footprint will adjust to keep a small clearance gap at all points. Airfloat bearings are essentially, self-adjusting seals which maintain a very small clearance gap, which provides for a thin lubricating film of air. Assuming sufficient air supply pressure, the pressure inside a given air bearing is determined only by the load applied, and the effective area of the bearing. It is not affected by the supply pressure. When air is supplied to a loaded bearing, the pressure increases as the bearing inflates and lifts the load. At that time it acts like a relief valve. As more air is supplied, the clearance gap increases to let the excess escape, and maintain a nearly constant pressure.

Why use air bearings? Easy to position the load exactly and precisely; No overhead cranes needed to move heavy loads; No wheeled carts, dollies or fork trucks using factory floor space to park or maneuver; No maintenance expense for the cranes, carts, or fork trucks; No wear and tear or floor and wall damage from fork trucks and carts, and Fewer workers required for the same amount of product movement.

Airfloat's high performance Air Bearings use a continuous flowing film of air between a flexible diaphragm and the floor surface, allowing virtually friction-free movement.

Rigid Air Bearing

The rigid air bearing can support large loads with small unit pressures when a film of air is forced between the support surface and the ground, but because this film is only a few thousandths of an inch thick, a very smooth and very flat surface is required. Principal advantages of the rigid air bearing are its low power requirements and low noise characteristics. Disadvantages are that the bearing and the surface must be very flat, smooth and parallel.

Skirted Plenum or "Hovercraft"

Air cushion devices were initially of the single plenum type. A large flow of low pressure air is supplied to counteract leakage out of the air gap. Improved forms employ the skirted plenum, which helps contain the air bubble, decreasing the amount of air required.

The main advantage of the air cushion is its high ground clearance, allowing it to move over objects up to several feet in height. The main disadvantage is the great air flow required, causing very high power consumption. The load capacity is limited by the low operating pressures.

Compliant Air Bearing

Airfloat compliant bearings combine the advantages of both the rigid air bearing and the air cushion. The load carrying capacity is high, the power requirements are low, and the noise level and dust disturbance levels are low. Airfloat air bearings can tolerate some surface imperfections and obstacles, and are omni-directional. Airfloat uses a flexible diaphragm beneath the load support surface. Air is pumped into the diaphragm and passes through the diaphragm holes and into the plenum beneath, raising the platform off the ground. The air that is forced out between the diaphragm and the ground forms a thin lubricating air film. Since the diaphragm is flexible, it can deflect as it encounters obstacles, or fill out as it passes over depressions in the surface. The following table shows an approximate comparison the three devices:

Comparison of Air Support Devices
(Approximate)

A Close Look At The Airfloat Air Bearing

Air flows into the space above the diaphragm and then flows through the restrictions of the communicating holes, so that pressure P1 is slightly greater than P2 (Figure 1-2). Air under the diaphragm tries to escape outward, under the footprint area where the clearance gap is small.

As air escapes through the small gap, its velocity increases. The Bernoulli (venturi) effect, reduces the pressure slightly, drawing the diaphragm closer to the operating surface. This results in a self-regulating clearance gap. If the operating surface is slightly undulating or wavy, the diaphragm footprint will adjust to keep a small clearance gap at all points. Airfloat bearings are essentially self-adjusting seals, which maintain a very small clearance gap, which provides for a thin lubricating film of air.

Assuming sufficient air supply pressure, the pressure inside a given air bearing is determined only by the load applied, and the effective area of the bearing; it's not affected by the supply pressure. When air is supplied to a loaded bearing the pressure increases as the bearing inflates and lifts the load. At that time it acts like a relief valve. As more air is supplied, the clearance gap increases to let the excess escape, and maintain a nearly constant pressure.

Floors

The air bearings and the equipment that supports and controls them, are actually only one half of the air floatation system. The floor makes up the other half. The best air bearing equipment will not function if the floor is not suitable.

The following discussion describes floor conditions which will allow air bearings to function. This discussion should not be used as a floor specification.

See the "General Specifications - Concrete Floors" document (PDF) and consult Airfloat, LLC for recommendations.

Floor Surface Requirements

The most critical single element in a successful air bearing application is the surface over which the air bearing operates. It's hard to set out general specifications, for the unusual nature of air bearings imposes some unusual floor requirements. Many factors of the surface condition have an effect on bearing performance, so there's no simple way to explain or specify floor requirements. We need to understand these factors fully to make a successful and practical application of air bearings. Let's look at individual aspects of the air bearing operating surface (floor).

The ideal surface would be smooth, level, flat, non-porous, and have low friction characteristics. Polished sheet metal is a good example. Commonly used floor surfaces that come very close to being ideal (when in good condition) are rubber, asphalt or vinyl floor tile, roll flooring in the non-textured types, terrazzo, and some smooth, steel-troweled concrete floors. On such an ideal surface, air bearings can carry very heavy loads with extremely low friction, (drag less than .1% of the load) and very low air consumption (under 0.25 CFM per foot of bearing perimeter). Under these conditions the lubricating air film is only 0.001-.002 inch thick.

How smooth does the floor have to be? A floor that looks good may be unusable, yet a floor somewhat pitted and spalled may work OK. These magnified drawings show the floor/air-bearing-diaphragm interface up close. The previous drawing shows a perfect floor with an air film of 0.002 inch. The drawing to the left shows a new troweled concrete floor that appears very smooth to a casual observer. Most such floors have minute trails of wet grout that followed the trowel, and caused minute sharp ridges. These are thicker than the normal air film. The diaphragm touches the sharp ridges, and high drag results. This condition can be identified by the “finger tip” test described later. BAD FLOOR.

Now let's look at the same new floor, but after a week of use. Foot or wheeled traffic quickly wears down the minute ridges, to less than air film thickness. VERY GOOD FLOOR. Note: Similar effect can be gained by rubbing the new floor with emery cloth, or light grinding.

As the floor wears, mortar is worn away slightly, exposing aggregate particles. Diaphragm may touch in spots, but projections are rounded, and friction is not high. MEDIUM FLOOR.

As the floor wears further, aggregate becomes more exposed. Some scratches develop. UNUSABLE FLOOR.

 

The above floor can be restored by grinding the surface with a terrazzo grinder. Small depressions may remain. These are not wider than the footprint zone, and have no effect on air bearing operation. VERY GOOD FLOOR.

 

Another way to restore a worn floor is by painting or roller coating with a liquid epoxy-which fills the small depressions, as seen in the cross-hatched area.

 

Pictured here is a floor once smooth, but subjected to local chipping by dropping of solid objects on it. If pitted areas are mostly isolated, and less than 1-2” in diameter, the diaphragm will control flow effectively, either on one side or the other of the pit. CAN BE USABLE FLOOR.

 

A smooth surface floor, but with mild gradual waviness allows the diaphragm to conform to the surface. GOOD FLOOR.

 

A floor with steep ridges but of the same height and spacing as the "waves" above, does not allow the diaphragm to conform. BAD FLOOR.

 

A broomed finish concrete, like sidewalk, may cause 100 times the normal clearance gap. UNUSABLE FLOOR.

 

The badly worn floor as pictured at right, has been covered with a half-inch of aggregate projects. POOR FLOOR. (To remove projections, lightly grind the epoxy or paint on a second top layer of unfilled epoxy).

The Finger Test and Floor Fixes

A simple “finger” test will help measure floor surface friction characteristics.  With the index finger at about 45⁰, press down with 1-2 pounds' force, and slide your finger forward.  If it slides easily, floor friction characteristics are probably acceptable.  If it doesn’t slide well and your skin wants to roll under, the floor condition may be a problem.  Some floor coatings leave a tacky or high-friction surface; a light dusting of talc can help until a normal dust/dirt film has developed.

Floor Flatness

One unique feature of compliant air bearings is their “compliance”, or ability to conform to undulations in the surface.  This compliance is exaggerated in Figure 6-13A.  Compliance increases as bearing size increases, and decreases as operating pressure increases.

As a rule of thumb, the limit of compliance is approximately 1 to 2% of the bearing diameter (somewhat greater at low pressures).  This means that a 36” bearing can operate if the surface under the bearing is 0.36” to 0.72” higher in one area than another.  Thus floor waviness can be 4% of bearing diameter, across a distance equal to a bearing diameter.

This does not mean a 24-foot bearing can traverse a half-inch tall step.  Here we are only talking about gradual undulations. Controllability will obviously be compromised.

When several bearings are mounted to a rigid frame, the undulations should be limited to about 1 to 2% of bearing diameter across the entire span of bearings.  If the frame is not rigid, waviness can be much greater over the entire frame span.

Steps

Air bearings have very little ability to traverse steps.  For example, a 36-inch bearing cannot readily go up, on, or down from a 1/8” thick square-edge sheet.  Three effects tend to prevent this.  Some of the air escapes from the corners of the steps, robbing air from the bearing.

The sharp edge of the step cuts off the lubricating air film from the advancing side of the bearing, and the diaphragm “runs aground” on the sharp edge.  If inertia or brute force carries the bearing some distance over on to the step, the internal pressure of the bearing forces the diaphragm down against the top of step.  Since no lubricating film reaches this area, it acts like a powerful brake.

In practice, the height of a square-edge step that can be crossed depends on bearing pressure and size.  Large, low-pressure bearings have greater capability than a small higher pressure bearing, but even a small square-edge step is a great obstacle.  Whenever possible, steps should be modified to ramps, if only by beveling the edges.

Ramps

While a given bearing may be unable to cross a 1/8” high step, it could cross a half-inch- high projection if it is ramped properly.  In this case, the ramp is gradual enough not to cut off the lubricating air film, and the bearing compliance can absorb the obstruction. 

When long ramps are required, they should be designed with regard to “Flatness” requirements.  In practice, long ramps whose length is equal to or greater than the bearing should be limited to 1-3% slope for most applications.

Cracks and Expansion Joints

Concrete floors are the most common surface on which air bearings operate.  These floors are often made with expansion joints, and are subject to cracks.  With many cracks, the floor on both sides of the crack is flush.  However, even a nearly hairline crack can be troublesome.  After several traverses with an air bearing, dirt can be blown out of the crack and air can escape through the crack to a porous gravel fill below.  This way, even a small crack can leak away most of the air supplied to the bearing. 

Expansion joints can also let air escape through the floor.  They are often wide enough, and recessed sufficiently, to allow much air escape out sideways from under the bearing.  They also may have the two slabs shifted, so a step results.  Thus unless expansion joints are properly filled they can cause a problem.

Porosity

Porous floor materials permit air to escape down through the floor surface itself, and leave insufficient air for bearing operation.

Floors that are sometimes encountered which show high porosity are hardwood plank and parquet, low density “Masonite” or particle board, and poorly painted plywood.

This porous condition is almost never encountered in good quality concrete floors, even when the surface is rather rough.  Some persons have the idea that concrete floors need to have sealers applied, to eliminate porosity to air flow.  Concrete sealers are often used, but their function is either to prevent unsightly oil absorption, to chemically harden the surface and prevent dusting, or for appearance reasons.  There are some concrete surface “sealers” which have a heavy body, and can level minute rough areas and improve bearing performance.

Airfloat welcomes an opportunity to carefully examine your floor conditions to get the best performance from your air bearing application.  Our years of practical problem-solving are at your service.

Disclaimer: This document covers only general specifications for concrete floors. Specific types of equipment; processes; sizes of, quantity and spacing of air bearings; may dictate more specific requirements. Please contact an Airfloat, LLC. representative for review of specific requirements. Airfloat, LLC. is not a flooring expert. New floor installation processes, coatings & refinishing processes are constantly being developed. It is the customer's responsibility to choose a floor contractor who can meet the above specifications and provide a floor that will be economical, durable and maintainable.