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Aerodynamic bearings are always dependent on the motion between the bearing surfaces and normally some type of spiral grooves to draw the air between the bearing gaps. This bearing action is very similar to hydroplaning in our automobile on a puddle of water at high speed. In just this way, aerodynamic bearings require relative motion between the surfaces, when there is no motion or when the motion is not fast enough to generate the air film the bearing surfaces will come into contact. Aerodynamic bearings are often referred to as foil bearings or self-acting bearings. Examples of this type of bearing include the read-write head flying over a spinning disk, crankshaft journals, camshaft journals, and thrust bearings for electrical generator turbines.
3.Orifice and Porous media technology
Air bearings are typically classified as ‘orifice’ or ‘porous media’ bearings. In orifice bearings the pressurized air is supplied to the bearing surface through a small number of precisely sized holes. Porous media air bearings are different that the air is supplied through the entire surface of the bearing (Figure 4.1). The porous material controls the airflow in the same way an orifice bearing would do if it had millions of miniature holes across its surface. Broadly speaking there are two techniques for achieving the compensating effect in air bearings. Orifice compensation is traditionally the most widely used method, but porous surface compensation is rapidly emerging as the method of choice due to its many advantages and increasing availability.
Exclusive Advantages of Air Bearings
The advantages air lubrication can offer is from the properties of gases: they are chemically stable over a wide temperature range and they have inherently low viscosities.
Low Friction
The use of gas allows a torque of magnitude smaller than that achieved by liquids, but a low-torque bearing with suitable large clearance can be made using gas lubrication. Air is employed as the exhaust can be released to the surroundings as it causes no damage to the atmosphere. High speed linear slides can operate at very high speeds for thousands of hours or repeatedly because of low friction aspects of air bearings. Any other rolling element bearing would never be able to achieve such a requirement. This low friction also finds uses in torque measuring equipment, dynamic balancing machinery, semiconductor positioning systems, micro or zero gravity trajectory simulators and other instruments requiring near-static conditions.
High Accuracy
The high accuracy of motion that can be obtained with air bearings is equally important in some applications. Considerable differences in motion accuracy exist between rolling element bearing supports and air bearing supports. In linear slides, for example, rolling element bearings witness noise error (or rumbling) due to the surface roughness and/or eccentric rotation of the rollers.
On the other hand, air bearings do not suffer from this difficulty. The reason for this lies in the absence of surface contact between the bearing parts and the averaging action of the air film over the various local surface irregularities present in the machined surfaces. Even the finest of rolling element bearings are orders of the magnitude less accurate than air bearings. In rotating air bearings, this effect produces high orders of rotational accuracy and smoothness of travel.
High Stiffness
At zero speed, air bearings provide considerably high stiffness characteristics. This same effect is seen at zero or low loads. For properly designed and manufactured aerostatic bearings, it is not uncommon to measure stiffness on the order of several million pounds per inch.
Zero Wear
The advantage of zero wear can be seen greatly in externally pressurized or aerostatic bearings. Although some properly designed rolling element bearings can achieve practical wear rates, none can match the zero wear characteristic of aerostatic bearings. With aerodynamic bearings, starting and stopping causes some rubbing within the bearing clearance, but this can be relieved by introducing a boost of air as the bearing begins translation. As compared with rolling element bearings, air bearings do not suffer from increased wear rates as the speed or load is increased. With proper care and maintenance, infinite life can be expected from air bearings.