Introduction to Air Bearings
Advances in engineering and technology frequently place severe new demands upon the support systems for machine components required to be in relative motion both in regard to improved performance and increasingly exacting operating conditions. The great majority of these demands are met by some combination of clever design of 'conventional' bearing systems, the development of improved liquid lubricants for a great diversity of specialized applications, and the devising of effective sealing techniques for circumstances in which it is essential to minimize the leakage of lubricant liquids or their vapors.
However, in a growing number of applications it is becoming necessary to achieve a result which is either technically or economically impracticable with conventional lubricants. Numerous alternative classes of bearing systems have been devised to meet new requirements by means which are tailored to like circumstances. For example, in a vacuum, a magnetic support may be employed (for low temperatures); for handling almost any liquid, the liquid itself may be used as a lubricant although it may lack numerous convenient qualities available with lubricating oils; and for numerous special purposes, including some very high temperature applications, solid lubricants such as graphite, or molybdenum or tungsten disulfide, may be employed. Another alternative class of bearings is that which makes use of a gas (air) as the lubricant. Air lubrication offers certain unique advantages, as in their own manner and place do the other alternatives, but likewise air bearings themselves have their own inherent limitations which must be clearly recognized in advance if the aspiring user is to avoid unwarranted failure.
In particular it must be recognized that air bearings of practically every type are somewhat more prone to instability difficulties than are liquid bearings, although most of these instabilities are to a huge extent the same general type that occur in liquid-lubricated systems; for example, the important case of half-speed whirl in self-acting 360 degree journal. Pressurized bearings also have their instability problems although these are sometimes of a very different type.
Air bearings fall into two main classes, which indeed correspond to the two major representative classes in liquid film lubricated bearings: aerostatic bearings, which require a feed of pressurized air for their operation, and aerodynamic bearings, which generate their own internal pressure differentials. The latter generate this pressure by the action of simultaneously shearing and squeezing the environmental gas between the surfaces in relative motion, whereas the former require an external pump to produce the pressure. Both these types of bearing can be employed to sustain either axial or radial loads in rotating systems (or direct loads in linear configurations), or to combine the functions in a single member. A bearing can operate either entirely aerostatically or entirely aerodynamically throughout its operating speed range, or the bearing can start its movement in one mode of operation and transfer to the other as the speed changes, or it can operate with a combination of both aerostatic and aerodynamic pressure generation. Under certain circumstances, moreover, it may be necessary to supply externally pressurized gas to a self-acting bearing to prevent instabilities from arising. The flow regime in either type is usually laminar, but turbulent flow conditions can occur.
In general, the externally pressurized bearings (aerostatic), for many purposes suffer an inherent disadvantage through requiring a pressure source and an exhaust sink; but they can be made to relaxed manufacturing tolerances and can provide support at low speeds, and sustain intermittent or stationary loads. The self-acting bearings (aerodynamic), on the other hand, are able only to support a few pounds per square inch of bearing area depending upon the speed, require careful manufacture and alignment and are only suitable for bearings whose surfaces are always moving when under load. However, such bearings do not require auxiliary equipment and there is no problem of disposing of the exhaust gas or arranging for appropriate pressure control of the working compartment. The pressure gas for the externally pressurized type may, however, offer special advantages in particular circumstances. For example, a pressure-fed bearing can work in a dust-laden atmosphere because the exhaust air prevents the entry of solid particles from the environment.
Despite some limitations of load carrying capacity compared to other mechanical or rolling element bearings there are numerous advantages, and some of them exclusive, that permit machine operation in conditions which would otherwise be quite impracticable.
The advantages which air lubrication can offer stem from the properties of gases: first, they are chemically stable over a wide temperature range and second they have inherently low viscosities. However, even in this category it is often practical to design an alternative device by a fundamentally different approach to the same job without air bearings, and so it is really not feasible to draw a hard line between exclusive advantages and advantages of degree. Nevertheless, it is believed that this way of thinking of the advantages available does avoid some confusion of thought since it focuses attention upon aspects of a proposed application in which the gas really may be doing something which no other lubricant could possibly do for fundamental reasons and those in which there is merely competition in regard to convenience or cost.
Of the more important exclusive advantages which are offered by air lubrication are those cases where the low viscosity of gases as compared with liquids can be exploited to special benefit. Particularly straightforward examples of this class of application are those which occur in near-static apparatus such as gimbal support, dynamometers, wind-tunnel balances and other specialized mechanical instruments which benefit from the extremely low static frictional torque which externally pressurized bearings can offer. The use of a gas permits a torque orders of magnitude smaller than could be achieved by liquids, but perhaps in practice often of more importance is the fact that a low-torque bearing with an appropriately large operating clearance can be made in a very simple and clean fashion using gas lubrication. Air is usually employed, since the exhaust from the bearing can be released to the surroundings and quite large flow rates can be employed.
In semiconductor machine tools, high-speed, acceleration and damping are key to product throughput. The use of air bearings in such an application has found widespread use. Experimental high speed linear slides of a composite lightweight structure have operated at 14g acceleration for thousands of hours or repeated because of the low friction aspects of air bearings. A mechanical rolling element-type bearing would never be able to satisfy 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.
On the contrary, 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. Typical T.I.R. for air bearing spindles are less than 1.0 µinch. For linear slides, pitch, roll and yaw errors of much less than a fraction of an arc second are attainable and straightness of travel errors on the order of nanometers have been achieved.
The advantage of zero wear can be seen greatly in externally pressurized or aerostatic bearings and to some large degree in self-acting or aerodynamic 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 alleviated by introducing a pulse of air just as the bearing begins translation. Furthermore, 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.
Note on crash resistance: allowing a bearing to crash or be overloaded to the grounded state should never be a design feature. Crashed bearings are a sign of a much wider system problem and should be corrected regardless of the type of bearing used.
Externally pressurized ceramic bearings were operated at temperatures of up to 1,500°F (800°C) at speeds of up to 65,000 rev/min. Low temperature applications of air bearings have been largely confined to various types of expansion turbine gas liquefiers and to refrigeration plant. A small high-speed expansion turbine for liquefying helium can operate at 350,000 rev/min. This unit employs bearings which are lubricated by helium gas at a temperature between 50° and 13°K, and the output of liquid helium is maintained at a pre-selected temperature of between 3° and 4°K to an accuracy of 0.05°.