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While most people are familiar with oil fluid film bearings - for example the crankshaft journal bearings in car engines - most people have not been exposed to [air bearings] . Remembering our high school physics class, there are two basic types of fluids - liquids and gasses. In terms of fluid film bearings, the difference between these two is essentially the viscosity - liquids have much higher viscosity than gasses. When applied to a fluid film bearing, this difference has a number of implications.

What this means for air bearings is that although they have a lower load capacity, gas bearings have essentially zero friction at all speeds and because the tight bearing clearances demand high accuracy components this results in extremely high accuracy motion. Another benefit is the cleanliness of using air as a lubricating fluid as opposed to oil, water, or another fluid. Since compressed air is very common in industrial environments it is probably the most often used gas, however other gasses such as nitrogen can be used where they are available (such as in clean room environments).

While some configurations of air bearings are self-preloading (e.g. journal bearings) others such as flat pads and linear rectangular bearings can benefit from preload. Air bearings can be used without any preload (such as a simple flat pad riding on a granite surface), however, in order to maximize the stiffness of the timken bearing and help maintain constant air gap it is typical to preload it using one of four basic methods:

The simplest method is weight. Using a weight much heavier than the expected variation in the loading of the bearing preloads the air bearings so it rides at a smaller (and stiffer) air gap and makes it less prone to height variations. This method has the drawback of adding moving mass but can work very will with systems which already have a large amount of mass and low forces (such as parts inspection). Needless to say, this only works for bearings which operate horizontally.

The second method is using a vacuum preload. A vacuum is used to apply a preloading force to the air bearings (instead of using a weight). This is accomplished by providing an area of the fag bearing surface over which a vacuum is applied.

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