Air Bearing Basics
Basically, air bearings use a thin film of pressurized air to support a load, the same way the puck on an air hockey table "floats" on air. This type of bearing is called a "fluid film" bearing. Fluid film bearings have no solid-to-solid contact under typical running conditions; instead, a film of lubricating fluid (in our case pressurized air) forms a layer between the solid machine elements and serves to transfer forces from one to the other. To compare this with ball bearings, in ball bearings the balls are in constant contact with and form a solid bridge between the machine elements.
Fluid film bearings offer a number of advantages over mechanical bearings. First, because there is no contact, air bearings do not suffer from wear or heat generation due to friction. They also exhibit no starting or running friction, even under their highest design loading. In addition, the fluid film acts to center and average out small scale errors in the components resulting in motion which can be more accurate than the individual bearing components. Air bearings also offer much higher stiffness than rolling element bearings because the air film fully supports the components, as opposed to balls or rollers which have point or line contact and are therefore limited due to Hertzian contact stiffness.
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.
First, lower viscosity means that for the same working pressure gas bearings have lower load capacity (liquid fluid film bearings typically support five times the load of gas bearings for the same pad area). Second, because of the extremely low viscosity of gasses, gas film bearings operate with essentially zero static and running friction where liquid fluid film bearings have much higher friction and pumping losses within the bearings, which can cause heat generation. And third, gas bearings require very tight bearing gaps for proper operation (10 µm for gas compared to up to 100 µm for liquid bearing) which translates into extremely high accuracy requirements on the components.
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).
The first question is how to generate the pressurized supply of fluid for the bearing. There are two ways to do this - one is using an external pressurized supply (hydrostatic) and the other is to use the relative motion of the machine components to generate the pressure internally (hydrodynamic or "self-generating"). While hydrodynamic bearings are common for oil fluid film bearings, which generate internal pressures quite easily due to the relatively high viscosity of oil, it is much rarer to see this technique used for air bearings because the pressure generated is quite low (although Nelson Air has built bearings of this type for low load, high speed rotary applications such as optical scanners).
