A bearing is the machine element that realizes the constraints of motion as such that only the desired motion remains in the system while reducing friction between moving components. A set of bearings can realize a rotational degree of freedom along an axis of rotation, while preventing undesired motion in other degrees of freedom, such as tilt motion or radial and axial linear motion. Bearings are classified broadly according to the type of operation, the allowed motion, or to the directions of the loads the element can withstand. Well-known type of bearings are the rolling element bearing and the sliding bearings. In this paper we will highlight air bearings as a part of the fluid film bearings.

In an air bearing, a thin film of pressurized air is used to provide an interface layer between surfaces that can transfer loads in a frictionless way. Although called air bearings, the utilized gas is not solely limited to air, e.g., also pure nitrogen is often used. Since the two surfaces are properly separated and contact-free, no wear, particulate generation or excessive friction is present, as opposed to sliding or rolling contact bearings. Due to the low friction in the film, air bearings are excellent in high-speed applications, but also in high-precision applications, where the lack of friction facilitates more precise positioning and a smoother motion.

A further distinction is made between aerodynamic bearings, where the air film is established through the relative movement between the surfaces, and aerostatic bearings, which are externally fed by overpressure to maintain the air film even when there is no relative motion between the surfaces. Aerodynamic (or self-acting) bearings are found in the magnetic read/write heads in disk memory storage devices but also in thrust bearings for electrical generator turbines. Aerostatic bearings have a wide range of applications, such as the linear guideways of coordinate measuring machines or wafer processing machines in the semiconductor industry. In the remainder of this article, we will focus on the precision motion applications where aerostatic bearings are employed, although certain aspects also hold for aerodynamic bearings.

The film in the aerostatic bearing is realized by supplying the gas into the bearing gap via a restriction. This restriction meters the flow of pressurized gas to an optimal level for the designed bearing gap, such that it matches with the flow through the bearing gap, which results into a load carrying capacity and stiffness of the film. The restriction can be realized in various ways, e.g., machined feed holes, glued orifices, porous media etc.

Air bearing components are available on the market in the form of flat bearing pucks, bushings, bearing slides, radial bearing segments etc. These components match with a high-precision counterpart, such as a precision ground shaft for the air bearing bushings, or a precisely lapped granite surface for the air bearing pucks. The designer can develop and build his mechanical design utilizing these off-the-shelf components. A typical example is the Coordinate Measuring Machine, that uses air bearing pucks for the linear guideways, which can easily be interchanged during a maintenance of the machine.