Foil bearings are a kind of specialized air bearings that use spring-loaded foil to support the journal of an axle or shaft until a certain rotational speed has been achieved. As a foil bearing rotates, it pulls air into the space between the journal and the sleeve. That creates enough air pressure within that space to push the foil out of the way, so that the bearing is entirely supported by air. Since there is no physical contact between the different components of a foil bearing at that point, very little wear occurs outside the initial start up period. Foil bearings can be used in any application that involves a high rotational speed, and turbomachinery is one prominent example.
Bearings are devices that are designed to allow one object to rotate within another object, and they are typically also expected to withstand some type of load. Traditional bearings consist of a sleeve and a bearing surface that are separated by balls, rollers or other components that must be lubricated on a regular basis. Other simpler bearings contain no roller elements, and the foil bearing is a variation that uses foil to replace both the roller elements and the bearing surface.
Most foil bearings consist of three main components, which are a bearing sleeve and two layers of foil. These layers are typically referred to as top foil and bump foil, and each serves a different purpose. The top foil typically acts as the bearing surface, which is what the journal of a shaft, or axle, rests on, while the bump foil offers necessary support. When a journal rotates within a foil bearing, air is drawn into the space occupied by the bump foil. That causes the foil to draw away from the bearing sleeve and the journal, leaving the shaft or axle suspended on a layer of pressurized air.
The main purpose of a foil bearing is to eliminate the requirement for oil, or other lubricating substances, that most other bearing designs have. Since the journal and sleeve are separated by pressurized air during operation, and new volumes of air are constantly being drawn in, little to no wear occurs as long as a sufficient rotational speed is maintained. Wear does occur whenever a foil bearing is put into operation or stopped, though some of these bearings can withstand over 100,000 such cycles.
There are also a few other advantages offered by foil bearings, such as an increased tolerance for misalignment. Bearings are often very sensitive to misalignment, but foil bearings typically correct for this automatically. Some foil bearings can also withstand temperatures in excess of 1,200°F (about 650°C), making them well suited to high temperature turbomachinery and other similar applications.