Foil bearings use pressurized air to support a pin or axle, eliminating the need for lubrication. They consist of a bearing sleeve and two layers of foil, with the top foil serving as the bearing surface. Foil bearings offer benefits such as greater tolerance for misalignment and high-temperature resistance.
Leaf bearings are a type of specialized air bearing that use a spring-loaded sheet to support the journal of an axle or shaft until a certain rotational speed is reached. As a leaf bearing rotates, it draws air into the space between the journal and the sleeve. This creates enough air pressure within that space to push the foil away, so that the bearing is fully supported by the air. Since there is no physical contact between the different components of a foil bearing at that point, very little wear occurs outside of the initial start-up period. Plain bearings can be used in any application involving high rotational speed and turbomachinery is a prime example of this.
Bearings are devices designed to allow an object to rotate within another object and are typically also expected to withstand some type of load. Traditional bearings consist of a sleeve and seating surface separated by balls, rollers or other components that need to be lubricated on a regular basis. Other simpler bearings do not contain roller elements and the foil bearing is a variant that uses foil to replace both the roller elements and the bearing surface.
Most foil bearings consist of three main components, namely a bearing sleeve and two layers of foil. These layers are usually referred to as top foil and relief foil and each serve a different purpose. The top foil typically serves as a bearing surface, which is what a shaft’s pivot, or axle, rests on, while the impingement foil offers the necessary support. When a pin rotates within a foil bearing, air is drawn into the space occupied by the impact foil. This causes the foil to pull away from the bearing sleeve and journal, leaving the shaft or axle suspended over a layer of pressurized air.
The primary purpose of a foil bearing is to eliminate the need for oil, or other lubricating substances, which most other bearing designs have. Since the pin and sleeve are separated by pressurized air during operation and new volumes of air are constantly being drawn in, little or no wear occurs as long as sufficient rotational speed is maintained. Wear occurs every time a foil bearing is operated or stopped, although some of these bearings can withstand over 100,000 such cycles.
There are also a few other benefits that foil bearings offer, such as greater tolerance for misalignment. Bearings are often very sensitive to misalignment, but foil bearings usually correct for it automatically. Some foil bearings can also withstand temperatures in excess of 1,200°F (approximately 650°C), making them suitable for high temperature turbomachinery and other similar applications.
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