Foil bearings are a type of hydrodynamic bearing. A shaft is supported by a compliant, spring loaded foil journal lining. Once the shaft is spinning fast enough, the working fluid (usually air), lifts the foil from the shaft so that there is no contact. The shaft and foil are separated by the viscous properties of the working fluid. A high speed of the shaft with respect to the foil is required to initiate the air gap.
Foil bearings require no pressure system for the working fluid, the hydrodynamic bearing is self starting. Anti-wear coatings exist that allow over 100,000 start/stop cycles for typical applications.
Foil bearings were introduced commercially in 1969 by Garrett Systems AiResearch on the DC-10's ventilation equipment. Capstone Turbine Corporation introduced the world's first oil free power generating microturbines in 1998, made possible by foil bearings. Turbomachinery is the most common application because it operates at high speed.
The main advantage of foil bearings is the elimination of the oil systems required by traditional bearing designs. Other advantages are:
- Increased reliability
- Higher (unlimited) speed capability
- Higher and lower temperature capability (40 K to 2500 K)
- No scheduled maintenance
- High vibration and shock load capacity
- Quieter operation
Areas of current research are:
- Higher load capacity
- Improved damping
- Improved coatings
The main disadvantages are:
- Lower capacity than roller or oil bearings
- Wear during start-up
- High speed required for operation
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