Foil bearing
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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, so the hydrodynamic bearing is self-starting. Anti-wear coatings exist that allow over 100,000 start/stop cycles for typical applications. New third generation bearings can hold 9000 times their weight, at extremely high speeds. Turbomachinery is the most common application because foil bearings operate at high speed.
Foil bearings were first developed using independent R&D funds to serve military and space applications in the late 1950s by AiResearch Mfg. Co. of the Garrett Corp.Some early history is reported in Giri L. Agrawal, "Foil Air/Gas Bearing Technology -- An Overview," American Society for Mechanical Engineers, Publication 97-GT-347 (1997) and Giri L. Agrawal, "Foil Bearings Cleared to Land," Mechanical Engineering 120 (July 1998): 1978-80. (See entry for Garrett Systems -- later acquired by Honeywell.) They were first tested for commercial use in United Airlines Boeing 727 and Boeing 737 cooling turbines in the early- and mid-1960s.Scholer Bangs, "Foil Bearings Help Air Passengers Keep their Cool," Power Transmission Design (Feb. 1973). Garrett AiResearch air cycle machine foil bearings were first installed as original equipment in 1969 in the DC-10's Environmental Control Systems. Garrett AiResearch foil bearings were installed on all U.S. military aircraft to replace existing oil-lubricated rolling-contact bearings. The ability to operate at cryogenic gas temperatures as well as at very high temperatures gave foil bearings many other potential applications.M. A. Barnett and A. Silver, "Application of Air Bearings to High-Speed Turbomachinery," Society of Automotive Engineers International, Technical Paper No. 700720 (September 1970), available at http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=700720.
Capstone Turbine Corporation introduced the world's first oil free power generating microturbines in 1998, made possible by foil bearings.
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
- Higher load capacity
- Improved damping
- Improved coatings
- Lower capacity than roller or oil bearings
- Wear during start-up
- High speed required for operation
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