Foil bearing
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Foil bearings are a type of air 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), pushes the foil away from the shaft so that there is then no contact. The shaft and foil are separated by the air's high pressure which is generated by the rotation which pulls gas into the bearing via viscosity effects. A high speed of the shaft with respect to the foil is required to initiate the air gap, and once this has been achieved, no wear occurs.
Unlike aero or hydrostatic bearings, foil bearings require no external pressurisation 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 in the late 1950s by AiResearch Mfg. Co. of the Garrett Corp[1] using independent R&D funds to serve military and space applications.[2].) They were first tested for commercial use in United Airlines Boeing 727 and Boeing 737 cooling turbines in the early- and mid-1960s.[3] 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.[4]
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
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
Current Generation foil bearings with advanced coatings have greatly exceeded the limitations of earlier designs, as described, for example, in the paper "Major Breakthrough in Load Capacity, Speed and Operating Temperature of Foil Thrust Bearings." [5]
[edit] References
- ^ See entry for Garrett Systems — later acquired by Honeywell.
- ^ 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.
- ^ Scholer Bangs, "Foil Bearings Help Air Passengers Keep their Cool," Power Transmission Design (Feb. 1973).
- ^ 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.
- ^ Hooshang Heshmat, "Major Breakthrough in Load Capacity, Speed and Operating Temperature of Foil Thrust Bearings," American Society of Mechanical Engineers, Technical Paper No. WT2005-63712 (September 2005), available at http://store.asme.org/product.asp?catalog_name=Conference%20Papers&category_name=%26nbsp%3b_WTC2005T-5&product_id=WTC2005-63712.
