Foil bearing

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.

Development

Foil bearings were first developed in the late 1950s by AiResearch Mfg. Co. of the Garrett Corporation [See entry for Garrett Systems — later acquired by Honeywell.] using independent R&D funds to serve military and space applications. [Some early history is reported in Giri L. Agrawal, "Foil Air/Gas Bearing Technology — An Overview," [http://www.rddynamics.com/foil-97-gt-347.pdf "American Society for Mechanical Engineers", Publication 97-GT-347 (1997)] and Giri L. Agrawal, "Foil Bearings Cleared to Land," [http://www.memagazine.org/backissues/membersonly/july98/features/foil/foil.html "Mechanical Engineering" 120 (July 1998)] : 1978-80.] 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.]

Current Generation foil bearings with advanced coatings have greatly exceeded the limitations of earlier designs. 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. [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.]

Applications

Turbomachinery is the most common application because foil bearings operate at high speed. [R. M. "Fred" Klaass and Christopher DellaCorte, "The Quest for Oil-Free Gas Turbine Engines," "SAE Technical Papers", No. 2006-01-3055, available at: http://www.sae.org/technical/papers/2006-01-3055. The authors report: "Since the 1960s, aerospace research and development (R&D) has been on a quest to eliminate oil lubrication systems from gas turbine engines. Beginning with small solar power dynamic engines for space applications, U.S. Government and industry have invested millions of dollars to mature this technology for incorporation into modern aircraft propulsion engines. . . . However, this technology has yet to be fielded in aerospace [propulsion] products."] The main advantage of foil bearings is the elimination of the oil systems required by traditional bearing designs. Other advantages are:
* More Efficient (Less losses)
* Increased reliability
* Higher speed capability
* Quieter operation
* Higher and lower temperature capability (40 K to 2500 K)
* High vibration and shock load capacity
* No scheduled maintenance
* No external support system
* Truly oil free where contamination is an issue

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 and stopping
* High speed required for operation
* Difficulty operating above critical speed

References

ee also

* Fluid bearing

External links

* [http://www.rddynamics.com/foil.html R&D Dynamics Corporation]
* [http://www.miti.cc/foil-bearings.html Mohawk Innovative Technology, Inc.]
* [http://www.grc.nasa.gov/WWW/Oilfree/bearings.htm NASA Foil Bearing Research]
* [http://www.nasa.gov/centers/glenn/about/fs14grc.html NASA Glenn Research Center]
* [http://phn.tamu.edu/TRIBGroup/ Tribology Group at Texas A&M]
* [http://ubisa.kist.re.kr/Teams/ubisa/afb.htm Korea Inst. Science and Technology]
* [http://tribo.kaist.ac.kr/bbs/res1c_1_e.html KAIST korean tribology lab]
* [http://www.kturbo.com/english/newhome/Core_Technology/air_bearing.html KTURBO]
* [http://www.neuros.com/02product/product_01.htm Neuros]