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[[Image:Foilbearing.jpg|thumb|A foil-air bearing for the core rotor shaft of an aircraft turbine engine.| alt= Foil-air bearing ]]

A '''foil bearing''', also known as a '''foil-air bearing''', is a type of [[fluid bearing#Air bearings|air bearing]]. A shaft is supported by a compliant, spring-loaded [[foil (metal)|foil]] journal lining. Once the shaft is spinning fast enough, the working [[fluid]] (usually [[air]]) pushes the foil away from the shaft so that no contact occurs. The shaft and foil are separated by the air's high pressure, which is generated by the rotation that pulls gas into the bearing via viscosity effects. The 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 aerostatic or [[hydrostatic bearings]], foil bearings require no external pressurisation system for the working fluid, so the hydrodynamic bearing is self-starting.

==Development==
[[Image:FoilBearing.png|right|thumb|Foil Bearing | alt= Sectional diagram of a foil bearing, showing the component parts (inner, moving outwards) of the shaft journal, a smooth top foil, the bump foil (both foils joined) and finally the bearing housing ]]
[[Image:Foil Bearing Capacity.png|thumb| Load capacity against rotation speed, for Gen I and Gen III bearings | alt= Graph of load capacity against speed for first and third generation bearings: The load is proportional to rotation speed, bearing length, and the square of shaft diameter. Third-generation bearings carry about three times as much load as first-generation ones. ]]

Foil bearings were first developed in the late 1950s by AiResearch Mfg. Co. of the [[Garrett AiResearch|Garrett Corporation]] using independent [[R&D]] funds to serve military and space applications.<ref >{{cite journal
|author=Giri L. Agrawal
|title=Foil Air/Gas Bearing Technology — An Overview
|url= http://www.rddynamics.com/pdfs/foil-97-gt-347.pdf
|publisher=[[American Society of Mechanical Engineers]]
|journal=Publication 97-GT-347
|year=1997
}}</ref><ref >{{cite journal
|author=Giri L. Agrawal
|title=Foil Bearings Cleared to Land
|url=http://www.rddynamics.com/pdfs/Foil-Bearings-Cleared-to-Land.pdf
|issue=120
|date=July 1998
|volume=1978-1980
|archiveurl = https://web.archive.org/web/20160415030128/http://www.rddynamics.com/pdfs/Foil-Bearings-Cleared-to-Land.pdf
|archivedate=15 April 2016
}}</ref> They were first tested for commercial use in [[United Airlines]] [[Boeing 727]] and [[Boeing 737]] cooling turbines in the early and mid-1960s.<ref >{{cite journal
|author=Scholer Bangs
|title=Foil Bearings Help Air Passengers Keep their Cool
|journal=Power Transmission Design
|date=February 1973
}}</ref> Garrett AiResearch [[air cycle machine]] foil bearings were first installed as original equipment in 1969 in the [[DC-10]]'s [[Environmental Control System|environmental control systems]]. Garrett AiResearch foil bearings were installed on all US military aircraft to replace existing oil-lubricated rolling-contact bearings. The ability to operate at [[cryogenic]] gas temperatures and at very high temperatures gave foil bearings many other potential applications.<ref >{{cite journal
|authors=M. A. Barnett and A. Silver
|title=Application of Air Bearings to High-Speed Turbomachinery
|publisher=[[Society of Automotive Engineers]]
|journal=Technical Paper No. 700720
|id=700720
|date=September 1970
|url=http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=700720
}}</ref>

Current-generation foil bearings with advanced coatings have greatly exceeded the limitations of earlier designs. Antiwear coatings exist that allow over 100,000 start/stop cycles for typical applications.<ref >{{cite journal
|first=Hooshang
|last=Heshmat
|authorlink=Hooshang Heshmat
|title=Major Breakthrough in Load Capacity, Speed and Operating Temperature of Foil Thrust Bearings
|publisher=[[American Society of Mechanical Engineers]]
|journal=Technical Paper No. WT2005-63712
|id=WT2005-63712
|date=September 2005
|url=http://store.asme.org/product.asp?catalog_name=Conference%20Papers&category_name=%26nbsp%3b_WTC2005T-5&product_id=WTC2005-63712
|access-date=2006-09-25
|archive-url=https://web.archive.org/web/20080214192746/http://store.asme.org/product.asp?catalog_name=Conference%20Papers&category_name=%26nbsp%3B_WTC2005T-5&product_id=WTC2005-63712
|archive-date=2008-02-14
|url-status=dead
}}</ref>

==Applications==
[[Turbomachinery]] is the most common application because foil bearings operate at high speed.<ref >{{cite journal
|authors=R. M. "Fred" Klaass and Christopher DellaCorte
|title=The Quest for Oil-Free Gas Turbine Engines
|publisher=[[Society of Automobile Engineers|SAE]]
|journal=SAE Technical Papers
|id=2006-01-3055
|year=2006
|url=http://www.sae.org/technical/papers/2006-01-3055
|access-date=2007-08-18
|archive-url=https://web.archive.org/web/20070930201550/http://www.sae.org/technical/papers/2006-01-3055
|archive-date=2007-09-30
|url-status=dead
}}</ref> The main advantage of foil bearings is the elimination of the [[oil]] systems required by traditional bearing designs. Other advantages are:
* Higher efficiency, due to a lower heat loss to friction; instead of [[friction|fluid friction]], the main source of heat is [[parasitic drag]]
* Increased reliability
* Higher speed capability
* Quieter operation
* Wider [[operating temperature]] range (40–2,500 [[Kelvin|K]])
* High vibration and shock load capacity
* No scheduled maintenance
* No external support system
* Truly oil free where contamination is an issue
* Capable of operating above [[critical speed]]

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

==See also==
* [[Fluid bearing]]
* [[Tribology]]

==References==
{{reflist|
<ref> Some early history is reported in Giri L. Agrawal (1997), "http://www.rddynamics.com/pdfs/foil-97-gt-347.pdf — An Overview" (PDF). Publication 97-GT-347. American Society of Mechanical Engineers.</ref>
}}

==External links==
*[http://www.nasa.gov/centers/glenn/about/fs14grc.html NASA Glenn Research Center "Creating a Turbomachinery Revolution"]
*[http://www.grc.nasa.gov/WWW/StructuresMaterials/TribMech/ NASA Tribology & Mechanical Components Branch]
*[http://www.rddynamics.com R&D Dynamics Corporation Foil bearing supported high speed turbomachinery]
*[http://www.uta.edu/mae/turboenergy/research.html Turbomachinery and Energy System Laboratory at UTA]
*[http://www.mohawkinnovative.com/products/compliant-foil-bearings/ Mohawk Innovative Technology, Inc.]
*[https://web.archive.org/web/20110720055621/http://rotorlab.tamu.edu/Tribgroup/ Tribology Group at Texas A&M]
*[https://web.archive.org/web/20080726082137/http://ubisa.kist.re.kr/Teams/ubisa/afb.htm Korean Institute for Science and Technology KIST]
*[https://web.archive.org/web/20100707181721/http://www.cerom.lsu.edu/projects_foil.htm Center for Rotating Machinery at LSU]

[[Category:Bearings (mechanical)]]
[[Category:Gas turbine technology]]

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