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Foil bearing

Type of air bearing

Foil bearing

Type of air bearing

Foil-air bearing
A foil-air bearing for the core rotor shaft of an aircraft turbine engine.

A foil bearing, also known as a foil-air bearing, is a type of air bearing. A shaft is supported by a compliant, spring-loaded journal lining made of foil. 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

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
Foil Bearing
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.
Load capacity against rotation speed, for Gen I and Gen III bearings

Foil bearings were first developed in the late 1950s by AiResearch Mfg. Co. of the Garrett Corporation using independent R&D funds to serve military and space applications.{{cite journal |archive-url = https://web.archive.org/web/20160415030128/http://www.rddynamics.com/pdfs/Foil-Bearings-Cleared-to-Land.pdf |archive-date=15 April 2016 |url-access=subscription}}

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.{{cite journal |author-link=Hooshang Heshmat |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

Applications

Turbomachinery is the most common application because foil bearings operate at high speed.{{cite journal |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 Commercial applications in production include microturbines,{{cite conference |book-title=Proceedings of the ASME Turbo Expo 2006: Power for Land, Sea, and Air |book-title=Proceedings of the ASME Turbo Expo 2025: Power for Land, Sea, and Air |book-title=Proceedings of the ASME Turbo Expo 2025: Power for Land, Sea, and Air

  • Higher efficiency, due to a lower heat loss to friction; instead of fluid friction, the main source of heat is parasitic drag
  • Increased reliability
  • Higher speed capability
  • Quieter operation
  • Wider operating temperature range (40–2,500 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

References

References

  1. (13 April 2020). "Compressors with foil bearings applied in fuel cells".
  2. (31 March 2025). "Oil-free Gas Foil Bearing Chiller".
  3. "ECC Refrigeration Compressor".
  4. 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.
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bearings-(mechanical) gas-turbine-technology
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