Foil bearing

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 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.

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

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.[1][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 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.[4]

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.[5]

Applications

Turbomachinery is the most common application because foil bearings operate at high speed.[6] Commercial applications in production include microturbines,[7] fuel cell blowers,[8] and air cycle machines. 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 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

See also
  • Fluid bearing – Type of bearings which use pressurized liquid or gas between the bearing surfaces
  • Tribology – Science and engineering of interacting surfaces in relative motion

References
  1. Giri L. Agrawal (1997). "Foil Air/Gas Bearing Technology — An Overview" (PDF). Publication 97-GT-347. American Society of Mechanical Engineers.
  2. Giri L. Agrawal (July 1998). "Foil Bearings Cleared to Land" (PDF). Mechanical Engineering. Vol. 120, no. 7. Archived from the original (PDF) on 15 April 2016 – via R&D Dynamics.
  3. Scholer Bangs (February 1973). "Foil Bearings Help Air Passengers Keep their Cool". Power Transmission Design.
  4. M. A. Barnett; A. Silver (September 1970). "Application of Air Bearings to High-Speed Turbomachinery". Technical Paper No. 700720. SAE Technical Paper Series. Society of Automotive Engineers. 1. doi:10.4271/700720. 700720.
  5. Heshmat, Hooshang (September 2005). "Major Breakthrough in Load Capacity, Speed and Operating Temperature of Foil Thrust Bearings". Technical Paper No. WT2005-63712. American Society of Mechanical Engineers. WT2005-63712. Archived from the original on 2008-02-14. Retrieved 2006-09-25.
  6. R. M. "Fred" Klaass; Christopher DellaCorte (2006). "The Quest for Oil-Free Gas Turbine Engines". SAE Technical Papers. SAE. 2006-01-3055. Archived from the original on 2007-09-30. Retrieved 2007-08-18.
  7. Lubell, D.; DellaCorte, C.; Stanford, M. (2006). "Test Evolution and Oil-Free Engine Experience of a High Temperature Foil Air Bearing Coating". Proceedings of the ASME Turbo Expo 2006: Power for Land, Sea, and Air. Vol. 5: Marine, Microturbines and Small Turbomachinery, Oil and Gas Applications, Structures and Dynamics, Parts A and B. ASME. pp. 1245–1249. doi:10.1115/GT2006-90572. ISBN 0-7918-4240-1. GT2006-90572.
  8. "Compressors with foil bearings applied in fuel cells". 13 April 2020.
  9. 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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