A foil bearing, also known as a foil-air bearing, is a type of
air bearing
Air bearings (also known as aerostatic or aerodynamic bearings) are fluid bearings that use a thin film of pressurized gas to provide a low friction load-bearing interface between surfaces. The two surfaces do not touch, thus avoiding the tradit ...
. A shaft is supported by a compliant, spring-loaded
foil
Foil may refer to:
Materials
* Foil (metal), a quite thin sheet of metal, usually manufactured with a rolling mill machine
* Metal leaf, a very thin sheet of decorative metal
* Aluminium foil, a type of wrapping for food
* Tin foil, metal foil ...
journal lining. Once the shaft is spinning fast enough, the working
fluid
In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
(usually
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
) 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
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.
They were first tested for commercial use in
United Airlines
United Airlines, Inc. (commonly referred to as United), is a major American airline headquartered at the Willis Tower in Chicago, Illinois. Boeing 727
The Boeing 727 is an American narrow-body airliner that was developed and produced by Boeing Commercial Airplanes.
After the heavy 707 quad-jet was introduced in 1958, Boeing addressed the demand for shorter flight lengths from smaller air ...
and
Boeing 737
The Boeing 737 is a narrow-body aircraft produced by Boeing at its Renton Factory in Washington.
Developed to supplement the Boeing 727 on short and thin routes, the twinjet retains the 707 fuselage width and six abreast seating with two u ...
cooling turbines in the early and mid-1960s.
Garrett AiResearch
air cycle machine
An air cycle machine (ACM) is the refrigeration unit of the environmental control system (ECS) used in pressurized gas turbine-powered aircraft. Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred as ...
foil bearings were first installed as original equipment in 1969 in the
DC-10
The McDonnell Douglas DC-10 is an American trijet wide-body aircraft manufactured by McDonnell Douglas.
The DC-10 was intended to succeed the DC-8 for long-range flights. It first flew on August 29, 1970; it was introduced on August 5, 19 ...
'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
In physics, cryogenics is the production and behaviour of materials at very low temperatures.
The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...
gas temperatures and at very high temperatures gave foil bearings many other potential applications.
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.
Applications
Turbomachinery is the most common application because foil bearings operate at high speed.
Commercial applications in production include microturbines,
fuel cell blowers, and air cycle machines. The main advantage of foil bearings is the elimination of the
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
systems required by traditional bearing designs. Other advantages are:
* Higher efficiency, due to a lower heat loss to friction; instead of
fluid friction
In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding ...
, the main source of heat is
parasitic drag
Parasitic drag, also known as profile drag, is a type of aerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is a combination of form drag and skin friction drag. It affects all objects regardless of ...
* Increased reliability
* Higher speed capability
* Quieter operation
* Wider
operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
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
*
*
References
{{reflist,
[ 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.]
External links
NASA Glenn Research Center "Creating a Turbomachinery Revolution"NASA Tribology & Mechanical Components BranchR&D Dynamics Corporation Foil bearing supported high speed turbomachineryMohawk Innovative Technology, Inc.Tribology Group at Texas A&MOil-Free Machinery, LLC
Bearings (mechanical)
Gas turbine technology