Oct 29 2024
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Uses air as the medium between moving part and stationary housing or vice-a-versa to ensure contactless motion. There are two major types of air bearings:

  1. Aerostatic
  2. Aerodynamic
    1. Aerostatic: A separate external supply of (compressed) air is supplied between the two surfaces being kept apart. The air cushion is formed by releasing pressurized air through orifices. This creates a consistent air film between the bearing surfaces. Continuous air supply ensures that the cushion is maintained, which is crucial for the high precision operation. For example, in linear motion in CMM (co-ordinate measuring machine), the carriage housing floats to traverse over the rails providing non-contact movement.  Air bearings’ ability to offer almost zero friction means measurements are not only precise but also consistent over repeated operations.


2. Aerodynamic: the air film is created by relative motion of two mating surfaces separated by a small distance.  These are self-acting type, and no external pressurized air supply is needed. For example, in an air foil 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. This is also called as Hydrodynamic type bearing. 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.

Turbo machines are the most common application because foil bearings operate at high speed.

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

The main disadvantages are:

  • High speed required for operation
  • Lower capacity than roller or oil bearings
  • Wear during start-up and stopping