What are Fluid Film Bearings?

The term describes a bearing system where the rotor or slider is separated from the stationary bearing surface, by a film of fluid. The working fluid can be a gas, most usually air, when they are commonly known as air bearings, or a liquid, most usually oil. Fluid Film Devices Ltd (FFD) design and manufacture bearings using any stable fluid.

All fluid film bearings rely for their method of working on the principle that an average mean pressure is created in the bearing clearance, which, when acting on the projected bearing area, enables a load to be supported. This latter statement is a simplification and ignores many other important parameters such as bearing stiffness and rotor stability, to name but a few.

The essential difference between using gas or a liquid, lays in the intrinsic properties of the two fluids. The viscosity of a gas changes little over a wide temperature range. The properties of a gas do not deteriorate with time and are stable in harsh ambient environments. Liquids generally do not have these properties. Air for example, has a coefficient of viscosity that increases by a factor of only1.7 over a temperature range of 300º C and is one fiftieth of water at NTP. Conversely liquids having a generally higher coefficient of viscosity can be used to advantage because higher film pressures can be generated within a bearing film and hence higher unit loads can be supported. The bearing specification and the needs of the end product critically dictate which fluid should be used.

Gases are compressible and liquids are not. The compressibility of a gas can lead to problems of self - excited instability within the gas film; whereas liquids can suffer from cavitation. Avoiding these latter problems is a function of detail design technique.

There are two main types of Fluid Film Bearing. One type relies on a supply of pressurised fluid being fed to the bearing, more commonly called Aerostatic or Hydrostatic Bearings, dependant on whether a gas or liquid is used. The other type relies on relative motion between the bearing surfaces to generate a pressure within the bearing film and are more commonly called Aerodynamic or Hydrodynamic Bearings . FFD design and manufacture either type.

There are many other types of Fluid Film Bearing such as Foil Bearings, sometimes known as compliant bearings, but they lay outside the scope of this discussion.