Reducing friction in bearings is often important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and premature failure of the bearing. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.
- By shape, gains advantage usually by using spheres or rollers, or by forming flexure bearings.
- By material exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
- By fluid exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.
- By fields exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.
- Air pressure exploits air pressure to keep solid parts from touching.
Combinations of these can even be employed within the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.
The frictional properties of any plain bearing depend on the lubrication system. Either hydrodynamic or hydrostatic lubrication can provide low friction. Gas bearings have the lowest friction levels.
Friction in hydrodynamic and hydrostatic bearings is a function of lubricant viscosity and shear rate. Shear rate increases with increasing rotational speed and decreasing film thickness. Friction coefficient is generally below 0.001.
Self-lubricated bearings vary widely. It is difficult to predict performance for a given bearing/lubricant system. The range of coefficients of friction is 0.01 to 0.10 for boundary lubrication and 0.01 to 0.3 for self-lubrication.
Caution must be used when applying friction coefficient handbook data. Conditions under which the values were measured should be known and duplicated in the application. Coefficient of friction tends to increase with increasing surface roughness, dryness, and cleanliness of surfaces, and decreasing temperature.