Friction. Plain bearings are simply a hole of the correct shape containing the relatively moving part. It uses surfaces in rubbing contact –often with a lubricant such as oil or graphite. Friction Plain bearings are very widely used. Particularly with lubrication, they often give entirely acceptable life and friction; however, reducing friction in bearings is often important for efficiency –to reduce wear, 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 of this would be using plastics that have low surface friction. By fluid – utilizes 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 –uses electromagnetic fields to keep solid parts from touching. Combinations of these can even be employed within the same bearing for instance, the cage is made of plastic and it separates the rollers/balls which reduces friction by their shape and finish.
Speed. Different bearing types have different operating speed limits. Speed is typically specified as maximum relative surface speeds, often specified ft/s or m/s. Rotational bearings classically describe performance in terms of the product DN where D is the diameter (often in mm) of the bearing and N is the rotation rate in revolutions per minute. Generally, there is considerable speed range overlap between bearing types. Plain bearings normally handle only lower speeds. Rolling element bearings are faster, followed by fluid bearings and finally, magnetic bearings which are limited ultimately by centripetal force overcoming material strength. Stiffness A second source of motion is elasticity in the bearing itself. For example, the balls in a ball bearing are like stiff rubber, and under load deform from round to a slightly flattened shape. The race is also elastic and develops a slight dent where the ball presses on it. The stiffness of a bearing is how the distance between the parts which are separated by the bearing varies with applied load. With rolling element bearings, this is due to the strain of the ball and race. With fluid bearings, it is due to how the pressure of the fluid varies with the gap (when correctly loaded fluid bearings are predictably stiffer than rolling element bearings).
Maintenance. Many bearings require periodic maintenance to prevent premature failure; though some such as fluid or magnetic bearings may require little maintenance. Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimize the effects of wear.