Bearings and Lubricants

Bearing Loads

Load and operating speed affect bearings. Service life is determined statistically from tests of numerous samples and is generally selected on the basis of a 10% failure rate (90% reliability). The result is the B₁₀ or L₁₀ life.

Hydrodynamic and hydrostatic bearings, however, operate with infinite service life below some critical value of load and speed. With self-acting, oil-lubricated (hydrodynamic) sleeve bearings, load capacity increases linearly with speed since rotation builds the supporting lubricating film.

For extremely pressurized (hydrostatic) sleeve bearings, there is a particular load capacity for infinite life, and this capacity is essentially unaffected by speed -- although at times there may be some hydrodynamic or speed-dependent contribution to load capacity.

If loads exceed the infinite-life value, the surfaces are assumed to come into rubbing contact, thereby setting up a condition of boundary lubrication. This condition often leads to accelerated wear and failure. However, bearings can be designed to operate satisfactorily in the boundary mode.

As a result of these interactions, rolling-element bearings are less sensitive than plain bearings to load variations. And because they do not rely on velocity effects to maintain a fluid film, they are well suited to supporting heavy load at low speeds. Fluid-film bearings tend to be a better choice if load increases with an increase in speed or if the load is dynamic. On the other hand, pulsating heavy loads on stationary rolling-element bearings can cause brinelling.

Both rolling-element and plain bearings can be vulnerable to severe impact loads. Often a case is presented that rolling-element bearings are more vulnerable because of their line or point contact and resulting high contact stress. The fluid film in plain bearings is said to provide a better "cushion" for impact. More realistically, however, resistance to impact for both types depends upon reserve capacity. Either type of bering can be designed to sustain high impact.

Bearings and Lubricants