Selecting bearing size using the static load carrying capacity
Bearing size should be selected on the basis of static load ratings C0 instead of on bearing life when one of the following conditions exist:
- The bearing is stationary and is subjected to continuous or intermittent (shock) loads.
- The bearing makes slow oscillating or alignment movements under load.
- The bearing rotates under load at very slow speed (n < 10 r/min) and is only required to have a short life. In other words, the life equation in this case, for a given equivalent load P, would give such a low requisite basic dynamic load rating C, that the bearing selected on a life basis would be seriously overloaded in service.
- The bearing rotates and, in addition to the normal operating loads, has to sustain heavy shock loads.
In all these cases, the permissible load for the bearing is the maximum load the bearing can accommodate without permanent deformation to the rolling elements or raceways. Permanent deformation is typically caused by:
- heavy loads acting on the bearing while it is stationary or oscillating slowly
- heavy shock loads acting on the bearing while it is rotating
Depending on the operating conditions and load, the resulting damage can be flattened areas on the rolling elements or indentations on the raceways. The indentations can be irregularly spaced around the raceway, or may be evenly spaced at positions corresponding to the spacing of the rolling elements.
Permanent deformations usually lead to higher vibration and/or noise levels and increased friction. It is also possible that the internal clearance will increase or the character of the fits may be changed.
The extent to which these changes are detrimental to bearing performance depends on the demands placed on the bearing in a particular application. It is therefore necessary to make sure that permanent deformations do not occur, or only occur to a very limited extent by selecting a bearing with sufficiently high static load carrying capacity, if one of the following demands has to be satisfied:
- high reliability
- quiet running, such as for electric motors
- vibration-free operation, such as for machine tools
- constant bearing frictional moment, such as for measuring apparatus and test equipment
- low starting friction under load, such as for cranes