A bearing can be viewed as a system of components: raceways, rolling elements, cage, seals (if present) and lubricant (fig. 1). The performance of each component contributes to or determines the performance and life of the bearing (diagram 1). Consider these aspects:
rolling contact fatigue (RCF) on the rolling elements and raceways – this is the primary aspect that dictates bearing life in most applications
permanent deformation of rolling elements and raceways because of heavy loads acting on the bearing, while it is stationary or oscillating slowly, or high peak loads acting on the bearing while it is rotating
cage type or cage material – these may limit the operating speed or the permissible acceleration or temperature1)
speed limit of contacting seal lips – this can determine the maximum allowable speed, which affects operating temperature, thereby affecting life
- lubricant life – when the lubricant deteriorates, the resulting poor relubrication condition quickly reduces bearing life
Therefore, the two main criteria that can be used for determining appropriate bearing size are:
Size selection based on rating life: This is based on the required bearing life, taking into account the possible effects of rolling contact fatigue, and requires calculation of the basic rating life L10, or SKF rating life L10m, for the bearing.
Size selection based on static load: This is based on the static load that the bearing can accommodate, taking into account the possible effects of permanent deformation, and requires calculation of the static safety factor s0 for the bearing.
Which selection criteria you should use depends on the operating conditions of the bearing:
For applications where bearings are running in typical operating conditions – i.e. normal speeds, good lubrication conditions and not highly or peak loaded – use Size selection based on rating life.
- For applications where bearings are running under very low speeds or which are used under stationary conditions, very bad lubrication conditions or where occasional peak loads occur, use Size selection based on static load.
- 1)Special cage executions are often available for bearing types that are commonly used in applications where such challenging conditions are present.