The size of a bearing must be sufficient to secure that it is strong enough to deliver the required/expected life under defined operating conditions.
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 ( fig. 2 ). Consider these aspects:
Lbearing = f (Lraceways, Lrolling elements, Lcage, Llubricant, Lseals)
fig. 1 - Bearing system life
fig. 2 - Performance and related bearing system components
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
The operating conditions of the application determine which of these factors most influence the performance and life of the bearing.
This section provides guidance on determining the required bearing size.
The effect of RCF or permanent deformation on rolling elements and raceways is directly related to bearing size. Effects of cage type and material are not related to bearing size. In capped bearings, the effects of the lubricant and integral seal are only indirectly related to bearing size.
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 rating life for the bearing under the expected operating conditions (Bearing rating life).
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.
fig. 3 shows these selection criteria and the related bearing ratings and static safety factor, which are described in detail in the relevant sub-sections.
fig. 3 - Main selection criteria for bearing size and related bearing ratings and safety factor
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.
Note that there are applications where both selection criteria must be considered, for example where a duty cycle has occasional peak loads. Also, in applications where the bearing is lightly loaded, the minimum load requirement must also be considered.
After determining bearing size, and before going to the next step, check the items listed in Checklist after the bearing size is determined.
Other attributes of the bearing components, such as strength and suitability, are addressed elsewhere in the Bearing selection process, including Lubrication and Bearing execution, as well as in the product sections. Consider these attributes, in addition to bearing size, to safeguard you obtain best bearing performance.
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