Selecting internal clearance or preload

The operating clearance/preload range in a bearing is determined by:
  • the specified initial internal clearance/preload class of the bearing(s)
  • the actual fits or the distance a bearing is driven up on a tapered seat
  • the effects of form errors
  • the resulting clearance/preload after one bearing is adjusted against another
  • the dimensional changes resulting from the operating temperature variations (start-up but also seasonal or application based)
Shaft deflection and axial displacement between inner and outer ring, such as in CARB toroidal roller bearings, might also need to be considered.
Among other factors, the operating clearance/preload in a bearing influences friction, size of the load zone and fatigue life. Diagram 1 shows the relationship between clearance/preload and the main operational criteria. The diagram is generalized and based on rolling bearings running at normal operating conditions and purely radially loaded.

The variation of interference due to the fits, as well as operational temperature differences, result in a bearing operational clearance/preload range that must be considered when selecting the appropriate bearing clearance/preload class. For general applications, bearings should operate with clearance.
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