Preload and internal clearance
Angular contact ball and thrust ball bearings
SKF super-precision universally matchable angular contact ball bearings, sets of angular contact ball bearings and angular contact thrust ball bearings are manufactured so that a predetermined amount of preload results when assembled immediately adjacent to each other. The preload values listed in the product tables represent the axial force required to press together the rings or washers of new unmounted bearings.
When mounted, and further when in operation, the preload will change. The main reasons are:
- An interference fit in the housing contracts the outer ring raceway while an interference fit on the shaft expands the inner ring raceway.
- Pressing the inner rings or shaft washers of bearings or bearing sets against each other causes deformation of the rings or washers. Especially when mounted on a solid shaft, the bore diameter cannot decrease and the lateral expansion increases preload.
Differences in thermal expansion of the bearing rings or washers, and mating components typically increase preload in operation.
For details about the preload in unmounted bearings and ways to estimate the preload in operation, refer to the relevant product section:
Cylindrical roller bearings
SKF super-precision cylindrical roller bearings are manufactured with radial internal clearance. Radial internal clearance is defined as the total distance through which one bearing ring can be moved relative to the other in the radial direction.
It is necessary to distinguish between initial internal clearance in the bearing prior to mounting and operating internal clearance, which applies to a bearing in operation that has reached a stable temperature.
In almost all applications, the initial clearance in a bearing is greater than its operating clearance. The difference can be attributed to interference fits on the shaft and/or in the housing, combined with thermal expansion of the bearing and mating components. In some cases, these factors can reduce clearance enough to create radial preload in the bearing.
For details about the internal clearance in new bearings prior to mounting and recommendations about clearance or preload in operation, refer to Radial internal clearance.