Angular contact ball bearings
Single row angular contact ball bearings are generally mounted as sets, in a back-to-back (fig. 1, fig. 2) or face-to-face arrangement (fig. 3) that is normally subjected to an axial preload. The preload is produced by displacing one bearing ring axially, relative to the other (fig. 1, fig. 3), by an amount corresponding to the desired preload force or by springs (fig. 2).
The standout of matched and universally matchable bearings is precision ground so that when two bearings are mounted immediately adjacent to each other, a given preload is obtained without further adjustment. Keep in mind that this preload is also influenced by the interference fit and the operating conditions. For additional information, refer to Preload in mounted bearing sets.
If it is necessary to change the preload, spacers between the bearing rings can be used. For additional information, refer to Individual adjustment of preload.
Influence of an external load on preloaded bearing sets
The influence of an external axial load on preloaded bearing sets is illustrated in diagram 1. The curves represent the spring characteristics of two bearings in a back-to-back arrangement. The blue curve represents bearing A, which is subjected to an external axial force Ka. The red curve represents bearing B, which becomes unloaded by the axial force.
The two bearings are each preloaded by an axial displacement δ0 of one bearing ring relative to the other, resulting in a preload force F0 acting on both bearings. When bearing A is subjected to an external axial force Ka, the load on the bearing increases to FaA while load on bearing B decreases to FaB. Axial displacement of the bearing rings follows the spring curves. δKa is the displacement of the bearing set while δKb is the remaining preload [µm] on bearing B.
When the axial forces on the spindle reach the natural lifting force Ka1, bearing B becomes completely unloaded. When this happens, there is a significant risk that the unloaded balls stop rolling and start skidding, which if it occurs for any length of time will result in premature bearing failure.
The lifting force varies depending on the preload and bearing arrangement (table 1). It is possible to avoid the lifting force phenomena in one of two ways: either increase the preload, or use bearing sets with different contact angles. For additional information, contact the SKF application engineering service.
Preloading with springs
Using springs to apply preload to angular contact ball bearings is common, especially in high-speed grinding spindles. The springs act on the outer ring of one of the two bearings. This outer ring must be able to be displaced axially. The preload force remains practically constant, even when there is axial displacement of the bearing as a result of thermal shaft expansion. For additional information concerning preloading with springs and values for preload force, refer to Preload with a constant force.
Preloading with springs is not suitable for applications where a high degree of stiffness is required, where the direction of load changes, or where indeterminate shock loads can occur.