Preloading with springs

By preloading bearings in small electric motors (up to frame size of 132) or similar applications, it is possible to reduce bearing noise levels. The bearing system in this case comprises a single row deep groove ball bearing at each end of the shaft. The simplest method of applying preload is to use a spring or spring package (fig. 1). The spring acts 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 expansion. The requisite preload force can be estimated using

F = k d
F=preload force [kN]
d=bearing bore diameter [mm]
=a factor, described below
Depending on the design of the electric motor, values of between 0,005 and 0,01 are used for the factor k. If preload is used primarily to protect the bearing from the damage caused by external vibrations when stationary, then greater preload is required and k = 0,02 should be used.
Spring loading is also a common method of applying preload to angular contact ball bearings in high-speed grinding spindles. The method is not suitable, however, for bearing applications where a high degree of stiffness is required, where the direction of load changes, or where undefined shock loads can occur.
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