Selecting preload


Depending on the application, there may be a need to preload a bearing arrangement. For example, if a high degree of stiffness or positional control is required then preload may be suitable. Similarly, where there is a very light or no external load on the bearing in operation then preload may be required to ensure a minimum load.
Applying the preload is typically done by measuring a force, sometimes a displacement over a distance or path, or by measuring the frictional torque during mounting.
Empirical preload values can be obtained from proven designs and can be applied to similar designs. For new designs, SKF recommends calculating the appropriate preload range by using SKF SimPro Quick or SimPro Expert and then checking it by testing in the application. The agreement between the calculation and the actual application depend on how closely the estimated operating temperature and elastic behaviour of the associated components – most importantly the housing – coincide with the actual conditions in operation. In this context, the effects of start-up at low ambient temperature must be included in the testing.

Considerations for preload

Depending on the bearing type, preload may be either radial or axial. Super-precision cylindrical roller bearings, for example, can only be preloaded radially because of their design, while angular contact ball bearings or tapered roller bearings can only be preloaded axially.
Single tapered roller bearings or angular contact ball bearings are generally mounted together with a second bearing of the same type and size in a back-to-back (load lines diverge, fig. 1) or face-to-face (load lines converge, fig. 2) arrangement. The same is true for single row angular contact ball bearings.
The distance L between the pressure centres is longer when the bearings are arranged back­-to­-back (fig. 1) compared to bearings that are arranged face­-to-face (fig. 2). The back­-to­-back arrangement can carry larger tilting moments.
If the shaft temperature in operation is higher than the housing temperature, the preload, which was adjusted at ambient temperature during mounting, will change. Since thermal growth of a shaft makes it larger both in the axial and in the radial direction, the back-to-back arrangements are less sensitive to thermal effects than the face-to-face arrangements.
When adjusting preload in a bearing system, it is important that the established preload value is attained with the least possible variation. To reduce variation when mounting tapered roller bearings, the shaft should be turned several times to ensure that the rollers are in correct contact with the guide flange of the inner ring.

Preloading with springs

By preloading bearings it is possible to reduce the noise in, for example, small electric motors or similar applications. In this example, the bearing arrangement comprises a preloaded single row deep groove ball bearing at each end of the shaft (fig. 3). The simplest method of applying preload is to use a wave spring. The spring acts on the outer ring of one of the two bearings. This outer ring must be able to be axially displaced.
The preload force remains practically constant, even when there is axial displacement of the bearing as a result of thermal elongation.
The requisite preload force can be estimated using 

F = k d

=preload force [kN]

=a factor, described below
=bearing bore diameter [mm]

For small electric motors, 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 for bearing applications where a high degree of stiffness is required, where the direction of axial load changes, or where undefined peak loads can occur.
For more information, refer to Bearing preload [PDF].
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