Rolling bearings are generally used with proper internal clearance during operation. However, depending on the application, axial loads are sometimes applied to the bearing during installation to cause negative internal clearance. This method is called preload

and is mostly suitable for angular contact ball bearings or tapered roller bearings.
1. The purpose of preloading
● Improve axial and radial positioning accuracy, reduce shaft runout and improve rotation accuracy.
● Improve bearing rigidity and gear meshing accuracy;
● Suppression of rolling sliding, revolution sliding and rotation sliding of rolling elements to reduce scratches;
● Prevent bearing abnormal noise caused by vibration and resonance;
● Ensure that the relative position of the rolling element and the ferrule is correct.
2. Preloading method
There are two methods of applying pretension: positioning pretension and constant pressure pretension. The representative examples are shown in Table 1.
Table 1 Preloading methods
Comparison of positioning preload and constant pressure preload:
● When the preload force is the same, the axial displacement of the positioning preload bearing is small, and it is easy to obtain high rigidity;
● The constant pressure preload can use the spring to absorb the load change and the shaft expansion and contraction caused by the temperature difference between the shaft and the shell during operation. Therefore, the change of the preload force is small, and a stable preload force can be obtained;
● Positioning preload can apply large preload.
Therefore, positioning pretension should be used when high rigidity is required. For thrust bearings that require high-speed rotation, prevent axial vibration, and are used for horizontal shafts, a constant pressure preload should be used.
3. Preload and rigidity
For angular contact ball bearings and tapered roller bearings, when preload is applied to increase rigidity, most of them use back-to-back assembled bearings. The back-to-back assembly bearing has a large distance between the points of action and the shaft system has strong rigidity. The relationship between preload force (positioning preload) and rigidity (axial displacement) of back-to-back assembled bearings is shown in Figure 1.
Figure 1 Preload curve of positioning preload
In FIG. 1, when a pre-tensioning force P is applied (the inner ring is axially locked), the bearing A and the bearing B respectively generate an axial displacement δao, so that the gap 2δao between the inner rings becomes 0. As in Fig. 1, the preload force and rigidity of the assembled bearing when applying constant pressure preload are shown in Fig. 2.
Since the rigidity of the spring is negligible, the rigidity of the assembled bearing is almost equal to the rigidity of the single bearing applying the preload P at this time.
4. Preload
The magnitude of the preload force must be determined within the range that has no adverse effects on bearing life, temperature rise, friction torque, etc. according to the different preload purposes. In addition, when determining the preload force, factors such as the reduction in preload force after running-in, the accuracy of the shaft and housing, installation conditions, and lubrication conditions must be considered.