I. What is Bearing Preload?

Bearing preload refers to the process of generating an initial pressure and pre-deformation between the rolling elements and the inner/outer rings during bearing installation. This ensures both rings remain in a compressed state, enabling the bearing to operate under negative clearance when subjected to working loads.

II. What is the Purpose of Bearing Preload?

The objectives of bearing preload are: to enhance bearing rigidity; to ensure correct axial and radial positioning of the rotating shaft, thereby improving shaft rotational accuracy; to reduce shaft vibration and noise; to minimise sliding of the rolling elements relative to the inner and outer raceways caused by inertia torque; to compensate for changes in internal bearing clearance due to wear; and to extend bearing service life. III. What are the methods of bearing preload direction?

Bearings may be categorised by preload direction into axial preload and radial preload. In practical applications, ball bearings predominantly employ axial preload, while cylindrical roller bearings utilise radial preload. When installing paired angular contact ball bearings or tapered roller bearings of identical specifications, axial preload is further subdivided into locating preload and pressure-setting preload based on the method of applying preload.

1. Positioning Preload

Positioning preload refers to an axial preloading method where the bearing's axial position remains unchanged during operation. A specific preload can be achieved by adjusting the width of the spacer between the two bearings.

If the preload is too small, the intended purpose is not achieved. However, excessively high preload does not significantly enhance bearing stiffness; instead, it increases friction within the bearing, raises operating temperatures, and reduces bearing service life. The magnitude of the preload should be determined based on load conditions and operational requirements. Generally, a lighter preload is selected for high-speed, light-load conditions, or to reduce support system vibration and enhance rotational precision. For medium-speed, medium-load or low-speed, heavy-load conditions, as well as to increase support system rigidity, medium or heavy preloads are employed. The magnitude of preload should generally be determined through calculation combined with operational experience.

2. Constant Pressure Preload

Constant pressure preload refers to an axial preloading method where a spring maintains a constant axial preload on the bearing during operation. A specific preload can be achieved by adjusting the spring's compression.

Compared to locating preload, constant pressure preload does not significantly increase the axial stiffness of the support system for the same preload deformation. However, factors such as axial length changes due to temperature differences between the shaft and bearing housing, or radial expansion caused by temperature differences between the inner and outer rings, can affect preload deformation in locating preload scenarios. These factors do not influence constant pressure preload. Therefore, the preload method must be selected based on specific technical requirements. Typically, locating preload is selected when high rigidity is required, while radial preload is chosen for high-speed operation.

3. Radial Preload

Radial preload refers to the method of eliminating radial clearance and inducing pre-deformation by expanding the bearing inner ring through an interference fit between the bearing and shaft journal.

Radial preloading enhances support rigidity. In high-speed cylindrical roller bearings, it reduces slippage between rolling elements and raceways under centrifugal forces. For bearings with tapered inner bores, radial preloading is achieved by adjusting the relative position of the inner ring and locking collar via a locking nut, thereby diminishing radial clearance.