Methods for setting preload in a bearing system

Adjustment

Adjustment means setting the internal clearance or preload in an adjusted bearing arrangement during assembly. In operation, a certain clearance/preload — or only preload —range then arises. Axial preload in an adjusted bearing arrangement with s ingle-row, angular, contact ball bearings, tapered roller bearings and deep-groove ball bearings is produced by displacing one bearing ring axially, relative to the other, by an amount corresponding to the desired preload force. There are basically two principal methods to adjust preload: individual adjustment and collective adjustment.

With individual adjustment, each bearing arrangement is adjusted separately using shims, spacers and sleeves, etc. Measuring and inspection procedures are used to ensure that the established nominal preload is obtained with the least possible deviation.
The Collective adjustment method can also be referred to as “random statistical adjustment.” Using this method, the bearings, shaft, housing and any other components are manufactured to tighter tolerances and the components (which are fully interchangeable), are assembled randomly. It is assumed that, given the limiting values of the tolerances, it is statistically unlikely that the extremes of the tolerance stack-up occurs. The advantage of collective adjustment is that no inspection is necessary and no extra equipment is needed when mounting the bearings.

Manufacture

Universally matchable bearings are specifically manufactured so that, when mounted immediately adjacent to each other, preload will result. In addition, Bearings can be supplied as a complete bearing set consisting of multiple bearings. They are matched to each other during production so that when mounted immediately adjacent to each other and in the specified order, preload within a predetermined range and effective load share results.

Springs

This is the simplest manner in which to apply preload, and either Conical Disc Springs (plain and slotted) or Wave Springs are used. To maintain the proper preload, calibrated springs are used between the bearing outer ring and housing shoulder. These springs are designed to have excllent fatigue cycle life and a linear load characteristic curve. With springs, the kinematic behaviour of the bearing does not influence preload (under normal operating conditions). However, a spring-loaded bearing arrangement has a lower degree of stiffness than an arrangement using axial displacement to set the preload. The spring preload method is standard for many applications.

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 where

F is the preload force in [kN]
is the bearing bore diamater in [mm].
k is a correction factor.

Values for k vary and it is best to consult your bearing supplier. 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, whereas for electric motors, values range between 0.01 and 0.005

Apart from Deep Groove Bearings, spring loading is also a common method of applying preload to angular contact ball bearings, however where a high degree of stiffness is required, where the direction of axial load changes or where indeterminate peak loads can occur, the two methodds introduced above should be considered.