Power Transmission
The importance of bearing preload and axial adjustment
Gary Hughes outlines the benefits of preloading and axial adjustment of bearings.
P
reloading and axial adjustment is a method of removing or controlling the internal clearance of a bearing. This is important because the degree of internal clearance within a bearing can influence noise, vibration, heat build-up and
fatigue life. When applied correctly, preloading reduces or overcomes
the causes of these problems: it controls radial and axial play; provides predictable system rigidity; reduces non-repetitive run-out; reduces the difference in contact angles between the inner and outer rings at very high speeds; and controls ball skidding under very high acceleration. In all bearing arrangements where preload is deemed
necessary, it should be applied as lightly as possible to achieve the desired result, avoiding excessive heat generation, which reduces speed capability and bearing life. It is also important to select the most appropriate method of preloading, of which there are three types.
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Spring preloading provides a relatively constant preload because it is less sensitive to differential thermal expansion than rigid preloading, and better accommodates minor misalignments. Also, it is possible to use bearings that have not had the added cost of preload grinding. One disadvantage is that springs cannot accept reversing thrust loads. Space must also be provided to accommodate the springs and spring travel. However, despite its disadvantages, spring preloading
is still popular. Numerous types of springs are available for preloading, including coil springs, Belleville, wave or finger spring washers. For mounting, the spring is normally applied to the non-rotating part of the bearing, typically the outer ring. This ring must have a clearance fit in the housing at all temperatures to ensure the preload force from the spring is effective. Axial adjustment calls for the mounting of at least two bearings in opposition, so that inner and outer rings of each
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