What is Bearing Pre-Load
When bearings are manufactured and assembled, they are done with clearances between the components to allow free movement. The free movement helps prevent bearing damage when in transit and absorb shocks. However, this free movement will be detrimental if present in bearings during service. The removal of the free movement is the role of bearing pre-load. Bearing pre-load is a force, axial or radial, applied to a bearing that removes the excess play.
Too Much or too Little Bearing Pre-Load
The rolling elements in the bearing will slide rather than roll if too little bearing pre-load is applied. The sliding is a result of the free movement remaining within the bearing. In addition to sliding, insufficient pre-load can cause the raceways to misalign and cause fretting on the raceway. On the other hand, too much pre-load will result in excessive heat, causing a dangerous knock-on effect of deteriorating the lubricant leading to failure. Too much pre-load will also likely lead to premature failure via fatigue.
How is it applied?
As noted at the beginning, pre-load can be applied axially or radially. The most common method, from my experience, of applying pre-load is elastic pre-load. Elastic pre-load is achieved by applying a constant force to the non-rotating parts of the bearing i.e. the inner and outer race. A load is achieved by shrink fitting an inner race onto an item, a shaft for example. Once cooled, the inner race is slightly larger, removing excess play. Also, the outer race is fit into housing which will contract it, removing more excess play.
The other method is rigid pre-load. This method involves pressing a bearing up to a shoulder/face of a component that is perpendicular to the axis of rotation. This pre-load is then fixed in place either using a bolt or other sold object. An example of this I have experienced is when wheels of a train are shrunk onto an axle, containing inboard bearings. After cooling, the wheel is pressed ,while floated using oil injection, and applied an axial load onto the bearing to remove the excess play. The load is maintained by screwing a plate into the end of the axle to maintain the pre-load. The plate is then removed after 24 hours once the pre-load has been established.
Here we have covered the main ways bearing pre-load is applied. I hope you understood the importance of proper bearing pre-load and the potential failures that can arise.
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