Preload can be defined as axial load applied permanently to bearings to ensure proper operation under external load. Preload can be rigid, as in the case of opposed bearings with our without spacers, or elastic, using springs.

Rigid preload involves the formulation of a gap between the faces of opposed bearings. This gap is eliminated when bearings are locked axially.

Rigid preload may change during operation due to the effect of thermal expansion. With elastic preload, expansion is compensated for by the springs. Bearing preload variation is minimal in this case.

Bearing preload is essential because of its influence on speeds limits, rigidity and applicable work loads.

The preloads commonly recommended to satisfy all requirements are Light [L], Medium [M] and Heavy [F] and are given in the bearing data table. Bearings can be supplied with special preload.

The tables also indicate maximum speed and axial rigidity Ra, for each preload, for paired bearings and for groups of three units.

In order to prevent premature failure, a lower initial preload should be selected so it reaches the proper value at full operation.