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.
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