SKF - Rolling Bearings

Design considerations
Frictional moment method
This method is common in series production
because it is fast and can be automated. Since
there is a relationship between bearing preload
and the frictional moment in the bearings, it is
possible to stop adjustment when a frictional
moment corresponding to the desired preload
has been reached. This can be done if the frictional
moment is continuously monitored while
setting preload. However, the frictional moment
can vary from bearing to bearing, and it
also depends on the preservative, the lubricant
and the sealing method.
Direct force method
As the purpose of bearing adjustment is to obtain
a specific preload, it would seem sensible
to use a method either to produce or to measure
the force directly. However, in practice, the
indirect methods of adjustment by axial displacement
or frictional moment are preferred
as they are simple and can be achieved easily
and more cost-effectively.
Collective adjustment
This method can also be referred to as “random
statistical adjustment”. Using this method,
the bearings, shaft, housing, and any other
components are manufactured to Normal
tolerances. The components, which are considered
fully interchangeable, are assembled
randomly. Where tapered roller bearings are
concerned, this interchangeability also extends
to the inner ring assemblies and outer rings.
To avoid high machining costs and the use of
precision bearings, it is assumed that given
the limiting values of the tolerances, it is statistically
improbable that tolerance stack-up
occurs. If, however, accurate preload is to be
obtained with as little scatter as possible,
manufacturing tolerances must be narrowed.
The advantage of collective adjustment is that
no inspection is required and no extra equipment
is needed when mounting the bearings.
Preloading with springs
By preloading bearings in small electric motors
(up to frame size of 132) or similar applications,
it is possible to reduce bearing noise levels.
The bearing system in this case comprises
a single row deep groove ball bearing at each
end of the shaft. The simplest method of applying
preload is to use a spring or spring
package († fig. 41). The spring acts on the
outer ring of one of the two bearings. This
outer ring must be able to be axially displaced.
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 = preload force [kN]
d = bearing bore diameter [mm]
k = a factor, † following
Depending on the design of the electric motor,
values of between 0,005 and 0,01 are used for
Fig. 41
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