Page 302 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

282 Major Process Equipment Maintenance and Repair

true as modern machine shop techniques can produce. In other words,
the shaft and bore must be round and free of taper and must completely
support the inner and outer rings. Also, the sizes of each must be correct
so as not to reduce the internal clearances in the bearing nor allow the
bearing to fret on the shaft or in the housing bore. When bearing fits are
damaged, they should be remachined and returned to their original sizes
by metalizing or plating.

Misalignment

Misalignment is a source of premature spalling of the bearing. This
condition generally occurs when the inner ring of a bearing is seated
against a shaft shoulder that is not square with the journal or when the
outer ring is seated against a housing shoulder that is out of square with
the housing bore. Also, misalignment can occur when the bores from
side to side of a gear housing are not parallel and square to the centerline.

Faulty Mounting Practice

Faulty mounting practice contributes greatly to premature bearing fail-
ure and usually results from abuse and neglect during the mounting pro-
cedure. The most common faulty mounting practices are foreign matter
in the bearing (not properly cleaned), impact damage during handling or
mounting resulting in brinelling, and overheating when expanding the
bearing to slip on the shaft. When heating a bearing for mounting, an oil
bath should be used if available; if a torch is used, care should be taken
not to overheat in one spot since localized overheating will actually
soften the bearing material.

incorrect Shaft and Housings Fits
A bearing may need to be fitted either with an interference fit or a slip
fit on the shaft and in the housing depending on the conditions present.
The degree of tightness or looseness in the bearing is governed by the
magnitude of the load, the speed of the journal, and the arrangement of
the bearing. In gear unit bearings, the inner ring usually rotates relative
to the load, and therefore, it will have an interference fit on the shaft and
a light slip fit in the housing. When it is incorrectly fitted, a bearing will
creep on the shaft or in the housing and cause wear to the journal or the
bearing seat.