Page 286 - Tribology in Machine Design

Rolling-contact bearings 271

A typical example is the sliding fit, spring preloaded bearing in an electric
motor where spring loads can barely be sufficient to overcome normal
levels of friction between the outer ring and the housing. A certain preload is
necessary to seat all of the balls and to ensure firm rolling contact, unless
this level of preload is applied, balls will intermittently skid and roll and
produce a cage-ball instability. When this occurs, vibration levels may be
one or even two orders of magnitude higher than that normally associated
with the bearing. Manufacturers catalogues usually give the values of the
minimum required preload for single radial ball-bearings.

7.6.5. Noise reduction and vibration control methods
Noise reduction and vibration control problems can be addressed first by
giving some consideration to the bearing type and the arrangement. The
most important factors are skidding of the rolling elements and vibration
due to variable compliance. These two factors are avoided by using single
row radial ball-bearings in a fixed-free arrangement with the recommended
level of preload applied through a spring washer. When this arrangement is
already used, secondary improvements in the source of vibration levels may
be achieved by the selection of bearing designs which are insensitive to
distortion and internal form errors. The benefit of this is clearly seen at
frequencies below sixty times the rotational speed. The ball load variation
within the bearing is a key issue and the problem of low-frequency vibration
generation would disappear if at all times all ball loads were equal. There
are many reasons for the variation in ball loads, for instance, bearing ring
distortion, misalignment, waviness errors of rolling surfaces all contribute
to load fluctuation. Design studies have shown that for given levels of
distortion or misalignment, ball load variation is a minimum in bearings
having a minimum contact angle under thrust load. Significant reduction in
low-frequency vibration levels can be achieved by selecting the clearance
band to give a low-running clearance when the bearing is fitted to a
machine. However, it is important to bear in mind that running a bearing
with no internal clearance at all can lead to thermal instability and
premature bearing failure. Thus, the minimum clearance selection should
therefore be compatible with other design requirements. Another import-
ant factor influencing the noise and the vibration of rolling-contact
bearings is precision. Rolling-element bearings are available in a range of
precision grades defined by ISO R492. Although only the external
dimensions and running errors are required to satisfy the ISO specification
and finish of the rolling surfaces is not affected it should be noted, however,
that the manufacturing equipment and methods required to produce
bearings to higher standards of precision generally result in a higher
standard of finish. The main advantage of using precision bearings is clearly
seen at frequencies below sixty times rotational speed where improvements
in basic running errors and the form of the rolling surfaces have a significant
effect. It is important to match the level of precision of the machine to the
bearing, although it presents difficulties and is a common cause of noise.