Page 263 - Tribology in Machine Design

7 Rolling-contact bearings

7.1. Introduction In contrast with hydrodynamically lubricated journal bearings, which, for
their low friction characteristics depend on a fluid film between the journal
and the bearing surfaces, rolling-contact bearings employ a number of balls
and rollers that roll, nominally, in an annular space. To some extent, these
rolling elements help to avoid gross sliding and the high coefficient of
friction that is associated with sliding. The mechanism of rolling friction is,
therefore, discussed first followed by the review of the factors affecting the
frictional losses.
As a matter of fact, the contact between the rolling elements and the races
or rings consists more of sliding than of actual rolling. The condition of no
interfacial slip is seldom maintained because of material elasticity and
geometric factors. It is natural then that the contact stress and the
kinematics of the rolling-element bearing are presented in some detail in
order to stress their importance in the service life of this type of bearing. The
advantages and disadvantages of rolling-contact bearings when they are
compared with hydrodynamic bearings are well known and shall not be
discussed here. Instead, more attention is given to the lubrication
techniques and the function of the lubricant in bearing operation. Finally,
vibration and acoustic emission in rolling-element bearings are discussed as
they are inherently associated with the running of the bearing. Some
methods to combat the excessive noise emission are also suggested.

7.2. Analysis of friction The resistance to relative motion in rolling-contact bearings is due to many
in rolling-contact factors, the basic one being rolling friction. This was long assumed to be the
bearings on ly resistance to motion in this type of bearing. It was established,
however, that the contribution of rolling friction is small though its effect on
wear and tear and operating temperature is important. These factors are
especially important for miniature instrument rolling-contact bearings
operating in the very accurate mechanisms of servo-systems, magnetic
recorder mechanisms and other precision parts of instruments.
Previously, rolling friction was treated as a mechanical process, i.e. the
interaction of rough surfaces of absolutely rigid bodies. In 1876 Reynolds
put forward the hypothesis according to which the frictional force due to
the rolling motion of a perfectly elastic body along a perfectly elastic
substrate was a result of relative slip between the contacting surfaces
resulting directly from their deformations. Figure 7.1 provides a graphic