Page 280 - Tribology in Machine Design

Rolling-contact bearings 265
Figure 7.20
7.5.7. Surface failure modes related to lubrication
As discussed earlier, the elastohydrodynamic film parameter, A, has a
significant effect on whether satisfactory bearing operation is attained. It
has been observed that surface failure modes in rolling-element bearings
can generally be categorized by the value of A. The film parameter has been
shown to be related to the time percentage during which the contacting
surfaces are fully separated by an oil film. The practical meaning of
magnitude for lubricated contact operations is discussed in detail in
Chapter 2. Here it is sufficient to say that a A range of between 1 and 3 is
where many rolling element bearings usually operate. For this range,
successful operation depends on additional factors such as lubricant/
material interactions, lubricant additive effects, the degree of sliding or
spinning in the contact, and surface texture other than surface finish
measured in terms of root mean square (r.m.s.). Surface glazing or
deformation of the asperity peaks may occur, or in the case of more severe
distress superficial pitting occurs. This distress generally occurs when there
is more sliding or spinning in the contact such as in angular contact ball-
bearings and when the lubricant/material and surface texture effects are less
favourable.
Another type of surface damage related to the film parameter A, is peeling,
which has been seen in tapered roller-bearing raceways. Peeling is a very
shallow area, uniform in depth and usually less than 0.013 mm. Usually this
form of distress could be eliminated by increasing the A value. In practical
terms it means the improvement in surface finish and the lowering of the
operating temperature. To preclude surface distress and possible early
rolling-element bearing failure, A values less than 3 should be avoided.
7.5.8. Lubrication effects on fatigue life
The elastohydrodynamic film parameter, A, plays an important role in the
fatigue life of rolling element bearings. Generally, this can be represented in
the form of the curve shown in Fig. 7.20. It is worth noting that the curve
extends to values of less than 1. This implies that even though A is such that
significant surface distress could occur, continued operation would result in
surface-initiated spalling fatigue. The effects of lubrication on fatigue life
have been extensively studied. Life-correction factors for the lubricant
effects are now being used in sophisticated computer programs for analysis
of the rolling-element bearing performance. In such programs, the lubricant
film parameter is calculated, and a life-correction factor is used in bearing-
life calculations. Up to now, research efforts have concentrated on the
physical factors involved to explain the greater scatter in life-results at low A
values. Material/lubricant chemical interactions, however, have not been
adequately studied. From decades of boundary lubrication studies, how-
ever, it is apparent that chemical effects must play a significant role where
there is appreciable asperity interaction.