Page 281 - Tribology in Machine Design

266 Tribology in machine design

7.5.9. Lubricant contamination and filtration
It is well recognized that fatigue failures which occur on rolling-element
bearings are a consequence of competitive failure modes developing
primarily from either surface or subsurface defects. Subsurface initiated
fatigue, that which originates slightly below the surface in a region of high
shearing stress, is generally the mode of failure for properly designed, well
lubricated, and well-maintained rolling-element bearings. Surface initiated
fatigue, often originating at the trailing edge of a localized surface defect, is
the most prevalent mode of fatigue failure in machinery where strict
lubricant cleanliness and sufficient elastohydrodynamic film thickness are
difficult to maintain. The presence of contaminants in rolling-element
systems will not only increase the likelihood of surface-initiated fatigue, but
can lead to a significant degree of component surface distress. Usually the
wear rate increases as the contarninant particle size is increased. Further-
more, the wear process will continue for as long as the contaminant particle
size exceeds the thickness of the elastohydrodynamic film separating the
bearing surfaces. Since this film thickness is rarely greater than 3 microns
for a rolling contact component, even extremely fine contaminant particles
can cause some damage. There is experimental evidence showing that 80 to
90 per cent reduction in ball-bearing fatigue life could occur when
contaminant particles were continuously fed into the recirculation lubri-
cation system. There has been a reluctance to use fine filters because of the
concern that fine lubricant filtration would not sufficiently improve
component reliability to justify the possible increase in the system cost,
weight and complexity. In addition it is usually presumed that fine filters
will clog more quickly, have a higher pressure drop and generally require
more maintenance than currently used filters.

7.5.10. Elastohydrodynamic lubrication in design practice
Advances in the theory of elastohydrodynamic lubrication have provided
the designer with a better understanding of the mechanics of rolling contact.
There are procedures based on scientific foundations which make possible
the elimination of subjective experience from design decisions. However, it
is important to know both the advantages and the limitations of
elastohydrodynamic lubrication theory in a practical design context.
There are a number of design procedures and they are summarized in Fig.
7.21. A simple load capacity in a function of fatigue life approach is used by
the designers to solve a majority of bearing application problems. The
lubricant is selected on the basis of past experience and the expected
operating temperature. Elastohydrodynamic lubrication principles are not
commonly utilized in design procedures. However, in special non-standard
cases, design procedures based on the ISO life-adjustment factors are used.
These procedures allow the standard estimated life to be corrected to take
into account special reliability, material or environmental requirements.
Occasionally, a full elastohydrodynamic lubrication analysis coupled with