Operation of a rolling bearing causes fatigue cracks within the subsurface material
of highly stressed roller-raceway contacts. The subsurface cracks propagate and
coalesce causing the removal of a portion of the contacting surface. This phenomenon is
known as fatigue spalling. When a bearing generates a fatigue spall, the contact
stresses, vibratory loads and heat generation rates are increased. This in turn causes
more fatigue cracks within the unfailed subsurface material of the contacts. The
propagation of existing subsurface cracks and the creation of new subsurface cracks
cause continued deterioration of the contact surface, as shown in Figure 1.
Repeated operation of the bearing progresses the fatigue spall until the entire contact area
has been roughened. The increased heat generation rates and vibratory loads within
a spalled bearing can lead to catastrophic failure of the mechanism. If the heat
dissipation is such that it causes internal clearances within the bearing to disappear, the
bearing could seize. Alternatively, the internal clearances could increase. This would lead
to larger roller loads and possibly component fracture. Finally, the increased
vibratory loads may be too high for the mechanism or the system surrounding the
bearing. Again, this could lead to a catastrophic failure (Michael and Tedric, 2001).
It is generally accepted that when two solid bodies of curved shape are brought
into contact, the maximum orthogonal shear stresses are developed somewhere
beneath the contact spots. The depth of these stresses is dictated by the magnitude of
applied loads and the elastic properties of the contacting bodies. It has been postulated
that fatigue failures or pits should originate at depths where maximum shear
stresses develop. Dislocation entanglement around inclusions, second-phase precipitates
and other types of volume defects that are located at depths where maximum shear
stress develops are thought to act as stress concentration points and hence initiate
subsurface microcracks, resulting in failure (Ali, 1999).
Rolling contact fatigue is manifested as flaking of metallic particles from the
surface of the raceways and/or rolling elements. For well lubricated, properly
manufactured bearings, this flaking usually commences as a crack below the surface and is
propagated to the surface eventually forming a pit or spall in the load-carrying surface
(Tedric, 2001). Subsurface cracks propagate to the surface and also connect with
surface cracks to form a network. The volume enclosed by this network crumbles,
thus producing a spall or fatigue failure (Shelley and Erwin, 1963).