Page 426 - Failure Analysis Case Studies II

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bearing (Fig. 2) [4] Under repeated loading, cracks ultimately initiate at the point of maximum
stress, and propagate parallel to the surface. At some stage, these cracks deviate and grow towards
the contact surface, resulting in the formation of steep-sided pits. These pits are usually microscopic,
but may, with continued bearing operation, act as stress concentration sites for further damage.
Under normal bearing operation, it is more common that contact between the rolling elements
and the raceway includes both rolling and sliding. The resulting stress distribution in the near-
surface material under these conditions changes, and the maximum stress point moves closer to the
surface. Again, this situation is similar to that encountered in the addenda and dedenda of gear
teeth [l]. Cracks initiate at the contact surface, and propagate to form small, irregular-shaped pits.
In some cases, the pits may form in the shape of an arrow-head pointing in the direction of load
approach [3]. This is similar to the “cyclone pitting effect” also observed in gear teeth [l].
The initiation of surface cracks under rolling-sliding contact can be significantly accelerated by
the presence of stress concentration sites on the contact surfaces [3]. These include corrosion pits,
handling damage, surface inclusions, and dents formed by solid particles entrapped in the lubrication
fluid. These geometric inhomogeneities lead to high localized stresses, rapid crack initiation, and
the formation of contact fatigue pits. In some cases, the cracks initiated in this way may propagate
through the bearing rings to cause complete fracture. An example of this is given in Fig. 3, which
shows the inner ring of a thrust bearing [5]. Extensive surface damage, probably resulting from the
action of solid particles entrapped in the lubricating fluid, is clearly noticeable, as is the through-
crack emanating from this damage. Figure 4 shows the crack face in the vicinity of the region
marked with an arrow in Fig. 3, and clearly indicates that crack growth was by fatigue.

3. FLAKING AND SPALLING

Under continued operation, the pits formed by rolling and rolling-sliding contact fatigue may
progress to form a more severe form of damage known as flaking [3]. This results in the formation
of large, irregular pits which cause rapid deterioration and failure of the bearings. Flaking is usually
first observed on the stationary ring of a bearing, since the surface of this ring is subjected to the
maximum stress every time a rolling element passes over it. In the case of the rotating ring, the

Fig. 2. Thrust bearing cup showing highly polished surfaces typical of the initial stages of rolling contact
fatigue.