Published: Jan 2007
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HOW DOES ROLLING ON A SURFACE WEAR IT? As mentioned in the discussion of rolling wear in Chapter 2, pure rolling only occurs in a fraction of the total footprint of a revolute shape (ball, roller, wheel, etc.) that rolls on another surface (Figure 9-1). Rolling means that both surfaces are at the same velocity; there is no relative slip. When there is slip, there is wear (and friction). It is really the same as sliding wear except that the relative slip can be very small. Therefore, rolling wear occurs by the nonuniform slip that accompanies a Hertzian contact.
Impact wear is somewhat related. It is material loss/damage produced by a solid surface repeatedly impacting another solid surface. Hookean mechanics dictates that elastic deformation of surfaces occurs under impact. Plastic deformation can also occur. Both produce relative slip of one surface on another-the requirement for sliding wear (Figure 9-2). Both surfaces deflect and both can wear. Even if the force (P) is insufficient to produce plastic deformation of either surface, there is relative motion between them because by Hooke's law, for every applied force there is deformation.
Impact and rolling often lead to surface fatigue. Surface fatigue is the localized fracture of material from a solid surface caused by the action of repeated compressive stressing of a surface. There are various ways to analyze the state of stress between conforming bodies, but for rolling, the maximum stress under a ball or cylinder is located below the surface (Figure 9-3). This phenomenon is what leads to surface fatigue. Repeated rolling over a surface or repeated impact can cause counterface material or counterface coatings to spall from subsurface cracking leaving pits or craters (Figure 9-4).