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CAM MATERIALS AND LUBRICATION 263

9.4.3 Abrasive Wear and Corrosive Wear

Abrasive wear occurs when two surfaces are in contact and one is considerably harder
than the other. Free particles of the harder surface are produced that by interaction remove
materials from the softer surfaces. This type of wear produces the removal of solid mate-
rial by gouging by the much harder body. In general these harder particles are the result
of the machinery action such as the cam and follower system. In addition, hard particles
may be found outside the system, such as dust, grinding action, and special contaminants.
One of the responsibilities of the lubrication system is the removal of these abrasive mate-
rials from the machine surfaces.

9.4.3.1 Corrosive Wear. Corrosion occurs in normal environments, and the most
common form is oxidation. Most metals react with oxygen in the atmosphere of air and
water and form oxides. In cam-follower surface action, the wear occurs due to the sliding
or rolling contact of the two bodies. As these oxides are loosely attached to the metal
surface, rubbing serves to remove them. These loose particles remove themselves and con-
tribute to the abrasive wear action. Proper lubrication can be utilized to keep corrosive
wear to a minimum. Adequate lubrication will protect the vulnerable surface from the cor-
roding environment and also react favorably and chemically with the surface.

9.4.4 Surface Fatigue
9.4.4.1 Introduction. Surface fatigue is produced by rolling elements in repeated
loaded contact. Fatigue is affected by many variables such as speed, load, material, tem-
perature, geometry of surfaces, lubricant, and amount of sliding. Rolling-element fatigue
can be of either surface or subsurface origin. General dislocation theory provides a tool
for understanding the rolling-element fatigue process. During rolling action, the accumu-
lation of dislocations will act as a stress raiser by concentrating the stress field in a small
local contact area. The result is to initiate a crack (called pitting) which in time will produce
a crack network and subsequently a spall. Figure 9.8a shows the phenomenon of fatigue,
producing mild pitting.
Spalling is the loss of large pieces from the surface and occurs when the pits are joined
and the metal surface is flaked. In other words, pitting produces spalling with the action
occurring rather suddenly after many cycles of operation. Surface fatigue exists with
rolling-element bearings, gearing, and cams. Figure 9.8b shows severe pitting and spalling.

(a) Mild pitting (b) Severe pitting and spalling
FIGURE 9.8. Example of surface failure by pitting and spalling (gear teeth). (Source: Graham, J.D.,
“Pitting of Gear Teeth,” Handbook of Mechanical Wear, C. Lipson, Ed., U. Mich. Press, pp. 138–43, 1961,
with permission.)