Page 272 - Cam Design Handbook

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260 CAM DESIGN HANDBOOK

Note that for a complete study, all points on the cam should be investigated to find the
maximum stress on the cam. This stress should be below the elastic limit of the cam steel
material with a proper factor of safety.

9.4 WEAR

9.4.1 Wear Phenomena

The action between two surfaces in contact is complicated, involving the statistical rela-
tionships between many variables. Among the parameters are the surface roughness, wavi-
ness, and stresses; prior history of machining; modules of elasticity; friction (rolling and
sliding); materials; lubrication; corrosion; and loads. Wear, in a broad sense, is related to
the friction between the two surfaces in contact.
Simply, wear may be considered the undesirable removal of solid material from rubbing
surfaces. Basically, the four kinds of wear in cam-follower mechanisms are: adhesive wear,
abrasive wear, corrosive wear, and surface fatigue wear. Each of these will be articulated
in the following sections. Also, other, less significant types of surface failure do not fit
into these four categories, such as erosive wear, cavitation wear, and fretting corrosion
wear. In cam-follower material design, test data will be presented including different per-
spectives for design. Much of the material in the following sections must be utilized with
discretion. Wear testing and experience of a particular design is critical to optimizing the
performance of the mechanism.
For the engineer, the study of wear mechanisms on a specific machine may be possi-
ble by three methods: examination of wear debris, examination of worn surfaces, and met-
allographic examination of surface and subsurface structures.
In the examination of the wear debris, usually collected from the lubricating oil, large
lumps imply adhesive wear; fine particles, oxidative wear; chiplike particles, abrasive
wear; and flakelike particles, delamination wear.
In the examination of the worn surface heavy tearing implies adhesive wear; scratches,
abrasive wear; and burnishing, nonadhesive wear.
Metallographic examination of the surface structure may reveal the type of deforma-
tion to the sliding process, the generation of subsurface crack, incipient delamination, and
other things.

9.4.1.1 Surface Definition. A fundamental understanding of the mechanisms interact-
ing with the tribological surface is of utmost importance in lubrication. A solid surface is
not a flat or smooth surface. When a surface is examined with an electron microscope or
surface profilometer, it is found to contain irregularities called asperities; see Buckley
(1997) and Fig. 9.6a.
The asperity surfaces contain surface films—they are not clean (Fig. 9.6b). For metals,
these films consist of oxides and absorbed gases—usually water vapor, carbon monoxide,
and carbon dioxide. Nonmetals may contain other adsorbates. All film materials strongly
affect the mechanical and metallurgical behavior of contacting solids. Also, the near
surface (surfacial) layers of the solid itself may vary from the bulk of the solid. Crystalline
solids may have recrystallized material and strain hardening. In metals, polishing or fin-
ishing the operation of a surface produces these surfacial layers. These layers are gener-
ally rich in impurities.
When the two solids touch, the area of contact is at the points of the contacting
asperities across the interface, Fig. 9.6c. Initial contact of the solids deforms the asperi-
ties elastically, later followed by plastic deformation when the load is increased. It is at