Page 274 - Cam Design Handbook

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262 CAM DESIGN HANDBOOK

Most of the energy released during the process of breaking the high spots is trans-
formed into heat, which causes a sudden temperature flash. When the heat of the temper-
ature flash is conducted through the interface, momentary diffusion occurs rapidly. If the
diffusion process is rapid enough to cause welding of the sheared-off peak to the oppo-
site high spot, a piece of transferred metal is formed. If the conditions are such that no
appreciable diffusion occurs, the adhesive force can still keep the metal particles on the
opposite high spot. The small piece of metal sheared from the high spot will leave a “loose
particle” when both diffusion and adhesion are weak. Also, the amount of adhesive trans-
fer and wear can be profoundly affected by the surface friction and ambient conditions.
Figure 9.7 shows adhesive wear on a shaft with an example of surface failure which is
similar for cams and followers.
Asignificant factor affecting adhesion is the compatibility of the mating materials. Tests
show that some combinations of metals are compatible for long wear life, whereas others
are poor. Table 9.2 indicates some interesting metal combinations’ wearing on unlubri-
cated surfaces. Note that these material combinations relate to adhesive properties only.
Adhesive bonding can occur if the materials surfaces have contaminants. These con-
taminants could be forms of oxide, oils from human skin, moisture coatings, or lubricants.
Obviously the lubricant is of positive value in isolating the two potentially adhesive
bonding surfaces.

FIGURE 9.7. Example of adhesive wear (shaft).

TABLE 9.2 Metal Combinations
Good Poor
Cast iron and phosphor bronze Hardened steel and hardened bronze
Hardened steel and phosphor bronze Hardened nickel steel and hardened nickel steel
Cast iron and soft steel Soft steel and bronze
Babbitt and soft steel Soft steel and soft steel
Soft brasses and soft steel Soft steel and laminated thermoset resins
Hardened steel and soft bronze Soft steel and nylon
Hardened steel and brass
Hardened steel and cast iron
Hardened steel and laminated thermoset resins
Hardened steel and nylon