Page 276 - Cam Design Handbook
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264 CAM DESIGN HANDBOOK
Rolling-element fatigue can be initiated by hard inclusions in the material; corrosion;
surface stress raisers such as dents, grinding imperfections, or geometric stress concen-
trations; microspalling; and surface interactions dependent on roughness and lubrication
film thickness. Sometimes thin layers of case hardening, surface checks from heat treat-
ment and high sliding velocities, excessive temperature, type of lubricant, or contamina-
tion can accelerate the action of fatigue.
If the surface is stressed in a corrosive environment, a phenomenon called corrosion
fatigue or stress corrosion occurs. Corrosion on either the cam or roller follower will ini-
tially roughen the smooth rolling surface and form minute pits from which a fatigue crack
will start due to the local stress concentration. The use of a lubricant has another function
besides the reduction of friction of the surfaces, which is to prevent corrosion and ensure
longer fatigue life.
9.4.4.2 Surface Fatigue Design. As mentioned previously cams, rolling-element bear-
ings, and gearing are machine elements that are similar in their performance, and they fail
most frequently by surface fatigue. Unfortunately statistical wear data have been devel-
oped for rolling-element bearings and gearing but not for cam-follower systems. The
diversity and complexity of cam-follower systems has prevented a complete wear design
methodology to evolve.
Nevertheless, this section presents a simplified design procedure for the cam
engineer. The data presented will be of initial value as a guide in selecting compatible
material combinations for the cam and follower. Ultimately, the proper materials,
wear performance, and lubricant must be confirmed in the field under actual operating
conditions.
Test data are presented for the comparison of cam and follower fatigue under pure
rolling and rolling with some sliding conditions. Experience has shown that some sliding
may occur, even at low speeds, affecting the wear life of the contacting surfaces.
Life surface fatigue tests were conducted by Talbourdet (1950), Morrison (1968),
and Cram (1956) utilizing radially loaded contacting cylinders with (a) pure rolling
and (b) rolling with 9 percent sliding. Three-inch diameter mating rollers were used; one
roller had a hardened steel surface (60-62 RC) and the other, softer roller was cast
iron, steel (of different hardnesses), bronze, and nonmetallic materials. The weaker roller
material wore out under the cyclical loading which established the life of the materials in
combination.
The stress test data algorithm relates s max to the hertzian compressive stress Eq. (9.11)
for dissimilar metals in surface contact.
For a Poisson’s ratio m = 0.3 and the normal force per cylinder length
K
P ¢ = lb in (9.13)
Ê 1 1 ˆ
Á + ˜
Ë r r ¯
1 2
where
s 2 Ê 1 1 ˆ
K = max Á + ˜ (9.14)
. 035 Ë E 1 E ¯
2
which is called the load-stress factor for cylinders in contact. Table 9.3 lists values of K
for 100 million stress repetitions for pure rolling and rolling plus 9 percent sliding action.
One roll is of hardened steel 60-62RC while the other is of various selected materials.
Values of K have been determined as a function of the number of stress cycles necessary
for surface failure. The test lubricant was mineral oil 280-320 SSU and 100°F at a surface