Page 318 - Cam Design Handbook

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306 CAM DESIGN HANDBOOK

manner, machine life cycle cost is controlled since a cam is much more complicated and
expensive than a roller-follower assembly. The roller is measured and replaced when a
reduction in diameter is observed. This approach to cam-follower life has yielded perma-
nently installed high-speed and high-load cams that last over 40 years.
Roller followers are commercially obtainable and comprise an assembly of separate
parts—a drilled stud (for bearing lubrication), an outer race, and a set of rollers. The outer
race is hardened to Rockwell C 60-62, having a highly polished hardened steel surface for
good wear life and a tough case for absorbtion of dynamic and shock loads. The rollers
are usually needle bearings. Sometimes cylindrical rollers are employed. Both kinds
supply purely radial loads.
Needle bearings have an advantage over cylindrical bearings when they are used alone
as cam followers; the large number of small-diameter, closely spaced rollers appreciably
reduces the bending stress in the outer bearing race. Needle bearings have a low mis-
alignment tolerance and moderate speed limits but use a smaller low-mass, low-inertia
roller assembly which is excellent in cam followers. Reduced slipping of the roller on the
cam surface is effected. Needle bearings are speed limited because the rollers skew at high
speeds. The bearings are made of AJSI 52100.
The cam roll mountings available are the stud type, which is contilever supported, and
the yoke or straddle type. Figure 10.16a shows the stud type supported on a steel stem
carrying needle bearings. Seals are included to return the grease which is inserted through
a grease nipple. Heavy stud types are commercially available (not shown) for heavy load,
shock loading, or applications requiring minimum deflection. Figure 10.16b shows a yoke-
supported type using needle bearings and used in the automobile industry as valve tappet
and fuel injection rollers. The yoke mounting is a better support and has reduced deflec-
tion and misalignment of the roller on the cam. The seals are excluded when external oil
lubrication is available. This is desirable also to reduce seal friction in bearings. Because
a yoke mounting distributes load uniformly over the bearing, the load limit of a yoke is
about equal to that of a heavy stud follower. Cylindrical roller cam followers are employed
for heavy loaded systems. They are also available in double-roller assemblies for extra-
heavy installations.
Sometimes standard ball, cylindrical roller, or needle bearings are utilized with the
outer race specially built. Figure 10.17 shows a needle roller follower with a laminated
thermoset resin outer roller used on a textile machine to reduce shock.
The cam-follower roller in contact with the cam will produce geometric stress con-
centrations that are dependent on the length of the contact. Figure 10.18a shows a straight
roller resting on a cam. The roller will deflect slightly into the cam surface, producing the
stress curve shown. Higher stresses will be induced at the ends and thus stress concentra-
tions occur. Fatigue pitting or spalling will initiate at the ends. Crowned rollers, having a
large radius of curvature, are used to distribute the stress more evenly. In Fig. 10.18b,
partial crown rollers and logarithmic rollers have been employed with impunity. Also, a
proper lubricated cam surface is required to minimize follower wear. If grease is used, it
must be “tacky” enough to cause the follower to roll instead of slide.

10.9.2 Roller-Follower Installations

Rolling function is explained in terms of the energy required to overcome the interfacial
slip that occurs due to the curved shape of the contacting area between the roller and the
cam surface. A portion of the elastic energy in rolling is lost because of hysteresis.
As stated, a roller is used because of its low friction and low rolling wear. Pure rolling
action is desired, although a tolerable amount of sliding is always evident. However,
certain difficulties may arise in which the roller, when running on a cam, may have a