Page 321 - Cam Design Handbook

THB10 9/19/03 7:28 PM Page 309

CAM MANUFACTURING 309

F = normal force, lb
R r = roller radius, in

Therefore to maintain rolling and reduce sliding at high speeds, a small lightweight roller
is required to minimize inertia. The angular acceleration cannot be changed easily because
it is inherent in the design. Also, the smallest cam gives the largest roller acceleration,
since the ratio or roller speeds (roller acceleration) is increased for the same rise and same
cam angle of rotation. Reducing sliding by increasing the coefficient of static friction or
the normal force F is limited, since wear and stress will be further increased.
Followers are held in contact with the cam in four ways:
1. preloaded compression spring
2. single roller in cam groove
3. double rollers in cam groove
4. external double rollers on cam

In commercial machinery, the preloaded compression spring is rarely used, primarily
because the spring adds an additional load to the system throughout its cycle, see Sec. 9.
single roller in cam groove: Figure 10.20 shows a single roller follower in an
internal groove in a radial cam (Fig. 10.20a) and a cylindrical cam (Fig. 10.20b). The roller
has necessary backlash for free movement in the cam groove. Sliding occurs at the
crossover point when the roller suddenly changes its contact cam, picking up the back-
lash. Wear and shock occur also. A kind of flutter movement sometimes exists with both
rolling and sliding for all single-roller positive drive cams.
For the cylindrical groove cam, another type of sliding may exist, depending on the
kind of follower. As mentioned previously, the cylindrical cam has either a cylindrical or
a conical follower. A cylindrical roller is preferred primarily because of the ease of cutting
the groove. In the end view of Fig. 10.20b, we see a cylindrical roller in contact with sur-
faces between radii r 1 and r 2. This means that the velocity of the path of travel in contact
with the surface at r 2 is more than the velocity of the path in contact at r 1 . Sliding must
occur to compensate for the velocity difference, which depends on the roller length. Obvi-
ously, this length should be kept as small as possible. In general, this kind of sliding is
not a seriously detrimental factor in the problem of surface life as compared to the action
at the radial cams or cylindrical crossover point.
To improve this sliding action in the cylindrical cam, a conical roller (frustrum) may
be utilized. The vertex of this roller should be located at the center of the cam for best
action (Fig. 10.20b). Thus, all points on the roller follower will be theoretically driven at
their proper linear velocities. The conical roller has been applied largely for small cams
in lieu of the simpler plain cylindrical type, because of the large ratio of radius r 1 to radius
r 2 for the same groove depth. A special cutter must be employed to obtain the groove for
the conical roller follower. Also, the conical roller in contact with a cam has a separating
force component that must be overcome to keep the roller in place. However, the conical
roller has the natural advantage that it can be moved radially inward to adjust for wear in
the roller and groove.
double roller in cam groove: Double roller or dual rollers in opposition with
positive-drive grooved cams have been employed. While this option has an advantage of
reduced rolling slipping compared to single roller, it also has limitations. The predomi-
nant shortcoming is the cantilever follower which deflects under load. This increases the
possibility of misalignment of the loaded roller on the cam surface and thus excessive
wear and reduced life. Figure 10.21 shows three different groove designs.
Figure 10.21a shows two eccentric rollers in a single groove. These rollers are of
equal diameter; they are free to rotate and ride on opposite groove surfaces. An eccentric