Page 234 - Cam Design Handbook

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222 CAM DESIGN HANDBOOK

FIGURE 8.3. Linear ball bearing for translating follower (Courtesy Warner Electric,
South Beliot, Ill.).

ous sliding followers, we have the following frictional losses: (a) oil viscosity, (b) hys-
teresis loss due to the damping of the rolling materials, (c) excessive working load impact
(if it exists), (d) number of follower rollers (either one or two) affected by change in inertia
force, (e) number of times of rotation of roller, (f) deflection of the follower stem, pro-
ducing misalignment and wear on the roller, (g) skidding loss of bearing in its container,
(h) flinging oil from rotating bearings, and (i) excessive oil in the bearing. Rolling-element
followers have a coefficient of friction 0.01 < m k < 0.15. This wide range is due to the
uncertainty as to the actual performance of this kind of follower.

8.6.3 Linear Ball Bearings
These commercially produced low-friction ball mechanisms are available in many appli-
cations, as they specifically are used for linear translation followers and have a coefficient
of friction m k > 0.010 < m k < 0.050, depending on the lubrication adversities previously
discussed. Fig. 8.3 shows a picture of a linear ball bearing used to guide and support a
translating follower.

8.7 SPRING FORCES

In cam-follower systems the follower must be held in contact with the cam at all times to
counteract the follower inertia. The follower inertia follows the acceleration curve with
the magnitude of the negative acceleration being of concern. The constraint of the fol-
lower should be accomplished by utilizing a preloaded compression spring, usually a
helical coil.
The spring force is directly proportional to the follower displacement. If the force is
too small it will allow the follower to jump off the cam. On the other hand, an excessive
spring force is reflected throughout the system during the total cycle of operation. This
excessive force will require a stronger design system and more wear of parts will result.
Also, one of the disadvantages of spring-loaded systems is that the spring force produces
an additional load on the system.
Figure 8.4 shows a cam-follower mechanism with its displacement and inertia force
curves, which are shaped as the acceleration curve. Also shown is a properly designed