Page 33 - Cam Design Handbook

THB1 8/15/03 2:42 PM Page 21

INTRODUCTION 21

1.9 DESIGN CONSIDERATIONS

This section is a collection of introductory thoughts relating to the proper design and per-
formance of cam-follower systems. Cam-follower systems have been built in an extreme
range of sizes, from small mechanisms that have been produced in microelectromechani-
cal systems (MEMS), in which forty of them could fit in the period at the end of this sen-
tence, to the largest machine being a four-story high newspaper printing and folding press
(heavy mass-moving parts) handling seventy thousand complete newspapers per hour. In
between these extremes is a small high-speed punching mechanism running at 12,000rpm,
with some dynamic loading and elasticity.
Figure 1.28 shows a high-production cam-operated press for sheet metal drawn parts
(eyelets) for the cosmetic industry. This machine has multiple cam followers for each stage
of operation and runs at 70rpm.
The following is a brief list of ideas to guide the engineer-designer in creating cam-
follower machinery.
• The first step in designing a cam system is to establish the proposed design speed of the
complete machine. This decision is a critical one and should be based on the best
experience and judgment available. The customer should be helpful in this decision.
After the design speed is determined, establish a time chart to synchronize the cam
system with other actions of the machine.
• Positive drive, closed-track radial cams or conjugate dual cams with roller followers are
the most popular choices.
• The cam contour should be smooth with no abrupt changes in its curvature. Note that
curvature at any point on a cam is directly related to acceleration of the follower.
• The minimum curvature or sharpness of a convex cam contour is dependent on the value
of the maximum negative acceleration of the follower. That is, the larger the negative
acceleration, the sharper the cam surface will be.
• The cam size should be as small as possible to minimize the cam-follower sliding veloc-
ity, surface wear, torque on the camshaft, and cost and space requirements.
• The pressure angle should be kept to a minimum; 30 degrees is a general arbitrary limit
for all followers.
• Proper dynamic cam-follower design necessitates the study of the cam-follower
acceleration curve.
• The maximum follower acceleration should be as low as possible to keep the inertia
forces and stresses small.
• The noise, surface wear, and vibration of a cam-follower system are dependent on
the shape of the follower acceleration curve; hence, smoothness and continuity of the
acceleration curve are essential.
• The moving parts of the cam-follower mechanism should be lightweight and as rigid as
possible to keep the inertia forces, noise, and wear at a minimum, especially at high
speeds.
• The torque curve should be investigated in addition to the force distribution of the
system.
• Manufacturing methods and accuracy of cam cutting and inspection are of paramount
importance in ensuring the anticipated performance of a system. Small surface errors
that are imperceptible to the eye may produce high stress and vibrations in the follower
linkages.