Page 371 - Cam Design Handbook

THB12 9/19/03 7:34 PM Page 359

CAM SYSTEM DYNAMICS—ANALYSIS 359

since a model is not a real representation of the performing machine. There is no real
model, only approximate representations of values. Since vibration is generally an unde-
sirable side effect, it seldom controls the primary design of the cam-follower machine
system. Such systems are designed first to fulfill their main function and are then analyzed
from a vibration viewpoint possibly for equipment damage or malfunction, noise, or
human discomfort or annoyance.
The most severe effects of vibration generally occur at resonance; therefore, one
usually is concerned first with determination of the resonance frequencies of the prelimi-
nary design. Damping is usually neglected in the pertinent calculations for all but the sim-
plest systems. If resonance frequencies are found to lie within the intended range of driving
frequencies, one should attempt a redesign of the cam and follower system to shift the res-
onances out of the driving frequency range. Added stiffness with little addition of mass
results in shifting of the resonances to higher frequencies. Added mass with little addition
of stiffness results in lowering of the resonance frequencies. Damping generally has little
effect on the resonance frequencies.
Excitation reduction of vibration may take the form of running a machine at reduced
power, isolating the resonating system from the source of excitation, or shielding
the system from exciting inputs. Also, increased damping may be obtained by addition
of energy-dissipating devices or structures. For example, one might use metals
with high internal damping for the primary structure or attach coatings or sandwich
media with large energy-dissipation capacities to a primary structure of common
materials.
In addition to the basic vibrations that result from compliant systems as discussed in
this chapter, vibrations may occur in the cam follower for the following reasons:

• As a result of separation of the cam and follower with backlash. With closed-track cams
impact of the roller on the cam is produced and is called crossover shock. With open-
track cams vibrations are due to the “jump” condition of the follower leaving the cam
surface.
• Because of surface imperfections or irregularities. These can affect the performance of
the machine depending on the loads or speeds.
• Due to the rate and phase of application of the external load. For example, a cam-driven
punch-indexing mechanism has its load applied suddenly as the punch starts into the
workpiece. This quick-load application cannot be eliminated, and therefore the design
by necessity must include it. Note that sometimes the application of load during the
minus acceleration period tends to reduce or even eliminate the reversal of forces acting
on the cam surface.
• Due to cam or linkage unbalance. These may be fabricated with blowholes or nonho-
mogenous contaminants.
• Due to installation of improper and worn parts and misalignment.
• Due to support structures (frame) being either too light in weight or elastic. These vibra-
tions may occur at high speed and may be of significant magnitude to affect the design
action. Incidentally, the author has observed a textile machine that performed properly
and used an elastic frame to its advantage. In other words, the machine only worked
utilizing the compliancy of the frame.
• Vibrations transmitted to the cam from a relatively external power source. These may
arise from electrical motor, gearing, or other machinery action as part of the whole system.
• Due to moving parts of complex linkages affecting the vibration mounts and substruc-
tures of the machine.