Page 460 - Cam Design Handbook

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448 CAM DESIGN HANDBOOK

4. Vibrations due to variations in the external load. For example, a cam-driven punch
indexing mechanism has its load applied suddenly as the punch starts into the work-
piece. This quick-load application cannot be eliminated, and, therefore, the ultimate
design must account for it.
5. Miscellaneous sources. One of these sources is camshaft unbalance. In this case, intel-
ligent practical design of the cam structure and the body to reduce the offset mass and
maximize camshaft stiffness and support will keep this vibration to a minimum. Vibra-
tion may be transmitted to the cam surface from the driving mechanism through the
frame from such sources as electrical motors, gears, and chains.
Vibrations may be transmitted from an external source through the foundation of
a machine base or the body structure of a vehicle to the cam-follower system. For
example, the vibration source could be terrain interaction on a ground vehicle or
aerodynamic loads on an airplane.
To reduce vibrations in a cam-follower system, all members from the driver to the
follower end should be as rigid as possible. The follower should also be as light as pos-
sible. Damping devices are rarely practical as a means of reducing vibrations in cam
mechanisms.

13.8.1 Design Guidelines and Rules of Thumb
Some generally accepted guidelines in designing cam curves for moderate to high-speed
operation are:
1. The maximum acceleration values should be kept as low as possible.
2. A continuous jerk function should be maintained, with the peak value preferably not
exceeding the maximum for an equivalent cycloidal curve.
3. Modified sine, modified trapezoidal, and cycloidal cam curves are reasonable choices
in most cases, usually giving lower peak forces, vibratory amplitudes, noise, stresses,
and, in general, smoother performance than other basic curves.
Other cam curves have the same characteristics that make cycloidal curves good. Like
cycloidal curves, they provide continuity of all derivatives throughout the lift and their
lowest order of discontinuity at each end of the lift curve is at the jerk level or higher.
Their peak acceleration is comparable to a cycloidal’s and they do not possess unneces-
sary relative maxima or minima in the jerk characteristic. Other curves such as polyno-
mials, for example, are also good choices.
Most methods for the design of high-speed cam profiles that do not involve a formal
optimization procedure rely strongly on rules of thumb such as those above. When a formal
design optimization procedure, such as one of those discussed in this chapter, will not be
performed, these guidelines are generally good ones to use. The resulting design is likely
to be much better than a design obtained without use of any guidelines or formal design
procedure, and will be obtained with minimal effort.
However, if an optimization procedure is employed, such as one of those given in this
chapter, it will provide a better design than use of these guidelines will. The optimal solu-
tion for a high-speed system will often violate guideline 2 to a significant degree; there is
a high likelihood of discontinuities in the acceleration, equivalent to infinite jerk, at the
ends of the cam curve. This is particularly true when the objective of the optimization is
to reduce the dynamic forces on the cam and follower while still maintaining low vibra-
tion characteristics over a wide range of operating speeds. Guideline 2 applies most accu-
rately to low-speed systems (where the cam lift duration is very long relative to the system