Page 65 - Cam Design Handbook

THB2 8/15/03 12:48 PM Page 53

BASIC CURVES 53

DRRD Cams—Symmetrical Rise Return

Let us now plot some basic curves for a DRRD cam. In Fig. 2.16a, for comparison, we
see the symmetrical parabolic, simple harmonic motion, cycloidal, and double harmonic
curves. We know that the displacement and velocity curves of all are smooth and contin-
uous between the dwell ends. However, the parabolic and simple harmonic acceleration
curves exhibit the same difficulty as those in the dwell-rise-dwell application, that is,
sudden change at the ends. This shortcoming is known to produce inferior performance at
high speeds. The cycloidal curve, on the other hand, which is one of the best dwell-rise-
dwell curves, has an abrupt change in acceleration at the maximum rise point. This is not
desirable, because vibration and difficult machining result. The magnitude of these factors,
of course, depends on cam speed and flexibility of the parts. However, we note that at the
maximum rise point neither the parabolic nor the simple harmonic curve has any abrupt
change. We also see that the best high-speed curve, from the standpoint of slow starting

FIGURE 2.16. DRRD cam application of basic curves.