9.6 KINETOSTATICCAMSHAFT TORQUE
A kinetostatic analysis assumes that the camshaft will operate at
some constant speed
. The input torque must vary over
the cycle if the shaft velocity is to remain constant. The torque can be easily
calculated from the power relationship, ignoring losses. Power in = Power out
Once the cam force has been calculated from either equation 9.10
or 9.11, the camshaft torque
Tc
is
easily found since the follower velocity
v
and camshaft
ù
are both known. Figure
917a shows the camshaft input torque needed to drive the forceclosed cam
designed in Example 92 (p. 234). Figure 917b shows the camshaft input torque
needed to drive the formclosed cam designed in Example 93. Note that the
torque required to drive the forceclosed (springloaded) system is significantly
higher than that needed to drive the formclosed (track) cam. The spring force
is also extracting a penalty here as energy must be stored in the spring during
the rise portions that will tend to slow the camshaft. This stored energy is
then returned to the camshaft during the fall
portions,
tending to speed it up. The spring loading causes larger oscillations in the
torque. Section 9.8 (p. 251) discusses the use of flywheels to reduce torque
oscillation.
One
useful way to compare alternate cam designs is to look at the torque function
as well as at the dynamic force. A smaller torque variation will require a
smaller motor and/or flywheel and will run more smoothly. Three different
designs for a symmetrical single dwell cam were explored in Chapter 4. (See
Examples 41, p. 58; 42, p. 60; and 43, p. 61.) All had the same lift and
duration but used different cam functions. One was a double harmonic, one
cycloidal, and one a sixthdegree polynomial. On the basis of their kinematic
results, principally acceleration magnitude, we found that the polynomial
design was superior. We will now revisit this cam as an example and compare its
dynamic force and torque among the same three programs.

Copyright (C) 2002 Industrial Press Inc.