9.8 CONTROLLING CAM SPEED—FLYWHEELS
shown in Figure 9-23, the typically large variation in accelerations within a
cam-follower system can cause significant oscillations in the torque required
to drive it at a constant or near constant speed. The peak torques needed may
be so high as to require an overly large motor to deliver them. However, the
average torque over the cycle, due mainly to losses and external work done, may
often be much smaller than the peak torque.
servomotors are used, we may need to provide some means to smooth out these
oscillations in torque during the cycle. This will allow us to size the motor
to deliver the average torque rather than the peak torque. One convenient and
relatively inexpensive means to this end is the addition of a
to the system. A flywheel can be sized, designed, and fitted to
the camshaft to smooth variations in torque. Program DYNACAM integrates the
camshaft torque function pulse by pulse and prints those areas to the screen.
These energy data can be used to calculate the required flywheel size for any
selected coefficient of fluctuation.
that if a servomotor is used to drive the cam, then a flywheel should, in
general, not be fitted to the camshaft as its added inertia will make it more
difficult for the servo system to accelerate and decelerate the shaft to
maintain near-constant velocity in the face of torque variations.
Figure 9-23 shows
the variation in the input torque for a cam-follower system over one full
revolution of the camshaft. It is running at a constant angular velocity of 50
rad/sec. The torque varies a great deal within one cycle of the mechanism,
going from a positive peak of 341.7 lb-in to a negative peak of –166.4 lbin.
average value of this torque
over the cycle is only 70.2
lb-in, being due to the
work done plus losses
large variations in torque are evidence of the kinetic energy that is stored in
the follower system as it moves. We can think of the positive pulses of torque
as representing energy delivered by the driver (motor) and stored temporarily
in the moving follower train as kinetic energy, and the negative pulses of
torque as kinetic energy attempting to return from the follower to the
camshaft. Unfortunately, most motors are designed to deliver energy but not to
take it back. Thus the “returned energy” has no place to go.
9-20 (p. 247) shows the speed torque characteristic of a non-speed-controlled
permanent magnet (PM) DC electric motor. Other types of motors have differently
shaped functions that relate motor speed to torque, but all drivers (sources)
will have some such characteristic curve as shown in Figure 9-21 and 9-22 (p.
248-249). As the torque demands on the motor change, the motor's speed must
also change according to its inherent characteristic unless a speed-controller
compensates for the variation. This means that the torque curve being demanded
in Figure 9-23 will be very difficult for a standard (non-servo) motor to
deliver without drastic changes in its speed.
computation of the torque curve in Figure 9-23 was made on the assumption that
the camshaft (thus the motor) speed was a constant value. All the kinematic
data used in the force and torque calculation was generated on that basis. With
shown, we would have to use a large-horsepower motor (or a servomotor) to
provide the power required to reach that peak torque at the design speed:
power needed to supply the average torque is much smaller.
would be extremely inefficient to specify a motor based on the peak demand of
the system, as most of the time it will be underutilized. We need something in
the system which is capable of storing kinetic energy. One such kinetic energy
storage device is called a