9.8 CONTROLLING CAM SPEED—FLYWHEELS

As 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.

Unless 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 flywheel 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.

Note 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.

TORQUE VARIATION 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. The average value of this torque over the cycle is only 70.2 lb-in, being due to the external work done plus losses . The 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.

Figure 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.

The 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 the torque

variation 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:

The power needed to supply the average torque is much smaller.

It 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 flywheel .