9.9 TORQUE COMPENSATION CAMS

Another way to balance torque on a camshaft is to add one or more cams driving dummy loads (essentially springs) that are designed to provide an approximately equal and opposite torque to the shaft, as is generated by the actual driving cams that do the machine’s

function. If the motivation is simply to hide the cyclic torque fluctuations from the driving motor (as with a flywheel), then only one torque compensation cam is needed to counteract the sum of the torques of all the driving cams on a given camshaft. If on the other hand, one wishes to reduce the torsional stresses and deflections in the camshaft, then it may be necessary to add a compensating cam adjacent to each driving cam, thus cancelling the torque locally before it has an effect on the relative angular positions between driving cams.

Figure 9-27 shows such an arrangement in the form of a test fixture designed and built to test the efficacy of this approach to torque compensation.[3] The two cams, one representing a “driving” cam and the other a “compensating” cam are seen at the right end of the camshaft each moving a translating, spring-loaded follower train. In a real situation, one of these would be driving tooling and the other driving a dummy load. At the left end of the shaft is a flywheel serving as an inertial reference, friction driven by a speed-controlled DC motor. Each free end of the shaft is fitted with a 5000 count per revolution shaft encoder. Their difference measures the instantaneous end-to-end torsional deflection of the shaft. The shaft is made in three sections. The two ends that, respectively, support the cams and flywheel are large in diameter for good torsional stiffness, and the long center section is small in diameter, thus torsionally compliant to exaggerate the cam-induced torsional deflections for ease of measurement.