SEED Unit Design - Cam Mechanisms < >

Referring to Figure 6, the cam is effectively a specially profiled disc shown keyed to a rotating shaft. The follower is a separate member which, in the configuration shown, is free to slide along a straight-line path co-incident with a centre-line of the driving shaft. A roller, freely pivoted to the follower, is forced against the cam profile by a spring to maintain a "higher pair" connection to complete the mechanism.

Thus the distance between the cam and roller centres determines the instantaneous location of the follower. Increasing cam radius with rotation, from A to B, drives the follower away from the camshaft to generate a rise movement; the converse action, from B to Q is the fall. The follower dwells with zero velocity whilst the roller traces the constant radius segment of the cam profile, from C through D to A. Each part of the cycle can be considered individually.

Fig 4 High-speed tape feed mechanism
Courtesy Creed-ITT Ltd.

In comparison with a linkage the cam mechanism has the advantages of absolutely stationary dwells, independent times for every part of the motion and, by shaping the profile appropriately, complete control of the output velocity and acceleration. The penalty for obtaining these benefits is the greater expense incurred in the precision manufacture of the cam profile.

In theory there is pure rolling between a roller follower, such as a special needle bearing having a re-inforced outer race, and the cam profile. In practice slip may occur. (Why? Study the forces and angular velocity of the roller during one cycle). Hence well-lubricated sliding followers, Figure 8, are sometimes preferred. They have good load capacity and can operate at greater cam speeds than roller followers. The knife-edge type, Figure 9, has poor load capacity and is found only in lightly-loaded instruments.

Fig 5 Timing Chart for the High-Speed Tape Feed Mechanism shown in Fig 4 Courtesy Creed-ITT Ltd.