Cam 1 rotates about fixed axis A and consists of two members having lobes a and b a long circular arcs. Follower 2 reciprocates in fixed guides B-B and carries rollers 4 and 5 which roll a long the working surface of cam 1. The members making up cam 1 can be shifted angularly with respect to each other a long circular slot d and be clamped in the required position with wing nut 3. This enables the parts of the working surface of the cam, described by an arc of a common radius from centre A, to be increased or decreased. Consequently, the dwell of follower 2 can also be increased or decreased within certain limits. Positive motion is achieved because the sums of opposinfg radius vectors to points on the theoretical, or pitch, curve o cam 1 are constant and equal to the distance between the centres of rollers 4 and 5.
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