Cam 1 rotates about fixed axis A and its theoretical, or pitch, curve is composed of two identical spirals a of Archimedes. Follower 2 reciprocates in fixed guides B-B and carries two rollers 3 and 4 which roll along the working curve (profile) of cam 1. When cam 1 rotates at constant speed, follower 2 travels at constant velocity with instantaneous dwells at the extreme positions. Positive motion is achieved because the sums of all opposing radius vectors of theoretical curves a are equal to the distance between the centres of rollers 3 and 4.
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