Cam 1 rotates about fixed axis A and its contour is composed of two symmetrical portions of spirals of Archimedes. Follower 2 reciprocates in fixed guides B-B and carries two rollers 3 and 4. Positive motion is achieved because all the diameters of the theoretical, or pitch, curve of cam 1 are equal, i.e. a͞a=a͞'a'=...=const, and equal to the distance between the centres of rollers 3 and 4.
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