Abstract: A plurality of translation rods are mounted in a cage for pivoting about a common axis and pass through the pivot axis, being at fixed circumferentially spaced positions but movable bidirectionally along their individual axes during rotation about the fixed axis shaft. Cam follower rods pivotably mounted on respective ends of the translation rods are slidable axially relative to their axis and through their pivot axis. The cam follower rods carry rotatable cam followers through tension springs, which normally maintain the cam followers at a predetermined distance from the cam follower rod pivot axis in the absence of cam follower contact with a fixed cam mounted in juxtaposition to the path of the rotation of the translation rods. The major surface of the fixed cam is generally at right angles to the direction of constant forces applied to respective ends of each translation rod.