A cam mechanism includes a cam 10 of basic circular formation and having a lobe formation 11 extending radially outwardly along part of its periphery; and a cam follower 16 mounted for reciprocating movement along an axis perpendicular to the axis of rotation of the cam 10; the cam acting against an end face 15 of the cam follower 16 so that engagement of the lobe formation 11 therewith will cause movement of the cam follower 16, the inclination of the face 15 of the cam follower 16 which is engaged by the lobe formation 11 being adjustable to vary the duration of movement of the cam follower 16 by the cam 10.