When crank 1 rotates, the tip of claw a, mounted on flat spring 3 which is joined to connecting rod 2 of four-bar linkage ABCD, describes a connecting-rod curve. At one of the portions of this curve, claw a engages a perforation of the film which it moves. At another portion of the connecting-rod curve, claw a is withdrawn from the perforation.
$772$LW,OC$