Link 1 of slotted-link mechanism ABC rotates about fixed axis A. Slotted link 2 turns about fixed axis C and has slot b along which pin B of link 1 slides. In the extreme upper position of slotted link 2, spring 3 inserts claw a of link 4 into a perforation of the film. Link 4 is connected by turning pair D to link 2. Upon further rotation of crank 1, claw a advances the film downward. When slotted link 2 starts to move upward from its extreme lower position, claw a is withdrawn from the perforation and slides upward along the film.
$1393$LG,OC$