Sprocket wheels 1 and 2 rotate about fixed axes A and B and their teeth engage the slots in endless steel belts 3 and 4 which run over pulleys 6 and 7, rotating about fixed axes C and D. When sprocket wheels 1 and 2 rotate, the first counterclockwise and the second clockwise, film 5 is gripped by the inner sides of links 3 and 4 and, owing to friction, advanced downward.
$3585$CFL,FD$