Sprocket wheel 1 rotates about fixed axis A and its teeth engage the slots in endless steel belt 2 which is in contact with either guide roller 5 or 6. When sprocket wheel 1 rotates counterclockwise, band 2 advances film 3, owing to friction, winding it onto drum 4. The provision of two guide rollers, 5 and 6, enables the film to be advanced in either direction by friction, winding it on or off of drum 4, by reversing wheel 1.
$3586$CFL,FD$