Film Transport

Design challenges

• Consistent and reliable vertical film frame registration
• Fragility of film
• Speed of operation
• Easy drive disengagement

My design of telecine film transport mechanism uses a simple cam driven claw pulling in line with the film path. Figure 2 shows the cam rider claw, its end was thinned to 0.7 mm thick for reliable entry into the 0.914 mm width of the sprocket hole of Super 8 mm film.

The advance of the film frame by frame is made by 360° rotation of the cam for each frame advance. The combined force of the bias spring and the rotational drag of the cam on the cam rider causes engagement by the claw in the sprocket hole. The sloped shape of the bottom of the claw raises the claw as it is pushed backwards to engage the next sprocket hole and causes disengagement of the claw from the current sprocket hole. The holding force of the film gate must exceed that exerted by the slope of the bottom of the claw on the film as it is moved backwards or the film will move too. The numbered sequence of Photographs 3.1 to 3.6 illustrates the operation. Figure 3 shows a diagrammatic representation of the operation.

Figure 4 shows the simple eccentrically mounted circular cam design used.

Photographs 4 and 5 show the 1.5 mm freedom of movement available at the claw tip that allows the claw to rise up out of the sprocket hole when the claw rider is pushed backwards by the cam and rises on the slope at the bottom of the claw. The movement also allows full disengagement of the claw drive from the film so that the film can be moved freely through the film gate or removed from it completely to facilitate repair of a failed film splice.

Photograph 6 shows the parts of the motor sledge followed by a naming of the parts list.

Naming of the parts