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Standards Committee Report
409
Film shrunk 0.75 per cent representing average film met with in service. Perforation pitch is sHghtly less than intermittent sprocket tooth pitch. The film is engaged by the last tooth B leaving a sUght clearance at the other teeth. As film comes off tooth B, it is engaged by the next tooth. Motion of film is aided by the snubbing action between the film and the base diameter of the sprocket.
Film is held against rewind pull by the last tooth D of the take-up sprocket. There is increased clearance at the back of tooth C, hence no interference at the entering tooth. Compared to new film there is decreased shpping from tooth to tooth due to rewind pull.
Film shrunk 1.5 per cent representing oldest commercially useful film. Intermittent sprocket moves the film by the leaving tooth B. No interference by the back of the entering tooth A occurs until the film is shrunk 2.92 per cent.
Take-up sprocket has same tooth pitch as film perforations; i.e., the theoretically perfect running condition six teeth are engaged. If film is shrunk more than 1.5 per cent interference will occur at the entering tooth C and the sprocket holes will be torn.
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CHART IV
Sprocket dimensions proposed by the Paris Congress
Intermittent and take-up sprockets both have the same base diameter, 0.9390 in. (23.85 mm.). The tooth pitch is that of the perforation pitch of film shrunk 0.78 per cent.
Film is shrunk 0.13 per cent. Entering tooth A of the intermittent sprocket engages the film perforation. When the next tooth engages film it pushes the film forward out of engagement with forward teeth. If tension is too great or film soft as in the case of new film the sprocket hole is Hkely to tear instead of pushing the film forward. This condition is to be avoided as it causes damage.