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32 Transactions of S.M.P.E., January 1927
bottom of the perforations. In the front portion of the film the teeth enter and in the rear they leave it. We must also distinguish, in film operated by a sprocket, between the portion under stress and that which is not under tension. That under stress is that which is in contact with the ^ pull down, or which meets with a resistance coming from friction in the track, or from the weight of the roll that the sprocket is pulling from. In the first case we have an idler sprocket, and in the second a pull down sprocket.
When the pull is in the direction of the arrow K in Fig. 1, the film touches only the back edges of the teeth 1-5. The film is not in contact with the front edges, for the teeth are smaller than the perforations. The words "front" and "back" must be understood in the direction of the movement. The sections of the teeth of the sprocket may be shown as in Fig. 2, where the teeth are shown inside the perforations of the film. When the pull is made from the left side, the right side of the perforations are in contact with the teeth. In the case represented by Fig. 2, the position of the five teeth in contact is exactly the same; that is, each of the teeth is in contact with the right side of the corresponding perforation, but this case is a special one occurring only when the pitch of the sprocket is identical with that of the film.
We will term a sprocket normal relatively to a given film when it is of the same pitch as the film and vice versa. A normal film in regard to a given sprocket is that which has the same pitch as the sprocket. If we consider a film that is stretched; that is, a film of pitch greater than the sprocket, the conditions of the pull down are shown in Fig. 3. Tooth 5 is no longer in contact with the edge of the corresponding perforation and the same thing is produced to a corresponding degree for tooth 4 and those which follow it. Only tooth / bears on the film, and in this case the tooth and the perforations are subjected to a strain five times greater than is the case in Fig. 2.
If we now consider a shrunken film, that is a film of which the ])itch is less than that of the normal film, we see in Fig. 4 that the first perforation is no longer in contact with the tooth. There is a space between the film and the tooth, a space which diminishes up to perforation 5, this perforation being the only one which bears with its following edge on the corresponding tooth exactly like perforation 5 in Fig. 2. Just as before, perforation 5 and the corresponding tooth are under a strain five times greater than is the case in Fig. 2.