Journal of the Society of Motion Picture Engineers (1930-1949)

Nov., 1930] A NEW RECORDER 655 we figure the rate of slip-back in terms of tooth shape. If the tooth surface were a circular involute no slip-back could occur while the film is sliding up along the tooth surface, and the inevitable readjustment would take place when the film is actually clear of the tooth, at which time a jump-back would occur. The tooth should evidently be cut back — more taper than an involute — so as to permit the film to slip back continuously while the stripping is taking place. Two suppositions may be considered, (1) that the tooth is so cut that the rate of slip-back is not constant, for example, that the slipping back is slow at first and more rapid later, and (2) that the tooth is cut to give a constant rate of slip-back. The first design obviously gives a net film speed which is not constant, although the departure from FIG. 2. Relation between sprocket teeth and film perforations when film is short. constancy may with good design be small. If the tooth is so cut that with stripping of the kind mentioned above, the slip-back is at a constant rate, then there are three analyzable cases to consider. (1) The film slips back sufficiently to touch the bottom of tooth No. 2 before it begins to strip from No. 2, or, in other words, before it pulls away from the cylindrical surface at the bottom of tooth No. 2. (2) The film slips back sufficiently to touch the bottom of tooth No. 2 just as the stripping from No. 2 begins, and (3) the film does not slip back sufficiently to touch No. 2 when stripping from No. 2 begins. These three cases might arise from use of different tooth pitch and shape with a film of given perforation spacing, or with a given sprocket they correspond to three different degrees of film shrinkage. In case (1), the film moves at non-slip speed so long as it remains