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under the pressure of the driving tooth revealed that the film was not staying down against the drum, but was sliding up on the inclined face of the tooth by an amount varying from 0.001 in. to 0.003 in. Some of the causes for this are illustrated in Fig. 4. They include greater compliance and sharper bending of the film in the region of the perforation; distortion of the film because of the component of force normal to the film created by the driving force of the inclined tooth face; plastic deformation of the film under pressure; and nonuniformity of coefficient of friction from tooth to tooth and from perforation to perforation.
An attempt was made to reduce these errors by providing an external holddown shoe of the same radius as the external surface of the film. While this resulted in substantial improvement, picture steadiness still did not meet expectations.
The Radial-Tooth Concept
With the above-outlined studies as a background, it was realized that for accurate longitudinal registration of the film the ideal driving face for the tooth
would be one which lay on a plane normal to the film (parallel to the sprocket axis), as shown in Fig. 5 A. Essentially this would mean that the driving face of each tooth would lie on a radius of the sprocket. A driving face of this type would have several important advantages. It would produce no radial component of force on the film, thereby eliminating outward distortion of the perforation from this cause. It would bear squarely against the full thickness of the film instead of against the sharp, somewhat irregular corner of the film. The position of the film on the face of the tooth would have very little effect on longitudinal registration of the film. Such a tooth would in many ways be substantially the equivalent of a registration pin in an intermittent mechanism. Under normal circumstances, the difficulty with using a tooth with a radial driving face is that it is impossible to get the film off the teeth, because the involute curve described by the driven edge of the perforation as it leaves the sprocket tangentially cuts into the radial tooth, as shown by the dashed curve in Fig. 5B. This difficulty can be circumvented by combining the radial tooth with the variable-pitch concept.
INVOLUTE
CENTER OF SPROCKET
B
Fig. 5A. Ideal tooth with driving face normal to film plane. Registration essentially independent of position of perforation on tooth face; greatly reduces errors from film distortion and bending.
Fig. 5B. Interference of film with radial tooth at the stripping point. Provision must be made for advancing perforation relative to tooth so that film can be disengaged from sprocket.
J. G. Streiffert: Radial-Tooth, Variable-Pitch Sprocket
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