Journal of the Society of Motion Picture and Television Engineers (1950-1954)

222 RECOMMENDATIONS February prepare new standards. * It was for that subcommittee that these recommendations were outlined. Figures 1 and 2 are the recommendations for 16-mm and 8-mm sprockets, respectively. Provision has been made for camera, printer, and projector sprockets having any practicable number of teeth. Particular attention has been given to the shape of the film path and to the lateral profile of the sprocket itself and also to the lateral profiles of guides, rollers, and film gates. For simplicity of illustration, the film is often shown entering and leaving the sprocket in straight lines tangent to the root diameter, but by far the more usual path is a curve, either away from the sprocket or toward the sprocket. A minimum of y± inch is proposed for Rit the radius of the path that curves away from the sprocket. The proposed value of 0.7D for R2, the minimum radius when the film is curved toward the sprocket, is derived analytically. Accommodation for changes in film caused by shrinkage is the principal factor in the design of sprockets. There are two reasons why this accommodation is necessary: (1) the film must riot be damaged prematurely by the sprocket (this is important for projection equipment in which the same film may be run many times), and (2) on sound and printing sprockets, the film must run at a relatively constant velocity in order to ensure freedom from flutter. Fortunately, it is possible to design for good results in both respects. Three aspects of sprocket design for which the potential shrinkage of the film must be taken into account are: (1) the circular pitch of the teeth, (2) the shape and thickness of the teeth, and (3) the lateral profile of the sprocket. Circular Pitch of the Teeth ' The circular pitch of the sprocket teeth can be made longer or shorter than the pitch of the film. Only in rare cases will the pitches be equal. The choice depends largely upon * O. Sandvik (Chairman), H. Barnett, J. A. Maurer, L. T. Sachtleben, and M. G. Townsley. the type of service expected from the sprocket. Types of Sprockets. In motion picture equipment there are two basic types of sprockets. With the first type, the film is urged forward against backward tension, and the sprocket is a drive sprocket. . The second type is the take-up or holdback sprocket. Here both the motion and the tension are forward; hence the film is held back by the sprocket. The important rules for drive and holdback sprockets are: (1) A properly designed drive sprocket should have a circular pitch equal to or greater than the pitch of the film. (2) A properly designed holdback sprocket should have a circular pitch equal to or less than the pitch of the film. These rules, upon which the formulas of the recommendations are based, are so chosen that all slippage between the film and the sprocket is in the same direction as the tension on the film. Thus any friction between the film and the base of the sprocket serves to assist in the functioning of the sprocket rather than to increase the load between the film and the teeth. Also according to the rules, the leaving tooth is the one that does the work of driving the film or holding it back. There is, therefore, clearance between the entering tooth and its mating perforation, as long as the thickness of the teeth is such as to avoid interference. In addition, there is the combination sprocket, which is often used in reversible apparatus where the function of the sprocket changes as the direction of motion changes. Also, in many cameras and in some projectors, one section of a single sprocket serves as a drive sprocket and another section as a holdback sprocket. Moreover, the function of a sprocket may change owing to the varying tension exerted by the take-up, or for other reasons. Combination sprockets are not recommended for precision apparatus such as printers or other professional equipment. The optimum pitch for a combination sprocket is a compromise, at best. If a prop