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

1950 16-MM AND 8-MM SPROCKET DESIGN 223 erly designed drive sprocket is used as a combination sprocket, all film operating under holdback conditions is forced against the direction of external tension by each tooth as it enters the perforation. The same is true of a holdback sprocket operating under drive conditions. Experience has shown that unless special attention is given to guiding the film as it engages the sprocket, a drive sprocket makes a poorer combination sprocket than does a holdback sprocket. Therefore, the combination sprocket was designed to favor its action when it is driving film; its pitch matches that of film having a shrinkage equal to the minimum shrinkage plus one-third (instead of plus one-half) the shrinkage range for which accommodation is being made. Shape and Thickness of Sprocket Teeth Importance of Shape of Tooth. In all cases except that of perfect mesh there must be some sliding of the edge of the perforation up or down the face of the tooth. The shape of the tooth is important not only from the standpoint of wear of the film at the point of contact, but also as it relates to the sliding of the film along the root circle of the sprocket and to the manner of transfer of the load from one tooth to the next. In the worst case, all the shrinkage differential is absorbed by a sudden jump of the film as it leaves the tip of one tooth and comes into contact with the next tooth. APPENDIX TO FIG. 1 These recommendations for film sprocket design have been developed to give the design engineer an opportunity to specify sprocket dimensions for particular applications and conditions. They consist of a number of simple formulas for the computation of tooth thickness, tooth shape, and circular pitch based upon the range of film shrinkage to be accommodated and the amount of contact between film and sprocket. The root diameter on which the film will run is computed from the circular pitch and the number of teeth on one end of the sprocket. In cases where the film pitch does not match the sprocket pitch, the formulas for the sprocket pitch are such that the slippage of the film will be in the direction of external tension on the film, backward on feed or drive sprockets, and forward on holdback sprockets. The optimum pitch of the combination sprocket has been established to specify a sprocket that meshes perfectly with film having a shrinkage of the minimum shrinkage plus one third of the shrinkage range. Film of less shrinkage is forced against the direction of the external tension when this sprocket is operating as a drive sprocket. Film of greater shrinkage is forced against the direction of external tension when this sprocket is operating as a holdback sprocket. Combination sprockets should be avoided wherever possible. It will be noted that formulas for the maximum permissible tooth thickness involve the use of fractions of a film pitch length. (See definitions for H and F.) This provides clearance for the teeth in partial engagement with the film. The shape determined by dimensions K and B provides clearance at the tip of the leaving tooth for paths between R\ and /?2. For sound and printing sprockets optimum conditions of flutter are obtained when the film path curves toward the sprocket and approaches the limit defined by /?2. A more precise formula for R* for sound and printing sprockets is l//?2 = 19/JV 26 IN* 0.060. The sprocket in this case should be designed and used as a drive sprocket. If it is necessary to use a sound sprocket as a holdback sprocket, it should be designed as a drive sprocket, and film guides must be provided to force the film onto the teeth. The dimensions shown in the lateral profile views provide clearance for film with lateral shrinkage from 0 to 1.0 per cent for cameras and from 0 to 1.8 per cent for projectors. Film of greater shrinkage can be used on these sprockets, but either the film will pull away from the guide or the fillet of the perforation will engage with the tooth. These lateral dimensions are applicable also to film gates, guides, and pull-down claws. The choice by the engineer of the range of film shrinkage to be accommodated must depend upon the experience of the manufacturer and the type of equipment being designed. Based on current film conditions, suggested ranges would be from 0 to 1 per cent shrinkage for equipment using unexposed film and from 0 to 1.5 per cent shrinkage for equipment using processed film. In most cases, particularly when tensions greater than 2 oz must be overcome, it is desirable to make H at least two film pitch lengths. An exception to this occurs when the film is positively guided through a path curving toward the sprocket. In this case, if the value of F is high — at least 4 — the value of H may be zero.