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June 1947 PROJECTION LlFE OF 16-MM FlLM 533
wedging on the holdback sprocket. However, when the pitch conditions are such that the entering tooth does not touch the film and the tension on the film is carried by the leaving tooth, the effect of the drive and holdback sprockets on the life of the film is significantly different, as is shown by a comparison of Figs. 7 and 8. As the sliding of the film on the holdback sprocket increases, the running life of the film also increases, at least up to a pitch difference of +1.5 per cent. In contrast, when the pitch difference between the film and the drive sprocket exceeds a certain optimum, the life of the film decreases.*
Thus, it appears that the conditions which permit increased sliding also cause increased damage to film running on the drive sprocket. The reasons for this damaging action and its relationship to the pitch difference between the film and sprocket are discussed in a later section of this paper.
Combination Sprockets and Reverse Projection. — It has been shown that the pitch of the drive sprocket should be greater than that of the film and the pitch of the holdback sprocket should be less than that of the film for the best operating conditions from the
* The failure of the maximum number of passages to occur at the point of calculated perfect mesh may be explained by the stretching of the film or distortion of the perforations when they engage the sprocket teeth under tension. If the film does stretch and becomes deformed on the sprocket, the effective pitch is greater than that measured on a standard pitch gauge, so that perfect mesh occurs when the measured pitch of the film is shorter than that of the sprocket. Experiments now in progress indicate that the stretch and deformation are surprisingly high Thus it may be that the maximum number of passages is obtained when the film and the sprocket are actually in perfect mesh.
It is possible that the peak in the curves of Fig. 8 is displaced too far from the point of calculated perfect mesh to be explained purely on the basis of stretching or distortion of the film. Another explanation appears quite reasonable. When the pitch of the film is shorter than that required for perfect mesh, the film slides backward on the circumference of the sprocket each time the leaving tooth disengages. On a drive sprocket, the film in contact with the sprocket circumference lies between the driving tooth and the source of tension. Thus, the film is wrapped under tension against the sprocket surface, and the frictional resistance to sliding can be appreciable. When the film slides on the sprocket, the friction between the film and the circumference of the sprocket helps drive the film, thus decreasing the strain on the perforations and allowing a higher number of passages before breakdown of the perforations occurs. The greater the amount of sliding, the greater the amount of strain absorbed by sliding friction. This is true up to a certain pitch difference. When this optimum pitch difference is exceeded, the life of the film decreases, indicating that some damaging action has overcome the beneficial effect of sliding on the sprocket.