International projectionist (Jan-Dec 1945)

FIGURE 1. Action of film on a sprocket when the pitch of the film perforations is greater than the pitch of the sprocket teeth. mesh, but, in general, so far as film wear is concerned, it is better to have the pitch of the film less rather than greater than that of the sprocket, as is the case today. This fact is illustrated in Figure 3, which shows graphically the increased number of projections, plotted along the ordinate, as the shrinkage* of the film, plotted along the abscissa, causes the pitch of the film to become equal to, or slightly less than the pitch of the intermittent sprocket. The shrinkage of film necesary to give perfect. mesh with a 0.935-in. sprocket is illustrated by the dotted line. All points to the left of this line represent Case I, in which the pitch of the film is greater than that of the sprocket, and all points to the right of the dotted line represent Case II, in which the pitch of the film is less than that of the sprocket. Film Shrinkage The actual number of projections required to produce complete breakdown of the film at the various film shrinkages will vary widely with machine conditions and the manner of operation. The maximum may occur under some conditions at several hundred to one thousand passages through the projector; under still better conditions, it may occur at several thousand passages. Cases have been noted in which the maximum occurred at 22,000 passages. These figures were obtained by projecting short loops of film continuously on a simplified machine under carefully controlled conditions. The number of projections, therefore, are probably higher than can be obtained in practice be * The values along the abscissa are expressed as "per cent shrinkage" of the film, for convenience. Since it would be impossible to obtain shrinkages of such great range with the present motion picture films, many of the samples were perforated less than standard pitch on specially designed equipment in order to s'mulate films having these different shrinkages. FIGURE 2. Action of film on a sprocket when the pitch of the film perforations is less than the pitch of the sprocket teeth. cause of the carefully controlled conditions under which such tests must be run. The main point is that there is a maximum and that this maximum! occurs on a 0.935-in. sprocket at a shrinkage of film far greater than exists in any present-day motion picture positive film. (The amount of shrinkage of most motion picture positive film lies between 0.0 and 0.6 per cent during its normal projection life.) Theoretically, the film is in perfect mesh with the 0.935-in. sprocket when it has a shrinkage of 1.2 per cent, and one might expect that the maximum should occur at this point. Frequently, it does occur close to this point, as indicated in Figure 3. However, it occurs often at a point well beyond the point of perfect mesh, as illustrated by Figure 4. The reason for the failure of the maximum passages to occur at the point of perfect mesh has not been clearly established. The following is offered as a possible explanation: In Case I, in which 2400 _ the .pitch of the film is greater than that of the sprocket, the film wedges on to the sprocket tooth, the film traveling faster thafiithe circumference of the sprocket. Consequently, all of the driving action is accomplished by the sprocket teeth. In Case II, in which the pitch of the film is less than that of the sprocket, the circumference of the sprocket travels faster than the surface of the film. In ■ this case, a considerable portion of the driving action is accomplished by contact along the circumference of the sprocket as well as by the sprocket teeth, thus causing the number of passages to increase to a certain point as the pitch of the film becomes less than that of the sprocket. Likewise, the failure of the maximum number of projections to occur at the calculated point of perfect mesh can be explained by the stretching of the film at the point of impact. If the film stretches upon impact of the sprocket tooth, and this appears to be a reasonable assumption, the effective pitch is greater than the calculated or measured pitch. Therefore, to obtain the optimum projections, the pitch of the film should be less than that required to give perfect mesh. Optimum Projections There is another and perhaps more satisfactory manner in which data of this type can be presented graphically. Since it is not the actual pitch of the sprocket and of the film, but only the difference between them which matters, we may express this difference graphically along the abscissa as the percent deviation of the film from the pitch of the sprocket, as in Figure 5. Zero on the abscissa, therefore, represents a perfect fit of the film on the intermittent sprocket in question, and, as before, all points to the left of the line represent Case I, in which the pitch of the film is greater than that FIGURE 3. Relationship between the pitch of the film and the number of projections necessary to produce film breakdown, using an 0.935-in. intermittent sprocket. Shrinkage or FILM 8 INTERNATIONAL PROJECTIONIST