The motion picture projectionist (Nov 1930-Oct 1931)

June, 1931 Motion Picture Projectionist 27 Novel Projector Reduces Film Injury The projectors described in the accompanying article feature a design which is intended primarily to reduce greatly the injury which is now done to the films during projection in noncontinuous projectors. This article appeared originally in the French paper Le Cinephose, Vol. XII, November, 1930, translation and abridgement from which ivas made for the S.M.P.E. for May, 1931, by A. G. Denis of the Eastman Kodak Co. IN this paper are described by "Codus" two motion picture projectors designed and marketed in Europe by M. Oemichen. The characteristic features of their design are intended to reduce the effects of wear and injury to the film as much as is possible in a non-continuous projector. Worn and torn sprocket holes, after a few hundred projections in the conventional type of projector, result in the screen image becoming too "jumpy" for continued use of the film. The designer attributes this mainly to the excessive pull exerted upon the edges of the film perforations by the sprocket drive. The pull is exerted at two places: First, the feed sprocket pulls the film out of the supply reel, against the heavy friction applied to prevent uncontrolled unwinding of this reel, and the strain of this pull is exerted upon the perforation edges at the feed sprocket. Second, the friction drive of the take-up reel exerts a similar pull upon the film which is resisted by the take-up sprockets, and the strain of this pull is similarly exerted upon the perforation edges at the take-up sprocket. The tension applied results in both cases in a tearing effect upon the perforation edges when the films is either forced down upon the sprocket teeth or pulled away from them. The action of the pull-down claws is similarly exerted upon the perforation edges. However, "Codus" indicates that the designer of the Oehmichen projectors does not consider the action of the pull-down claws as one of the primary causes of wear, and, if his theory may be considered as correct, the action of the pull-down claws is not resisted by a heavy pull like that of the driving sprockets but is resisted only by the friction applied at the gate. Furthermore, although the Oehmichen machines embody every feature calculated to reduce to a minimum any wear of the film which may be due to the action of the pull-down claws — multiple claws, a special gate of patented design, independent pressure pads, and a very low pressure of 3 ounces — the Oehmichen gate and pull-down mechanism present no feature constituting a radical departure in design, as do the feed and take-up sprockets. New Roller Arrangement On one type of the Oehmichen projector (Type GC4), there are provided feed and take-up sprockets of conventional design whose action is supplemented by that of driven friction rollers. The function of these rollers is not to drive the film positively but solely to take up, in a manner which cannot result in injury to the film, the strains required to pull the film from the supply reel and to resist the pull of the take-up reel. This avoids putting heavy strains upon the perforation edges at the sprockets themselves and the film enters and leaves the sprockets without appreciable injury. The friction rollers are cylindrical drums of the same width as the film, covered with rubber on their edges. The perforated edges of the film are in contact with a rubber surface on both sides and the picture area of the film does not come into contact with the surface of the rollers. It will be seen by referring to Fig. 1, which illustrates in schematic manner the Oehmichen Type GC4 projector, that no pressure rollers are used to maintain the edges of the film in contact with the friction rollers, but that the film is merely looped around the greater part of the periphery of the friction rollers by means of standard TAKE-UP ROLLER V= RATE OF TRAVEL OF THE FILM Fig. 1. Schematic arrangement of Type GC4 projector guiding rollers. As a matter of fact, no continuous positive contact such as would allow no slippage is sought between the film and the friction roller. The friction roller intended to supplement the action of the feed sprocket revolves at a peripheral speed slightly higher than that of the feed sprocket itself, and the friction roller used in conjunction with the take-up sprocket revolves at a peripheral speed slightly lower than that of the latter. It will be obvious that if continuous positive contact were established between the friction rollers and the film, loops would at once form between the friction rollers and the sprockets with which they are associated. However, since there is no mechanism provided to maintain continuous positive contact between film and friction rollers, and since a heavy pull is exerted by the film in the case of both friction rollers on the side away from their associated driving sprockets, no such loops can actually be formed. This is due to the fact that any slackness of the film between the driving sprocket and the friction roller must manifestly, long before a loop can be formed, cause the loss of positive contact between film and friction roller and must result in slippage. Film Slippage Factor The greater or lesser degree of slackness of the film between friction roller and driving sprocket must provide a continuous and instantaneous regulation of slippage on the friction roller. This slippage decreases, and a positive contact may even be established momentarily whenever an appreciable tension of the film happens to develop between the friction roller and the driving sprocket. The slippage increases rapidly as soon as a certain degree of slackness of the film between friction roller and driving sprocket has been exceeded. That the friction roller can effectively be made to assume the pulling strains exerted by the film and relieve the driving sprockets of all but a negligible fraction of this strain, will become apparent when it is realized that the mechanism constitutes an ingenious and simple application of a familiar principle in the operation of the capstan. It is well known that if a belt be looped around a rotating shaft, the continuous pull which it is required to maintain on the outgoing side of the belt in order to prevent slippage and maintain positive contact between the belt and the shaft is only a fraction of the pull which it is desired to exert on the ingoing side of the belt. This fraction, which is a