The motion picture projectionist (Nov 1930-Oct 1931)

28 Motion Picture Projectionist June, 1931 Fig. 2. Schematic arrangement of Type BAG projector function of the area of contact, the ratio of the latter to total peripheric area of the shaft, and the coefficient of friction may easily be made small. Moreover, the ratio of the pull transmitted by the shaft to the pull applied to maintain contact of the belt increases rapidly with the latter. In the case of film which is looped around a well-designed friction roller, a pull of a small fraction of an ounce exerted on the film on the outgoing side of the friction roller is sufficient to permit the latter to transmit a pull of several pounds to the film on the other side of the friction roller. The friction roller thus does not relieve the driving sprocket — and the perforation edges — of all pulling strain, but limits the pull on the perforation edges to the tension required to maintain slipping contact between film and friction roller. In other words, it limits the pull to a negligible fraction of its normal value. The same explanation applies as well to the action of the friction roller associated with the take-up sprocket, which revolves more slowly than the latter and resists a pull instead of applying one, as to the action of the friction roller associated with the feed sprocket. The Recent Type BAG On a more recent type of Oehmichen projector, Type BAG, a much more radical departure from conventional design is attempted. The BAG projector appears to have been designed primarily for use in schools or by amateurs, and it would seem legitimate to claim for it the advantages of great simplicity and cheapness of construction as well as those of the reduction of film wear. It has a pulldown mechanism with self-engaging claws which presumably does not differ much from that of its predecessor. It contains no sprocket at all, and has no positive drive. Friction rollers with automatically-regulated slippage take the place of both feed and takeup sprockets. As in the case of the GC4 projector, the speed of rotation of the feed friction roller is so adjusted that it tends to pull the film out of the supply reel and feed it to the pull-down mechanism faster than the latter requires. The take-up friction roller revolves at such a speed that, allowing no slippage, it would feel film to the take-up reel more slowly than it receives it from the pull-down mechanism. A certain amount of slippage is thus neeessary to prevent the formation of loops of unreasonable size. Since there are no driving sprockets of which the positive drive can be used, as in the case of the GC 4 machine, their regulated pull cannot be used to control the degree of contact between film and friction roller and thus regulate their slippage. Moreover, the pull exerted by the pulldown mechanism is obviously too abrupt to permit its use to regulate slippage directly. As a matter of fact, loops of a certain size must be maintained both above and below the pulldown mechanism to permit the smooth operation of the latter. Fig. 2 shows schematically how the problem of maintaining these necessary loops and simultaneously utilizing the pull of the pull-down mechanism to control slippage has been ingeniously solved. Between the puldown mechanism and both the feed and take-up rollers, the film is passed back and forth between the friction roller and a guide roller placed in close proximity to the friction roller. On both sides of the pull-down mechanism loops are thus formed which permits smooth and effortless operation. Effective Slippage Control It would seem, however, that since no pull is applied on the film at the points where it leaves the friction rollers to form these loops, there would be nothing to check slippage once the projector is put in operation and that these loops could not be maintained. Such, however, is not the case. In order that the loop should be lost, it would have to be reduced to a very small size in order to slip out between the friction roller and the guiding roller. A very simple experiment, illustrated in Fig. 3, which consists of attempting to slip a loop of film from between two fingers held almost in contact, will show at once that this cannot be done easily. The film resists bending at a sharp angle, and the two sides of the loop begin to exert a very appreciable pressure on the fingers as soon as the radius of curvature of the loop falls to certain limits. In the same way, as soon as the action of the pull-down mechanism, or that of the take-up reel, have reduced the size of the upper and lower loops to a certain radius of curvature which is a function of the thickness and elasticity of the film, but apparently never falls below safe limits, an appreciable pressure is exerted by the film against the friction roller. This pressure, increasing the adherence of the film to the friction roller, is identical in its effects to the steady pull of the driving sprockets in the GC4 projector. It permits the friction roller to transmit to the film a steady pull which is out of proportion to the pressure exerted by the loop. In other words, the loop adjusts its own size by controlling the slippage. It is asserted that this automatic regulation of the size of the loop is a further great advantage of this projector and greatly tends to make its threading fool-proof and increase the reliability of its performance. No care whatever need be taken to adjust the size of the loops before starting since they automatically adjust themselves within a few revolutions of the machine. Many irregularities and faults in the film, the occurrence of which would result in disastrous damage to the film in the ordinary projector, cause in the sprocketless projector only a momentary disturbance followed instantaneously by a return to normal operation. It is claimed for both the GC4 and the BAG projectors that they may safely be run at speeds as high as 180 feet per minute, and that, by renewing splices after every thousand projections, it is possible to project the same film 30,000 or 40,000 consecutive times without producing more than a normal amount of wear on the film. Fig. 3. Illustrating the principle by means of which the loop is maintained in the projector; due to the pressure exerted by the loop it is difficult to pull it through the fingers Proper S. R. P. Markings Any information the individual may wish to furnish the projectionist should be printed in black letters on a white background in the 32 frames immediately following the Part Title in the Identification Leader of the Standard Release Print. This information may also be placed in the 32 frames immediately following the End-of-Part Title in the Identification Trailer.