International projectionist (Jan-Dec 1947)

than that of the sprocket, the film wedges onto the sprocket teeth, the film travelling faster than the circumference of the sprocket. Consequently, all of the driving action is accomplished by the sprocket teeth. When 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. Films of any pitch whatsoever may be run on projectors with intermittent sprockets of different diameter and, if other factors are controlled, the results will still be comparable. Sprocket and Film Wear These principles have been recognized by film manufacturers and projector manufacturers alike, and this general agreement was the basis for the approval of the .0945-inch diameter spocket and its adoption as an American Standard in 1930. Why, then, has this larger sprocket not been in general use in this country. The answer is that projector manufacturers have been concerned over the possibility of increased wear of the sprockets, their fears being based on the theory then held. It is quite obvious that if the film be considered as a steel tape, the wedging action of the film onto Leans Toward .940-Inch Sprocket By KARL BRENKERT Brenkert Light Projection Company I have always felt that the intermittent sprocket diameter should be greater than .935-inch; but I am inclined to doubt that it should be as great as .943. I believe our attitude will be to make these sprockets to about .940-inch diameter, which we feel will not cause any difficulty and will be an improvement over the .935 diameter. With this larger sprocket the entering tooth will not have to climb the film perforation as far as it now does on the .935inch diameter. .943 Sprocket Entirely O.K. By LARRY DAVEE Century Projector Corporation We believe that the .943-inch sprocket is entirely satisfactory. You will recall that the Waller Gunnery Trainer (1941 et seq.) , using nine Century projectors operating in synchronism, used the .943 sprockets. This equipment was to operate 24 hours per day continuously (not intermittently as in the theatre, i.e., two-projector operation). We thus obtained operating records which could not be duplicated in regular theatre operation except over long carefully controlled periods. The experience of a few years ago anent the larger sprocket was that it created "undesirable noise". This may have been true for many reasons, none of which, however, were adverse to picture quality or to the life of the projection equipment or the film. 'Noise-Conscious' Projectionists Some projector manufacturers have long concentrated on a selling campaign of "quietness of operation". This has gone so far as to make the projectionist unduly conscious of "necessary noise," and especially those originating in and around the intermittent movement. The noise with the .945 sprocket consisted of a "tearing" sound caused by shrunk, brittle film leaving the sprocket. It caused no damage to sprockets or film, but to the sensitive ears of the "noise-conscious" projectionist it certainly sounded bad. Eastman Kodak Co., however, has since improved the shrinkage characteristics of film to a point where the .943 sprocket is wholly desirable. A .945-inch diameter is, I believe, the exact dimension corresponding to the sprocket hole pitch of unshrunk film, thus the .943 dimension allows a little margin of safety. The aforementioned "noise condition" will occur only when the pitch of the sprocket holes of the film is less than the pitch of the sprocket teeth. the entering tooth should wear away the face of the tooth uniformly. The principal objective of the S.M.P.E. sub-committee previously mentioned was the determination of the comparative wear of the .935-inch and the larger intermittent spEockets. Fortunately, the theory formerly held as to sprocket wear was somewhat in error. In every theatre in which the oversized sprockets were compared with the .935-inch ones, the latter showed the most wear. Wear on the .943-inch sprocket is predominantly at the base, but it is slight and not sharply defined. There is no evidence of a hook. The wear on the .935-inch sprocket, however, shows that the* film cuts deep grooves into the face of the teeth. The striking thing about these grooves is that they vary in depth and in the distance from the base of each of four teeth. The increased extent of wear on the .935-inch sprocket as compared to that on the oversized sprocket is explainable. In the case of the former, the additional force necessary to thrust the film ahead of the rotational speed of the sprocket manifests itself in increased wear of both film and sprocket teeth. Proper Film-Sprocket Pitch When the pitches of film and sprocket are equal, the driving force is spread over two, or even three, teeth simultaneously, resulting in decreased wear on both film and sprocket. This condition should hold true even when the two pitches are nearly equal, as a result of the local stretching of the film upon impact. Fortuitously, therefore, the use of larger intermittent sprockets enable substantially increased film life with no increase in sprocket wear. It is impossible to align the film on the intermittent sprocket so that the latter will strike the film in the center of each row of perforations. This is because the transverse pitch of the sprocket is less than that of the film. If the film be centered on the intermittent sprocket on one row of perforations, the sprocket will ride very near the inside corner of the other row of perforations and wear or rupture will then almost invariably occur at the point nearest the sprocket tooth. It is believed that the best alignment with the present intermittent sprocket occurs when the sprocket is centered as nearly as possible on each row of perforations. Even under this condition the preponderance of wear occurs in the corners, and occasionally at the outer corners also. Only rarely does it occur directly under the point of impact in the case of the .935 sprocket. This is because the perforation is weakest at the points of curvature, the wedging-on action causes a strain throughout this en( Continued on page 27) O.K., But Dictates Caution By HERB GRIFFIN International Projector Corporation I approved the .943-inch intermittent sprocket because of the voluminous work done by the special S.M.P.E. committee, by Eastman Kodak Company, and by the projector manufacturers themselves. The ; trouble experienced in the '30's with the .945 sprocket was caused, I believe, by the amount of shrinkage present in release prints at that time. However, following extensive work by Eastman, film shrinkages are now of an extremely low order, and there should be no difficulty experienced with the .943 sprocket. It will readily be understood, of course, that projector manufacturers, mindful of their past experience with larger sprockets, would want to make extensive experiments, preferably under actual theatre operating conditions and under the watchful eyes of first-flight projectionists, before going allout on the proposed new standard .943 sprocket. INTERNATIONAL PROJECTIONIST May 1947