Motion Picture Herald (1953)

damage runs a close second to film scratching. Although it may not be visible to the audience, sprocket hole damage may produce even worse effects — film jamming and film fire. During projection, sprockets may produce damage to film due to one of the following causes : (1) Badly worn sprocket teeth; (2) damaged sprocket teeth; (3) poor alignment betiveen sprockets, film, shoes and guide rollers; (4) excessive tension hy upper magazine brake, on film gate shoes, and from take-up clutch; (5) shrunken or deformed plm. I'he teeth in a projector sprocket are supposed to register exactly in the film sprocket holes so that the film is engaged and pushed at the base of the tooth, not by the narrower tip or intermediate section of the sprocket tooth. In a properly fitting sprocket, the tips of the entering and leaving teeth should never touch the film, otherwise chattering and jumping will surely occur. INTERMITTENT SPROCKETS Intermittent sprockets may suffer damage in excess of normal wear produced by film. The most common is caused by the apron shoes rubbing against the sides of the teeth, or by the shoe frame rubbing against the tips of the teeth. This metal-to-metal contact will quickly ruin a sprocket and can be produced only by mis-shaping the gate apron, or by inserting the gate in the wrong position. Either of these errors is inexcusable and should be remedied immediately. The metallic clicking produced certainly cannot go undiscovered for very long. Should this happen, the intermittent should be removed and the sprocket teeth should be examined one by one, with a magnifying glass under a strong light, to make certain that no gouges or sharp edges have been developed, because these would cut into the film like so many knives. If the film guides and rollers In the film gate are out of line with the sprocket teeth, the sides of the sprocket holes will be forced against the tooth bases causing fast wear of the teeth and corner tears In the film itself {see accompanying photographic reproductions of sprocket tooth damage). OTHER SPROCKETS The only other sprockets which are loaded to any extent during projector operation are the upper feed sprocket and the sound hold-back sprocket. The upper sprocket in the projector has to pull the film from the upper magazine, and here the wear of the sprocket teeth is directly proportional to the restraining force of the upper spindle brake mechanism. It goes without saying that this mechanism should be checked and lubricated at intervals to Insure a minimum of retarding action to permit the smooth feeding of film to the projector. Any extra braking action above this merely serves to increase wear on the upper sprocket and possibly deform the film sprocket holes. The hold-back sprocket In the sound head, as the name Implies, serves to prevent Figure D Figure B FIGURE "A" pictures a brand new tooth with its full curvature. FIGURE "B" shows the face cut at the base of the tooth, indicating the progression of normal wear. FIGURE '"C" shows a tooth with excessive wear. Here the concave indentation half way up the tooth means that the film was beginning to strike higher and higher as the tooth was entering the sprocket hole; all due to the fact that the wear at the tooth base had progressed too far. FIGURES "D" and "E" definitely indicate very hard rubbing of apron shoes against the teeth. It seems impossible that the racket produced by this kind of metallic striking should go unnoticed by any one with normal hearing. Much film can be damaged by the sample shown in Figure "D." FIGURE "F" shows the beginning of side film rub, or of side metal scrape. (Photos courtesy of Eastman Kodak Co.) Figure C BETTER THEATRES Section, Guide Number of 1953 21