The motion picture projectionist (Nov 1931-Jan 1933)

22 Motion Picture Projectionist March, 1932 Th eory and Fundamentals By W. W. Jones — — Mr. W. W. Jones, whose Department is a monthly feature of this magazine, has long been actively associated ivith the Motion Picture Industry. At the present time Mr. Jones is a member of the Engineering Department of RCA Photophone and has been closely identified with the educational activities of that organization since the time of its inception. He is a graduate of the Milwaukee College of Engineering and was at one time Instructor of Mathematics and Electrical Design at that institution. — The Editor. Some Facts Concerning Film Drive Sprockets (Cont. ) IN last month's issue of this magazine our department was devoted to the consideration of feed sprockets. We now take up the discussion of hold-back sprockets. For the sake of clarity it may he well at this point to repeat the definitions applying to these two main sprocket classifications as furnished in our preceding article on the subject: "For purposes of this discussion, sprockets as used in picture projectors and sound reproducers are classified under two main divisions, viz.: (1) Feed Sprockets, and (2) Holdback Sprockets. "The first division, feed sprockets, may, from the point of view of operation, be classified further as follows : (1) The upper feed sprocket — used to pull the film from the upper magazine and feed it into a loop just ahead of the picture gate, (2) The intermittent sprocket — used to pull the film through the picture gate and feed it into a loop just ahead of the lower take-up sprocket, and (3) The sound sprocket (often called constant speed sprocket) — used to pull the film through the sound gate or trap. The Hold-back Sprocket "The second division, hold-back sprockets, when applied to sound and picture projection equipment, may be sub-divided as follows: (1) Lower hold-back sprocket — used as the last drive sprocket in the projector to hold the film back and maintain a loop of film between itself and the intermittent sprocket to prevent damage xo the film, and (2) The Lower take-up sprocket— used as the last sprocket in the sound reproducer to hold back the film from the pull exerted by the lower take-up reel, and to maintain a loop of film between itself and the sound sprocket to prevent interference with the sound." The hold-back sprocket is primarily intended to maintain a free loop of film in the film path just ahead of itself. For example, the hold-back sprocket (commonly called the takeup) is located in a projector just ahead of the take-up magazine, and its purpose is to maintain a loop of film between itself and the intermittent sprocket, so as to prevent damage to the film caused by the intermittent film drive at the intermittent. SPROCKET HOLE — -TOOTH prrCH Further, the hold-back sprocket in a sound attachment is intended to maintain a loop of film between itself and the constant speed sprocket, which will prevent the uneven pull exerted by the take-up drive from interfering with the uniform speed drive at the sound sprocket. Design Considerations In the design of a hold-back sprocket it is not necesary to consider the question of uniform speed of film. It is necessary to consider only those factors which will affect the life ot the film driven by the hold-back sprocket. Ordinarily for maximum film life, precision work in the manufacture of a hold-back sprocket is just as important as in the manufacture of a feed sprocket. That is, the shape of the sprocket teeth should be uniform, and each succeeding tooth pitch should be equal within very close limits. One of the most important considerations of a hold-back sprocket is that of diameter. In operation it is possible to have a hold-back, because of variation in diameters, operating under the following three conditions: (1) An ideal condition wherein the sprocket hole pitch is equal to the tooth pitch, (2) A condition wherein the tooth pitch is smaller or less than the sprocket hole pitch as illustrated in Figure 4, and (3) a condition wherein the tooth pitch is greater than the sprocket hole pitch. The effect of operating a sprocket under these conditions will be discussed in the following: In the discussion of the ideal feed sprocket, it was pointed out that its diameter should be such that its tooth pitch is equal to the sprocket hole pitch of the film. The same statement holds true for the hold-back sprocket. From this it can be said that for a given film the ideal holdback sprocket is identical in diameter or tooth pitch with the ideal feed sprocket. However, in practice it is necessary to drive film of which no two lengths have the same shrinkage, or, stated differently, no two lengths of film have the same sprocket hole pitch. Since the sprocket hole pitch varies from film to film, it would be impossible to use an ideal sprocket, thus making the ideal sprocket only a subject for discussion. From previous discussions it has also been pointed out that the practical feed-sprocket tooth pitch was made equal to the sprocket hole pitch of new or unshrunk film. In the case of the practical hold-back sprocket, the tooth pitch is made equal to the sprocket hole pitch of maximum shrunk film. The feed-sprocket always operates with film which has a smaller sprocket hole pitch than its tooth pitch, and the hold-back sprocket always operates with film which has a sprocket hole pitch greater than its tooth pitch as shown in Figure 4. Figure 4 shows a take-up or holdback sprocket which is receiving film from a film loop, and is feeding it to a take-up magazine which is exerting a pull on the film. It is to be noted that the tooth pitch is smaller than the sprocket hole pitch. It is to be noted further, that only one tooth is engaged (No. 6), and this tooth is engaged with the trailing or lagging edge of the sprocket hole. Operation of Hold-back Sprocket In operation the film becomes disengaged with tooth No. 6 during the stripping process, and at the same time the film slips ahead on the periphery of the sprocket until tooth No. 5 becomes engaged. Thus the stripping and slipping process repeats itself for each tooth. Since uniform film velocity at the take-up sprocket is not important, no attempt has been made, as in the case of the feed sprocket, to adjust the shape of the teeth in order to reduce the effect of film slippage. However, for the sake of uniformity the tooth shape of all sprockets is made alike. By further reference to Figure 4, it will be noted four teeth are included within the film wrap on the sprocket. The film wrap in this particular diagram could be increased to include tooth No. 2. However, if