The motion picture projectionist (Nov 1931-Jan 1933)

24 Motion Picture Projectionist February, 1932 uf actured within certain tolerances in order to preclude the possibility of excessive wear on film or possible damage to film. It is true, however, that a set screw may be used to fasten the upper-feed sprocket to the shaft and thus it may be identified from a sound or constant speed sprocket. Careful Manufacture Required In order to obtain proper sound reproduction the sound sprocket presents a more or less difficult problem in design. Once the design is complete a second problem of manufacture exists. In the manufacture of sprockets special machinery and tools are required and properly trained and experienced mechanics are needed to operate the machinery. The intermittent sprocket and the sound sprocket are the most important sprockets in the projector and sound head film drive. The intermittent sprocket is used to pull the film through the picture gate, intermittently stopping each successive picture frame accurately in the picture aperture and in the same relative position as the preceding frame. Certainly if the sprocket is eccentric or if the teeth are not spaced uniformly around the circumference of the sprocket, it will be impossible to fulfill the above conditions. It may be repeated that the tolerances of eccentricity and tooth pitch must be held within very close limits, since any variations or inaccuracies in manufacture will be magnified on the screen in an amount equal to the degree of magnification of the picture on the screen. The sound sprocket is used to pull the film through the sound gate at a constant or uniform speed. As in the case of the intermittent, very close tolerances must be maintained in manufacture, since any variations will result in a varying film speed which introduces distortion in sound reproduction. This distortion can be recognized as a harsh or raspy sound noticed particularly on higher frequency notes produced by such instruments as the piccolo, flute or violin. It might also be pointed out here that even though the proper tolerances may have been maintained, poor or distorted sound will result if the sprockets have not been designed properly or if sprockets are not used for the purpose for which they were intended. Effect on Film In the following will be discussed the effect of using a sprocket having improper diameter and improper tooth shape. While all of the comments are made with reference to the sound sprocket, they are applicable as well to the intermittent sprocket and the upper-feed sprocket. The sound sprockets will be considered under actual operating conditions for the following three cases, viz.: (1) in which the sound sprocket tooth pitch is equal to the sprocket hole pitch as shown in Figure 1, (2) in which the tooth pitch is greater than the sprocket hole pitch as shown in Figure 2, and (3) in which the tooth pitch is less than the sprocket hole pitch as shown in Figure 3. Figure 1 shows a feed sprocket rotating in a clockwise direction and pulling film from a magazine or through a picture or sound gate. It is to be noted that Figure 1 represents an ideal case, in which the sprocket hole pitch is equal to the tooth pitch. Also, it is to be noted that each sprocket hole is engaged with a tooth and is sharing its equal portion of the load. For a condition of this sort it is necessary to have a sprocket diameter or sprocket tooth pitch which corresponds and equals the sprocket hole pitch. With this condition fulfilled it will be found that the sprocket will at least theoretically impart uniform film speed, since for the particular wrap in Figure 1, tooth No. 1 will assume that portion of the load previously shared by tooth No. 6. However, it will be found that in practice this ideal condition does not exist. It is common knowledge that film shrinks a certain amount depending upon its age, use, and the treatment it has received. This, of course, would indicate that, in general, no two reels of film have shrunk an equal amount. Therefore, the ideal case shown in Fig. 1 does not exist. Selecting a Sprocket In selecting a sprocket to be used as a sound sprocket, it is necessary that its tooth pitch be equal to and correspond to the tooth pitch of new or unshrunk film. If this is done, the tooth pitch will be greater than the sprocket hole pitch when the sprocket is used with shrunken film. While this condition is not ideal, it is the condition under which all feed sprockets operate. Figure 2 shows a sprocket whose tooth pitch is greater than the sprocket hole pitch. It represents for this discussion a sprocket of proper diameter operating with film which has shrunk by an amount (D) shown in the figure. In operation the film will travel at uniform speed as long as tooth No. 6, which carries the entire load, is engaged. However, the film will slip back a distance (D) on the periphery of the sprocket or run at a reduced speed when the film is stripped from tooth No. 6, thus transferring the load to tooth No. 5. This process repeats itself each time a tooth is stripped or disengaged from the film. If the tooth shape were a circular involute as shown by the dotted line at tooth No. 6, there would be no slipback while the film was stripping off the tooth. However, there would be a sudden change in film speed or slipback the moment the film disengaged with the top of the tooth. While this change in speed cannot be easily determined, it is known to be a rapid and sudden change. It is possible and common practice to change the shape of the tooth from a circular involute curve to one which recedes more rapidly in order to reduce the rapid change in speed which occurs at the period of slip-back. The tooth shape can be adjusted so that for a given, film shrinkage the top of the tooth is a distance (D) from the circular involute curve. By so doing, the film will have slipped back during the stripping period so that when tooth No. 6 disengages, the film will be in contact at tooth No. 5, which will take up the load. Further, the tooth shape may be adjusted so that the slip-back speed will be uniform during the stripping period. Also the tooth shape may be adjusted so that the slip-back speed is low at the beginning and increases as the stripping progresses. Allows for Maximum Shrinkage It is to be observed here that the tooth shape, if adjusted for a particular film shrinkage, in accordance with the above discussion, the sprocket will operate as intended only when used with a film having the particular shrinkage considered in the design. If a film having a greater shrinkage is used, the film will be stripped off tooth No. 6 before tooth No. 5 becomes engaged, and will result in a sudden slip-back when the film leaves tooth No. 6. If a film has less shrinkage than that used in the design, tooth No. 5 will become engaged before the stripping is completed on tooth No. 6. Since it is undesirable to have the film slip back suddenly, it is, therefore, customary to adjust the tooth shape for a film having maximum shrinkage so that tooth No. 5 will, for all values of film shrinkages, become engaged before or just at the time stripping is complete on tooth No. 6. By further reference to Figure 2, it is to be noted the diagram is drawn so the film wrap includes four teeth. The circular involute curve shown by the dotted line at tooth No. 2 indicates that it is possible for the film wrap to include tooth No. 2. However, the circular involute on tooth No. 1 indicates that tooth No. 1 could not be included in the film wrap. If the film wrap were to include tooth No. 1, the lagging edge of the sprocket hole would strike the lagging face of the tooth and would result in nonuniform film speed as well as possible damage to the film in the form of torn sprocket holes. Also, if it were found desirable to include tooth No. 1 in the wrap, this would be possible by decreasing the base thickness of all the teeth to a value equal to the distance from the involute curve at tooth No. 1 to the leading face of the tooth. It should be further noted that for a given tooth base thickness, the wrap on the sprocket can be increased for (Continued on page 31)