International projectionist (Jan-Dec 1950)

VOLUME XXV MARCH 1950 NUMBER 3 The Geneva Intermittent Movement THE Geneva intermittent movement is used in all modern theater projectors, although claw-type movements are used in studios for process work. The Geneva movement changes continuous rotary motion into an intermittent motion, the camshaft turning at a uniform speed, while the intermittent sprocket stands still part of the time and moves very rapidly the rest of the time. The intermittent must transport about eight inches of film a distance of % of an inch at. an average speed four times normal, and stop each frame in exactly the same position at the aperture. When moving at a uniform speed (as it does on the upper sprocket) each frame is in motion four times as long as it is when in motion at the aperture. The PIN FIGURE 1 average film speed at the aperture is four times normal, or 360 feet per minute. Due to inertia, the film, the sprocket, and the intermittent shaft must be brought up to this speed gradually, consequently By A. C. SCHROEDER The first of a series of three authoritative articles on the "heart" of the professional motion picture projector. the maximum speed must exceed 360. In Fig. 1 the pin is shown at the instant it engages the slot in the star: exactly one-half the pin is in the slot. Until this moment the star has not moved. A represents the cam center and B the star center. A, the center of the pin, and B form a right-angle, as the dotted lines indicate. The arrow indicates the direction of the pin travel for an infinitesimal fraction of time at the exact moment it engages the slot. Notice that the pin travel coincides with the direction of the slot, consequently there still is no star movement. This holds true only for the shortest possible moment, immediately following which the star begins to move very slowly, but the acceleration constantly increases until the cam has turned about 30 degrees. The speed of the star, and consequently the film, is still increasing even after 30 degrees of cam rotation, but the rate of acceleration has decreased. Figure 2 shows the movement when the speed of the star is greatest but acceleration has stopped. The pin is in line with the shaft centers, A and B, and is comparatively close to B. For a brief instant the pin and the sides of the slot act nearly like gear teeth. The radius of the corresponding large gear would be the distance A to the pin center; the radius of the corresponding small gear being B to the pin center. Since the latter is much less than the former, the star is turning at the rate of many more revolutions per minute than the cam. As the pin leaves the position of Fig. 2, the. star, the sprocket, and the film begin to decelerate. This part of the cycle is as important as the first half. If the parts fit and are adjusted so that operation of the first half cycle is perfect, the conditions will also be correct for the last half. If the parts fit poorly, the tension shoes must decelerate not only the film but also the star, its shaft, and the sprocket, at least part of the time between the position of Fig. 2 and completion of the film transfer, thus putting an enormous load on the sprocket holes FiGURE 2 and preventing proper positioning of the film at the end of its travel. The star, its shaft, and the sprocket, having attained such a high speed at the 45-degree position (Fig. 2) tend to main INTERNATIONAL PROJECTIONIST • MARCH 1950