International projectionist (Jan 1959-Dec 1960)

The Geneva Intermittent Movement: Its Construction and Action The fourth in a series of articles which detail the what, the why and the how of the "heart" of the motion picture projector. ELLIPTICAL gears are costly and difficult to cut without special equipment. Thomas A. Edison, on his last machine, used a system of levers to obtain similar results. Fig. 11 shows a layout which produces a varying angular velocity of the driven shaft. E is the flywheel, fastened to shaft F, on the other end of which is the lever G. In the upper end of G is the pin H, the dotted lines being a continuation of the pin, which portion is hidden by /, a forked member fastened to the camshaft and shown better in the end view in Fig. 12, the flywheel and the cam / not being shown here. The dotted lines show portions of lever G which are hidden by the forked member /. Pin H is the only means of transmitting motion from G to /. H is continually sliding in the slot in /. When both levers are vertical (the position they are approaching in Fig. 12) //is very close to the camshaft K, and the speed of K will be much greater than that of the flywheel shaft F. After leaving the vertical position, the lever / decelerates, because the pin slides farther away from K as the motion progresses. In Fig. 13 the levers have almost reached the vertical position again, but in the downward direction. The pin is nearly at the end of the slot, and K is turning relatively slowly. When the levers point straight down, K is moving at the slowest speed; from here on it again accelerates. Star Slot Engagement I. in Fig. 12, is 45 degrees from the vertical, and the cam pin is just entering By A. C. SCHROEDER Member, IA Local 150 the star. The star movement is completed when / is 45 degrees past the vertical, a total of 90 degrees. Lever G is 16 degrees from the vertical position, and when the star movement is completed it will be the same distance past the vertical, a total displacement of 32 FIGURE 11 degrees. Here the 9 degrees of cam rotation is obtained by only 32 degrees of flywheel travel. Thus, we have 328 degrees of flywheel travel in which the film remains stationary, allowing more time for projection to the screen, deducting that lost due to the flicker blade on the shutter, of course, as we also do in conventional projectors. If we place the shafts F and K in line with each other, we find that the pin does not slide in the forked member /, but remains fixed in one position, relative to /, although both levers are revolving. The angular velocity of /, then, is constant and equal to the driving member G. No advantage is obtained from the levers, and the movement has a three-to-one ratio, requiring 90 degrees of flywheel travel to effect the film transfer. If camshaft K is displaced only slightly from the in-line position, the pin moves slightly within the forked member and the speed of / begins to vary, increasing as the levers move in an upward direction, decreasing as they move in the downward direction. The amount of speed variation depends on how far the shafts are out of line, and if /, /, and K are arranged so that they can be moved relative to shaft F, then FIGURE 13 the variation of camshaft speed can be changed at will. The Old Edison Method This is what Edison did. Obviously, the cam and camshaft cannot be moved very easily. (These parts actually move in all our machines, but this is done for framing.) Edison used two more levers, as in Fig. 14. Both driving levers are marked G, both pins are indicated by H, FIGURE 14 12 INTERNATIONAL PROJECTIONIST • MARCH 1959