International projectionist (Jan-Dec 1950)

cide with the horizontal line, therefore A turns 12 degrees from the position in Fig. 9 to reach the position in Fig. 8; while B turns 45 degrees during this time. After the gears pass the position of Fig. 8. the action is similar, hut in re FIGURE 10 verse order. Consequently B turns 45 degrees past the position of Fig. 8, while A turns 12 degrees beyond this position, and the cam pin is now leaving the star. The total movement of B was 90 degrees, while A turned only 24 degrees, during which time the film was pulled down. Shutter Movement Relationship The shutter turns at the same speed as gear A, and for each revolution of A one frame of film is pulled down. A acts just as the flywheel and the camshaft in our conventional machine, so we must calculate the speed of the movement from A. Since the film is moving only while the flywheel turns 24 degrees, it remains stationary for 336 degrees of flywheel travel. 336 being 14 times 24. we have a 14-to-l movement which is some speed and requires a shutter with about a 24-degree blade. Our regular shutter blade is about 90 degrees. Obviously, a 14-to-l movement is hardly practical, and in an actual movement the gears would be more nearly circular. The angular velocity of B would not vary so much, and we would produce a movement with a speed of about 5 or 6-to-l. Elliptical gears are costly and hard to cut without special equipment. Edison, on his last machine, used a system FIGURE 11 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 J 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) H 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, and from here on it again accelerates. /, in Fig. 12, is 45 degrees from the vertical, and the cam pin is just entering 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 degrees. Here the 90 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 thre-to-one ratio, requiring 90 degrees of flywheel travel to effect the film transfer. The Old Edison Method 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 /, J, and K are arranged so that they can be moved relative to shaft F, then the variation of camshaft speed can be changed at will. This is what Edison did. Obviously, the cam and camshaft cannot be moved very easily. (These parts actually move FIGURE 12 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, and the forked levers are /. Notice the third L, which is moveable in an up-and-down direction, indicated by the doublepointed arrow. When L, together with the two levers fastened to it, / and G. is moved downward so that it is in line with F and K, the drive is straight through, just as though F and K were FIGURE 13 connected by a shaft, and no variation in the angular velocity of K results. As shown in Fig. 14, pin H in the first lever is very close to shaft L, causing a large variation in the speed of L, the latter being shown at the position INTERNATIONAL PROJECTIONIST • April 1950 15