International projectionist (Jan-Dec 1950)

The Geneva Intermittent Movement ii IN THE extremely fast movement no bounce occurs when the pin leaves the slot, but a tendency to bounce exists when the curved surface of the star contacts the lock ring, which may be quite serious. The pin will not completely stop the star, due to the same conditions which started the star so violently, shown in Figs. 4 and 5 in the last issue of IP (March, p. 8). Since the pin leaves the star before the star has stopped, the latter must be stopped some other way. Unless the film tension is abnormal, the point of the star strikes the cam ring at A, Fig. 6. and rebounds, causing the star and sprocket to turn backwards. This is more serious here, because no film tension alleviates the trouble. While the star is turning clockwise, due to the rebound, the cam is also turning clockwise, so the second contact occurs when the end of the cam ring is somewhere between the position shown in Fig. 6 and the dotted line. The end of the cam ring and the star are moving in opposite directions, resulting in a collision, and due to the position of the parts, a terrific wedging action takes, place, tending to force the star and cam apart and also to bend the two shafts, causing high pressures between the journals and their bearings. Figure 7 shows the parts at the second impact. The dotted line connects the shaft centers; solid lines are drawn from the star center to the point of impact, and from there to the cam center. The solid lines bend only slightly at the point of impact, being nearly in a straight line and causing extremely high pressures for an instant. A small opening exists at E, showing that the star has bounced and must again turn in the normal direction (counter-clockwise) to close this gap. When this gap has again closed, the end of the cam ring has moved past the dotted line, and the star is locked in FIGURE 6 CAM RING CAM CENTER ' DIRECTION OF PIN AS IT LEAVES SLOT position. Such a movement undergoes a severe beating, and failure is only a matter of time. Of course, this treatment does the film no good: a few times FIGURE 7 through the machine causes checked sprocket holes, and complete failure soon follows. Effecting Faster Film Transfer A faster film transfer may be accomplished in other ways. Although results are similar, the parts and the film last longer. In effect, it is as though the projector speeds up — say, to 180 feet per minute — while the film is pulled down. The projector then slows down below 90 feet per minute and runs at this speed until the next film transfer. Actually. FIGURE 8 the machine runs at 90 feet, but the cam is accelerated when about to move the star, and again decelerated at the completion of the star movement. The star and cam are to designed that a three-to-one movement results. The cam is driven by a pair of elliptical gears having a one-to-one ratio. The shaft is located at one of the foci of the driving gear, making the gear appear to wobble as it revolves. Angular Velocity Ratio of Gears In Fig, 8, A is the shaft center of the driving gear, and B is the shaft center of the driven gear, to which the cam is fastened. Shaft A has a comparatively heavy flywheel, to maintain a constant angular velocity of the driving gear. The driven gear varies greatly in its angular velocity, and the cam is r.o arranged that it moves the star when the velocity of the driven sear is greatest. By A. C. SCHROEDER The angular velocity ratio of the two gears varies according to the respective radii of the gears at the point of contact. (The gear teeth are not shown; the gears are shown as two blanks* or friction gears, that would roll on each other as the toothed gears do.) The radius of thedriving gear at this instant is the distance A to C, and BC is the radius of the driven gear. The ratio is about five-to-one. In other words, B is turning about five times as fast as A, and consequently the cam is also turning at this speed. At this instant the star and cam are in the position shown in Fig. 2 (IP for March) ; the star has turned through a 45-degree angle and is turning at its greatest speed, and simultaneously the cam also is turning at its greatest speed, thus producing an extremely fast movement. The angular velocity ratio of the two gears is constantly changing: for each fraction of a degree that gear A turns, gear B turns a different amount, except at two positions of A where both gears have the same angular velocity. This holds true, however, for only an instant. The speed of B has been increasing up to the moment shown in Fig. 8. From here on B slows down and continues to do so until both gears have turned onehalf revolution. Then B accelerates again, until the position of Fig. 8 is reached once more. In Fig. 10 both gears have turned onehalf revolution from the position of Fig. 8. B now turns about one-fifth as fast as A, thus allowing a long time before the cam pin enters the star again. Figure 9 shows the gears at the instant the cam pin enters the star, which is 45 degrees before the position shown in Fig. 8. The line BC is drawn through the foci of the driven gear, being 45 degrees from the horizontal line. Line AD is drawn through the foci of the driving gear, being approximately 12 degrees from the horizontal line. In Fig. 8 the foci of both gears coin FIGURE 9 14 INTERNATIONAL PROJECTIONIST • April 1950