International projectionist (Jan-Dec 1950)

\CAM RING FIG. 3. Notice that radius on sides of this star is much greater than in Figs. 1 and 2; also that the slots are not so deep, due to large cam and the larger circle in which pin travels. tain that speed, but the cam pin prevents this, forcing the star to gradually slow down. Immediately following the position of Fig. 2, the deceleration is low. The rate of deceleration then increases (remember the pin is decelerating the star all this time) . The rate of deceleration decreases toward the end. The star is almost stationary when the pin is about to leave the slot. The lock ring (C in Fig. 1) contacts the curved portion of the star snugly, thus holding it in position while the film is projected on the screen. The end of the lock ring is approimately at line A-B when the pin enters the slot. When the end of the ring is above this line, the star cannot move in either direction. As the pin proceeds farther into the slot, the end of the cam ring moves down and away from this line, allowing the star to turn, as Fig. 2 shows. If the lock ring extends beyond the dotted line when the pin enters the slot, the star is still locked, while the pin exerts pressure on the star attempting to turn it, and the movement will not work. If the lock ring does not extend to the dotted line when the pin is the position of Fig. 1, the star is free to turn before the pin enters the slot, and the pin might strike the point of the star, thus wrecking the movement. We can now study the possibilities of a faster movement. In the ordinary movement the cam and pin turn trrough 90 degrees while the star and sprocket move the film one frame. To increase the speed of the movement we must allow less than 90 degrees cam rotation to complete the star movement. The pin still enters and leaves the slot at the same two points, and to do this in less than 90 degrees the cam center must be farther away, decreasing the angle A, as shown by the dotted lines in Fig. 3. The corresponding angle in Fig. 1 is 90 degrees. Notice how much smaller this angle is in Fig. 3. The arrow in Fig. 3 shows the direction of the pin travel as it enters the slot. Notice that it does not correspond to the direction of the slot, as it did in Fig. 1, resulting in the pin contacting the slot with a bang, and the star immediately turns at a certain finite speed: a miniature collision occurs and the shock is transmitted to the film. This extremely rough treatment soon shows not only on the film but on all the parts, including the cam pin. The larger the cam, with the same size star, the harder is the impact. If both the cam and star are larger, in proportion, there is no increase of speed, although the parts are more able to stand the strain imposed on them. Another problem enters the picture when we increase the cam size. To illustrate this we make the drawing as though the cam were inordinately large in proportion to the star, as in Fig. 4: the three solid lines represent the side and end of the slot, the circle represents the pin, and the dotted line with the arrow at the end continues on to the cam center. The lock ring is not shown. Arrow D indicates the direction of pin travel at this instant. Note that the pin just clears the upper edge of the slot; but E, which is closest to the lower edge of the slot, still is some distance away, as shown by the space between E and F. As the pin travels further it again leaves the upper edge of the slot and the distance E-F becomes less, until we reach the position in Fig. 5, showing the instant that contact occurs. Arrow G indicates the direction of the pin and shows the point of contact. The distance between the pin and the upper edge of the slot has increased to H-I. The lowest extremity of the pin is now below the lower edge of the slot, as the SLOT IN STAR TO CENTER OF CAM FIGURE 4 dotted line indicates. One might say that the pin has moved ahead of the star, so the star must snap ahead at a terrific speed, instantly, to make way for the pin. Since the pin is positively-driven and cannot rebound, this impact causes the star to rebound, resulting in again opening a space at G. What occurs next depends upon the weight of the star and associated parts, the weight of the intermittently-moving portion of the film, and the friction due to the tension shoes. With heavy parts and light tension the star flies ahead until it "catches up" with FIGURE 5 ind/, the pin, resulting in contact of H an with the possibility of another rebound here. The pin will now be far enough into the slot to prevent further erratic action, such as rebounding. On the other hand, with very light parts and relatively heavy film tension, H and / will not touch; but the second contact occurs at G, where there is possibility of another rebound. The degree to which the foregoing action takes place depends upon the speed of the movement. If the speed is only slightly above normal (3-to-l), rebound might not occur because of the lighter impact at G. The direction of arrow G would more nearly coincide with the direction of the slot. Also, the pin would be practically half way in the slot when contact occurs, thus stopping any tendency to rebound. However, there still is a sudden jar in any conventional movement where the speed has been increased beyond the 3-to-l ratio, and the effects will be seen on the parts and the film. In this discussion an intermittent in which the transfer of film is completed during a cam rotation of 90 degrees is termed a 3-to-l movement. This same intermittent is sometimes spoken of as a 4-to-l movement. This is apparently due to considering the ratio of the entire circle, 360 degrees, in relation to that part of the circle during which the pin is moving the star, or 90 degrees, which gives a ratio of 4-to-l. The writer prefers to consider the ratio of that part of the cam rotation during which the star stands still, or 270 degrees, in relation to the part of the cam rotation during which the star moves, or 90 degrees, giving a ratio of 3-to-l. [TO BE CONTINUED] 800 Million c.p. in Eveready N. Y. Sign Some 800 million candlepower produced by a carbon arc contained in a giant flashlight features the new Eveready Beam which is the latest "spectacular" electric sign to be unveiled in the Times Square area of New York City. Unveiled on New Year's Eve, this gigantic ad display utilizes a flashlight tube 26 feet in length which, under favorable atmospheric conditions, pierces the heavens for 5 miles and is visible for as much as 100 miles encompassing an area of 31,000 square miles having a total population of 20 million people. 8 INTERNATIONAL PROJECTIONIST • MARCH 1950