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AMGULAR DISPLACEMENT OF CAM PIN IS DECREES
FIG. 3. Angular displacement of star wheel in re
ao «5 » lation to cam-pin rotation.
blades of the shutter could be reduced from over 90 degrees to a little more than 60 degrees, and overall transmission efficiency would be increased from less than 50% to over 65%. However, the flicker frequency remains at 48 cutoffs per second, and a more disturbing flicker may be created than with a conventional 93-degree shutter if the illumination reaches high levels and the theatre has a short projection throw to a large screen. The reason for this is partly brought out in research done by F. C. Mathieu in France with respect to flicker and eye fatigue. Eye fatigue tends to occur when the screen image covers the whole retinal field since the secondary or side-vision area of the retinal field is more sensitive to flicker. When we consider further that flicker always becomes more noticeable as the light level rises, it becomes evident that a situation where the side areas of the retina receive a very bright image could cause irritation.
Three-Bladed Shutter
Therefore, the 5-to-l intermittent would probably give best results when combined with a flickerless 3-bladed shutter providing 72 60-degree cutoffs per second. An exception would be large drive-in theatres. The 2-bladed 60-degree shutter would be adequate there because the light level would not be high enough to introduce disturbing flicker.
Two excellent articles dealing with intermittent movements have appeared in past issues of IP*. However, for comparative purposes it is essential to describe the geneva movement again, especially with regard to motion study and geometrical design. Fig. 1 shows the familiar geneva star wheel and cam pin, the 3-to-l movement of 90 degrees
cam action which is universally employed in present-day 35-mm projectors.
Fig. 2 illustrates the five stages of cam-pin engagement with the curves plotted graphically to show what happens during the pulldown cycle. Two curves are plotted as follows: A represents the star wheel angular displacement against the cam-pin angular rotation, and B the rate of film acceleration from zero to 90 degrees. The curve A shows the gentle action of the driving pin as the pulldown cycle starts. From the beginning of the acceleration phase of the pulldown stroke to the middle of the cycle or 45 degrees cam displacement, the star is moving with accelerated motion until it reaches its maximum velocity. Past the middle of pulldown stroke, the star is moving with decelerated velocity until it reaches 90 degrees or rest position. This rapid action is achieved in l/96th of a second if the machine is running 24 frames per second.
The curve in Fig. 3 shows this action plotted in terms of angular displacement. We can see at a glance the displacement of the star wheel against the cam-pin displacement in terms of degrees for any relative position of the two components in motion. The curve plotted in Fig. 4 illustrates the velocity ratio between the star wheel and cam pin in terms of cam-pin rotation in RPM (revolutions per minute) for the
two mechanical elements. The cam-pin shaft rotates at the uniform or constant rate of 1,440 RPM for 24 frames per second at sound speed. When the cam pin reaches the 25 degrees of angular displacement, the star wheel is moving at the same speed as the continuous velocity of the cam-pin shaft, or 1,440 RPM.
It can also be seen that at the beginning of pulldown, 10 degrees of cam rotation, the star wheel is moving at 300 RPM speed, while at the middle of pulldown the star wheel reaches its greatest velocity, or 3,456 RPM. The curves have been plotted mathematically by trigonometry** and are the main basis for analyzing any type of geneva motion.
These facts are essential to a clear understanding of the reasons why the geneva star and cam pin give the film such desirable acceleration transport action in overcoming the inertia of film, and the frictional forces created by the gate pads. In these simple principles lie the advantages of the geneva movement.
Possible Solution
When the designer faces the question of speeding the pulldown action of the standard geneva star, the matter becomes insoluble due to the fixed conditions imposed by the geometry and mechanical properties of the four-slot geneva star wheel. However, there are two possible variations in design that can solve the problem. The first is to increase the locking-ring diameter. This solution was very popular in old Pathe projectors. In fact, the Pathe 60-degree geneva intermittent designed by Cotinsouza (the manufacturer of Pathe projectors) was known as "Systeme Cotinsouza." This system, due to (Continued on page 27)
** "Application of Pure Mathematics to the Solution of Geneva Ratios," hy Ron. H. Jones, Journal of the SMPE, July, 1946.
* "Heart of the Projector Mechanism," by R. A. Mitchell, IP for July, Aug., and Sept., 1952; "The Geneva Intermittent Movement," by A. C. Schroeder, IP for July, Aug., and Sept., 1952.
FIG. 4. Velocity ratio between star wheel and cam pin in terms of revolutions per minute during pulldown cycle.
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3*.56 3168 2880 2592 2304 2016 1278 1U0 1152
10 15 20 25
30 35 W 45 50 55 60 65
CAH PIN DISPLACEMENT IN DECREES
70 75 80 85 90
INTERNATIONAL PROJECTIONIST
DECEMBER 1956
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