International projectionist (Jan-Dec 1957)

Faster Pulldown Geneva Movements By JOSE M. RUIZ In this second installment, designer Ruir considers three basic ways to speed up the geneva intermittent, and suggests the best solution for present problems. BACK IN 1926, Publix Theatres in New York City tried a projection system called "Magnascope." The Magnascope was first used at the Rivoli Theatre in New York in showing certain scenes of the historical film "Old Ironsides." These scenes were magnified to almost four times the conventional aspect ratio. The Simplex projector was of special design and fitted with a fast intermittent movement of only 72-degrees cam action. The revolving shutter blades were trimmed down as close as possible, or near 78-degree angle of coverage. The practice followed by Publix engineers was intelligent and rationa If the enlarged picture had an area approximately four times that of small est one (old aspect ratio of 1.33 to 1) it was essential that a maximum amount of light be projected on the screen Speeding up the intermittent move ment for a longer exposure cycle gave an efficient level of screen illumination without increasing the light source. The question of pulldown reduction time may be attacked in another way. It can be achieved by means of an additional acceleration mechanism act FIG. 11. Diagram of method for accelerating the pulldown phase of the standard-size geneva star and cam. This is regarded as the most promising method of obtaining a faster pulldown speed in modern projectors. S/&U/A4>*/ 4ti4/s> of Jsy> £///>£*'- £/ocf' FIG. 13. Three positions in the pulldown cycle of the Radion II, a 35-mm projector manufactured in France that uses the acceleration system diagrammed in Figs. 11 and 12. The offset driving shafts and "slipper block" described in the accompanying article make it possible to accelerate the pulldown action of a standard-size geneva star wheel and cam. ing upon the pulldown cycle of standard-size geneva star and cam and taking advantage of the well-known mechanical principle of "offset" drive — two displaced shafts linked by means of one universal joint. This gives a periodic acceleration to the cam pin at the instant of the pulldown cycle. Accelerated Geneva Action In 16-mm work, the offset drive has been successfully employed in many excellent professional machines. However, in 16-mm work this method was chosen due to some inconvenience in the geometric cam action which would occur if a geneva with more than four slots were employed. One of the first to use this idea was Thomas Edison who designed a sort of "offset" system to achieve an accelerated motion in the pulldown cycle of the star wheel. The components tried by Edison were combined stages of accelerator elements. Two lever accelerators achieved a very short pulldown of only 32 degrees of cam action, leaving the remainder for projection or exposure time. However, this achievement was impractical. Incidentally another mechanical expedient to accelerate the intermittent cam is to employ elliptical gears, but for many technical reasons, elliptical gears are not practical when interposed between the cam-pin shaft and constant velocity flywheel drive shaft. The offset mass of gears in motion with pre cipitous changes from positive to negative accelerations and the forces created by masses in high sjseed motion, introduced terrific vibration and shock incompatible with good projection operation. It is an important fact that when gears are interposed between natural intermitent mechanical elements, a high degree of precision is required in machining the components. Any inaccuracy in the tooth shape creates oscillating hums due to the angular velocity change between gears. Therefore the gears must be cut with a high degree of precision to attain correct angular velocity. However, in the case of accelerated mechanisms the film acceleration is perhaps the most important thing to DRIVING SHAFT ACCELERATOR— £ CATCH PLATE— C FIG. 12. Another view of the acceleration system shown in Fig. 1 1 . INTERNATIONAL PROJECTIONIST JANUARY 1957 13