American cinematographer (Feb-Dec 1929)

Four AMERICAN CINEMATOGRAPHER June, 1929 FILM FILM APERTURE FLOATING TENSION PLATE FIXED TENSION PLATE. FINISH TIME FILM TENSION IN 5ILENT HIGrW-SPEED INTERMITTENT MOVEMENT FILM MECHANISM Fig. 6 — Film tension in the new B. & H. speed intermittent movement mechanism. sion plates, precent any possibility of the film sagging or curling, even under very severe temperature conditions. The film is thus kept under control througout its path in the channel, and the presentation of a perfectly flat film surface at the focal plane of the photographic lens is thus assured. The Film Channel consists of two members. The film is inserted in the space thus provided for and which is so designed as to provide a frictionless surface passage of the film through it. A light spring pressure of the pareture plate against the body of the movement prevents possible injury to the film or to the mechanism, should be threaded improperly. The aperture plate is provided with a lock and the film cannot be introduced into the channel if the lock is not in its engaged position. This arrangement eliminates all possibilities of neglecting to lock the aperture when the mechanism is replaced in the camera after having been withdrawn from it for the purpose of cleaning, even if this operation is performed under the greatest possible pressure of urgency. A very light tension at one side of the film keeps it registered sidewise against a solid rail. Figure 6 shows the arrangement of such tension, which is exerted only on the portions of the film which assume the curved shape of the film channel. The point on the floating tension plate at which the pressure is applied was determined by careful calculation, and is so located as to assure an equally distributed pressure along the entire side tension-producing surfaces. THE FILM-FEEDING FINGERS:— The function of the Film-feeding Fingers is to engage in the perforations, carry the film downward or upward, according if it is desired to record the motion of the subject normally or reversed, to withdraw from the performations and resume its original position as at the beginning of the stroke. The ideal IN and OUT movement of the Film-feeding Fingers would be the one in which the motion occurs at a time when there is no contact between the fingers and the faces of the perforations, since the inevitable wear which occurs at the end of the fingers, is caused by the rubbing of the surfaces which contact with the faces of the perforation. In the new mechanism here presented, the entire feed forward movement of the fingers has been held to only .012 inch and only 1/6 of this total displacement, or cL ACCELERATION BY A CONSTANT FORCE (GRAVITY) t> •• •• OBTAINED BY THE MOVEMENT Fig. 7 — Comparative curves of the acceleration by a constant force with the acceleration obtained through the movement. .002 inch, is the extremely short motion which takes place from the time at which the fingers begin to engage in the perforations until the entire IN movement is completed. In Figure 7 curve "A" represents the acceleration due to a constant force, that is to say, GRAVITY. Curve "B" is plotted from the downward movement of the film. It will be noticed that during a complete from top to bottom stroke, the acceleration and deceleration of the feeding fingers lie very close to the ideal. The variation is very slightly greater in the deceleration. It is because the ideal acceleration condition is so closely met that the time necessary for the IN and OUT movement of the feeding fingers has been reduced to the smallest extent. The amount of noise produced in intermittent movements by the rapping of the Film-feeding Fingers against a stationary film, is determined by the amount of "play" between finger and perforation. In the movement here presented, the Film-feeding Fingers are .008 inch narrower than the film perforation, which condition limits the overthrow to .004 inch, that is to say, the Film-feeding Fingers move along a path only .004 inch long before touching the faces of the perforation. The extent of displacement is so small that the downward movement of the fingers is extremely slow at this moment, so that they may practically be considered as stationary at the instant in which they actually come ly come into contact with the film perforation. To insure a still greater resistance to wear, the ends of the Film-feeding Fingers are chrominum plated. In Figure 8 are shown five schematic drawings of the mechanism, at five different phases of movement. It is well to remark that the schematic condition of the drawings does not permit to illustrate the extremely small overthrow referred to. In position 1, Feeding Fingers drawn from the perforations and the Registration Fingers are holding the film stationary by spring pressure. Film-feeding Fingers are shown The film is still held in START previously the are Filmwith In position 2, the during their upward motion, position by the Registration Fingers and remains so while Continued on Page 11 SCHEMATIC DRAWINGS OF PROGRESSION OF MOVEMENTS OF THE. SILENT HIGH-5PEED INTERMITTENT MOVEMENT MECHANISM. P05IT10N-S FILM CHANNEL FILM FEEDING, FINGERS. REGISTRATION FINGERS. IN grOUT CONTROLLING CAMS REGISTRATION FINGERS SPRING POSITIONS POSmON-5 Fig. 8 — Schematic drawings illustrating different phases of the cycle of movements performed by the new B. & H. high speed intermittent movement mechanism.