International photographer (Feb-Dec 1929)

Twenty The INTERNATIONAL PHOTOGRAPHER June, 1*2* Silencing the Bell & Howell (Continued from Page 14) The intermittent mechanism of a motion pictures camera is, so to speak, the heart of the whole instrument, since upon it depends the precise registering of the rapidly succeeding photographic records. The intermittance of movement has from the very beginning of motion pictures, put to task the inventive gen us of motion pictures Engineers, in order to solve the problems involved in the designing of a mechanism which would, while working at a speed of 16 pictures per second, perform a cycle of four main and distinct movements. I. — The engaging of the film-feeding fingers into the film perforation. II. — A downward movement of the fingers to bring an unexposed portion of the film in the proper position in front of the camera aperture. III. — A backward movement to withdraw the fingers from the film perforations. IV.— An upward movement of the fingers to bring them in position to reengage into the film perforation and repeat the cycle. Dur'ng this cycle of movements, the shutter of the camera was to make one complete revolution. The mechanical problems inherent to this rather complex cycle of movements, had been happily solved with a reasonable disregard of the noises resulting from the functioning of its parts all attention being paid to the accuracy of registration and the elimination of any possibility of damaging the surface or the perforations of the film. The advent of sound Motion pictures demanded a mechanism capable of noiselessly completing at least 24 cycles per second and also capable of withstanding a much more strenuous usage than the movements in existence, due to the fact that the average length of scenes taken for sound purposes is at least four to six times greater than the average length of scenes photographed for the silent drama. The main prerequisites of such mechanism are the absence of noise and the ability of performing at a minimum speed of 24 pictures per second (90 feet per minute) while conserving intact the indispensible attributes of perfect registration, forward and backward movement and an as complete as possible elimination of friction upon the surfaces of the film, in order to avoid the evils of scratches and abrasions. In order to simplify its description, we shall consider one after another its principal parts, namely: The Film Channel The Film-feeding fingers The Registration fingers and conclude with a brief description of features of general interest, such as the lubricating system and the general assembly of the mechanism. THE FILM CHANNEL:— The film channel can be divided into three main sections, an upper and a lower curved section each having a radius of 1 V± inch and a central plane section. Figure 5 shows a schematic drawing of the mechanism plainly illustrating the above mentioned essentials, Film channel, Film-feeding fingers and registration fingers. The film-feeding fingers operate in the upper curved section, while the plane section comprises the camera aperture and the location of the registration fingers. The central plane section of the film channel has been kept within the shortest possible length in order to insure perfect flatness of the film surface at the time of exposure. The longitudinal curves impressed upon the film by the curved sections of the channel, in conjunction with the transverse guide and tension plates, prevent any possibility of the film sagging or curling even under very severe temperature conditions. The film is thus kept under control throughout 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 aperture plate against the body of the movement .prevents, possible injury, to. the . film or to the mechanism, should the film 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 i-eplaced 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 determnied 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 perforations 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 .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. The ideal movement, is the one in which the total shutter covering time and the film advancing time are equal with the acceleration produced by a constant force. 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 .088 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 considei'ed as stationary at the instant in which they actually come into contact with the film perforation. To insure a still greater resistance to wear, the ends of the film-feeding fingers are chromium 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 previously referi'ed to. In position 1, the film-feeding fingers are withdrawn from the perforations and the registration fingers are holding the film stationary by spring pressure. In position 2, the film-feeding fingers are shown during their upward motion. The film is still held in position by the registration fingers and remains so while the shutter (not shown in the drawing) is functioning in the exposure condition. ... . ... .....