British Kinematography (1952)

72 BRITISH K 1 N E M A T< )GRAPHY Vol. 20, No. 3 of 54 per rent, (against about 45 in most projectors), without any sign of travel ghost. The Automatic Film-loop Correction during Framing It is important that the sizes of the film loops above and underneath the film gate are not altered during framing. There are two factors which affect the sizes of the loops: 1 . the intermittent sprocket being turned round its shaft and '2. the speed of the shaft of the locking cam being changed. These two factors counteract each other and we found that they compensate each other completely by making the ratio between the pinion on the shaft of the locking cam and its driving gear 4:1. This may be illustrated by the following example : Let us suppose the framing knob is turned from the extreme right-hand to the extreme left-hand position, i.e. the framing disc is turned U0G, thus shifting the film one full image in its running direction. By doing so, the pinion on the shaft of the locking cam revolves around its driving gear and when the transmission ratio between these gears is 4:1, the shaft of the locking cam will be turned backwards 4 x 90° =360° = one revolution = one image. A displacement of the film in one direction is therefore completely compensated by the displacement of the film in the opposite direction. The advantage in practice is that framing can be carried out without any risk of the film loops becoming too large or too small. The Lubricating System Lubrication of the Maltese cross and of the driving mechanism is by spur-gear oil pump located in the oil receptacle of the projector. This makes a suction duct superfluous, and moreover, the pump can never fail. Having passed a fine-mesh gauze, combined with a magnetic filter, the oil is forced into a duct with outlets leading to the various lubricating points. The duct ends at the top of the projector, just above a second magnetic filter, over which the oil flows towards the intermittent mechanism. This second filter is placed behind a viewing glass for checking the oil circulation. The magnetic filters hold back all steel and iron particles from the oil, reducing wear of the mechanism and acidification of the oil ; they are readily removable. The rear of the projector is closed by an oil-tight cover provided with a special oil screen, which prevents the oil from penetrating between the contact surfaces of the cover and the projecting housing. Consequently, it is unnecessary to use a gasket, which is liable to break when the cover is removed, or a liquid packing which takes 12 to 24 hours to dry. Oil retainers prevent the oil from leaking along the shafts of the driving mechanism, so that there is no risk of the oil soiling the film. The shafts of the upper and lower spool boxes run in self-lubricating ball bearings. The Film Path All the sprockets are made of chrome-nickel steel which has a high resistance against wear. They are easily removed from their shafts and when remounting need only be pushed against the stop and fixed in that position, when they are automatically aligned. The three pad rollers (Fig. 7) are of exactly the same sturdy construction. They are blocked in their released position, thus facilitating threading the film. Two velvet-covered runner-plates (Fig. 8), which can be interchanged without tools, are supplied with each projector. The velvet tape is stretched over the running surfaces of the plate by means of a spring. A big advantage of velvet-covered runnerplates is that accumulations of film dirt when running new prints is avoided. When a plain steel plate is used, the dirt may form hard ridges which are liable to damage the film. Another advantage is that, since the friction coefficient between film and velvet is mucl higher than that between film and smoothh polished steel, the pressure of the skates cai be reduced considerably, thus ensuring longer life of the skates. The third advantage is that only the chea]