British Kinematography (1948)

17 Mechanical Design Turning now to the mechanism proper, one is reminded of Kipling's dictum : " There are five and forty different ways of composing tribal lays, and every single one of them is right." One has certain units to couple together — sprocket, shutter, intermittent unit and so forth, the relative dispositions and speeds of which a're more or less fixed ; how best to gear them up ? Certain considerations must be borne in mind. For convenience of manufacture and service, the intermittent is made as a self-contained, detachable unit. In order to permit of racking the film, provision must be made for movement of the cross box — preferably for rotation of the whole unit, about the axis of the sprocket. The supporting structure must be rigidly designed, since any looseness or vibration is certain to show up in unsteadiness in the projected picture. Since the timing of the intermittent in relation to the shutter is important, the more direct the gearing between them the better, on account of reduction in inevitable backlash, but this gear train must include some form of differential arrangement to provide automatic compensation for the disturbance in the timing otherwise introduced by the rotation of the cross box. The commonest and probably the simplest solution is the employment in the gear train coupling together the intermittent and the shutter of a sliding spiral gear moved endwise in step with the cross box. Some projectors are arranged with a manual adjustment superimposed upon the automatic operation of the compensation movement. This enables fine adjustment of the shutter timing during the show. This complication is, however, of very doubtful practical value, since the timing, once set, should never alter. When one turns to consider the drive arrangements themselves, one has broadly a choice between trains of gears on parallel axes, or shaft transmission by spiral gears, and a good case can be made out for either system. A shaft transmission type of layout can be very simple, with only a few gears. This is probably the most inherently silent drive, but due to the considerable sliding action at point of engagement of the spiral gears, friction and wear are increased, involving greater driving power, and adequate lubrication is of great importance. Whatever the layout, points of importance are rigidly supported accurately located bearings ; suitable gear materials, preferably alternate metallic and non-metallic gears with employment of hunting-tooth gear ratios ; and adequate lubrication arrangements. Ample and continuous lubrication is essential if trouble is to be avoided. At the same time, cleanliness is essential, since oil must not reach the film or the sound system. One has alternatives of oil bath construction and " dry type." Advantages of the " dry type " are greater accessibility and ease of replacement in event of, say, stripping of a gear. Of course, gears should not strip in a well conducted projector, but such disasters do sometimes happen — ■ maybe due to a cleaning rag caught in a shutter or sprocket ; maybe dirty oil leading to a seize-up. It is also a cheaper construction, since the problems attendant upon provision of satisfactory oil-tight enclosures are avoided. Paradoxically, perhaps, the oil-bath machine is inherently the cleaner type ; in the " dry " machine, it is difficult to avoid " excess " oil draining out somewhere. Invariably this is traceable to dirt or grease bridging the gap between a spindle and the surrounding catcher, and resulting in a capillary seepage of oil. The obvious remedy is careful cleaning.