British Kinematography (1948)

]f>7 Flywheel Damping The method of providing the flywheel damping is all important, since any method which imposes a steady drag will, by increasing the film load, tend to eliminate the loose " S " loop and so degrade the filtering performance. This means that, in practice, the damping must be effected between the flywheel and some additional component revolving at about the same speed. Thus by using a copper flywheel in conjunction with an independently driven group of magnets, the eddy currents induced in the flywheel, when its speed differs from that of the magnets, produce a friction drag between the two parts. A similar effect can be obtained by using the viscous drag of an oil film between the two members. However, a relatively " tight " coupling between driving and driven members is required to suppress satisfactorily the tendency to oscillate, and in consequence, any speed variation of the independent driving member tends to be transferred to the film drum. To avoid this new potential source of speed fluctuation, it may be necessary to adopt elaborate means of mounting and rotating the controlling member. A somewhat simpler solution is offered by the so-called " rotary stabiliser," wherein the desired damping takes place between the film-driven outer shell and the free running inner flywheel, through the medium of a viscous fluid which fills the small clearance between the two parts. In an alter COnOiTiOM CivimC iCaST C0w#li4ȣt CONDITION CIVIMC OCOUAtf COH'OkNCf Fig. 3. Compliance of Film Loop. Fig. 4. Loaded Film Loop. native design, the inner solid flywheel is replaced by a heavy liquid of low viscosity, which has to force its way through narrow channels within the shell. The resulting viscous resistance provides a degree of damping quite similar to that of the previously described type4. Film Compliance The successful elimination of high frequency flutter, and particularly the 96 c.p.s. tooth ripple, depends largely on the compliance of the loop of film between the drum and the sprocket, and this is governed by the bending stiffness of the film, the tension in the film, and the relative diameters and placement of the drum and sprocket. Obviously the compliance will be least when the film is under such tension that it follows a straight path from sprocket to scanning drum (Fig. 3). Film stiffness is known to vary according to the thickness and age of the stock and the humidity of the atmosphere, whilst the film tension depends, in most designs, on the friction in the bearings of the scanning drum. In practice, there has been little indication that these variable factors have any noticeable effect on operation, but the performance may be made independent of these variables by providing a spring loaded and damped roller to bear on the film between the sprocket and the drum, as shown in Fig. 4.