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The braking device should have just sufficient tension to prevent the spool from "free-wheeling" when the motor is stopped.
The take-up may be worked either by direct drive or by friction. In the latter case the ordinary wire belt is favoured, and with some kind of friction device within the head of the take-up arm. In its simple form the wire belt is allowed to skid over one or more pulleys, and thus maintain an even pressure thrust to the spool axis. But this method is not without a number of disadvantages, and most projectors now include an additional friction device within or as part of the take-up head. Friction may therefore be imparted by a number of different methods, and operators should find out for themselves the type of mechanism within the take-up. But this does not mean, of course, that it should be taken to pieces for examination.
One type of take-up head is apparently held together by a single screw, but it will throw out quite a large number of springs and roller-bearings when released by the unwary. (The author speaks from experience.)
Another type incorporates a cork lining within the head, a fact seldom known by many of its users, and with the result that it is not long before the cork lining becomes thoroughly soaked with oil. The cork then breaks up and the take-up fails. The moral, of course, is that you must only oil the correct points, and nowhere else.
But all these different types of take-up heads have one thing in common, and that is the slipping clutch. This consists of two parts: (a) that which is driven by the motor and which revolves at an even rate, and (b) that part which is in contact only (but not mechanically linked in the true sense of the term) and which drives the spool around by its spindle. These two parts may consist of two discs in contact, or a steel spring in contact with a cork lining, and motion is imparted from one to the other by pressure only. But why a "slipping" clutch? The answer is because it has to slip as the spool slows-up with the increasing diameter of film taken up, and it thus provides compensation for varying speed of the spool plus compensation for variations in the "pull" of the film against the last take-up sprocket.
Where such a clutch is fitted, and this includes all modern projectors, note that the wire belt does not skid over the pulleys
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