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One of the important characteristics of a drum is that it is grooved. This groove is helically cut into the drum outer surface. (Think a tightly wrapped spiral here and you have the right picture). This groove insures that the cable wraps up neatly on the drum when you are lifting or moving the piece of scenery. It's very dangerous to use a drum that isn't grooved. The cable can flop around wherever it wants to and it's a safe bet that it will go where you don't want it to go. (Flopping around may be ok in a mosh pit, but not on a cable drum.) The cable can cross over on itself, wrap around the shaft or around some other piece of equipment associated with the drum. Any of those scenarios can cause problems ranging from bouncing scenery to cable failure. (Generally, depending upon their mood, audiences hate it when the scenery falls down and smacks their favorite actor in the head). Always use a grooved drum and make sure the groove is the same size as the cable you are using. Another important requirement, as one of my editors just pointed out, is that the drum has to be long enough to allow all the cable in the rig to wrap on it in a single layer. Cable wrapping over itself on a too short drum can cause jams, kinks and possibly even breaks in the cable.
So that's the basic assembly. A motor spinning a shaft connected to a gearbox that adds torque to the spinning shaft and a drum that has the room to collect the cable. You could do the job with just these three things. But you can't do it safely. To do that you need at least three more devices. Two are brakes and the third is a communication device. If you can remember way back when I was talking about the motor, I said there was a shaft sticking out of one end of it. Well, I lied. That shaft actually runs through the motor and sticks out on both ends. One end goes into the gearbox and the other end just sort of , well, sticks out. Some bright soul saw the shaft waving in the breeze as an opportunity and stuck a brake on it. This brake clamps down on the shaft when the power is turned off. It doesn't matter if the power was shut down on purpose or accidentally. If the shaft doesn't turn, nothing else in the system is going to move either. This is a good thing. Think of it as leaving your car parked on a hill with the gear shift in neutral. Without a brake engaged the car will start rolling down the hill. If you happen to be in San Francisco (or some sections of Cincinnati) your car will start rolling really fast and, even if you are an Olympic sprinter, you'll never catch it.
The other brake is a safety device called an "overspeed brake". It's located on the opposite end of the drum from the gearbox. This is a device that slows down and stops the drum rotation in the event of a problem. For example, you are lowering in the drawbridge in Man of La Mancha (now there's a golden oldie for you!) and suddenly the bridge starts moving too fast. Maybe the shaft driving the drum broke or the drive chain between the motor and gearbox gave way. In any case, the drum is now freewheeling, able to turn without any encumbrance. The overspeed brake will engage the drum when the drum reaches a certain speed. See? It's a safety device.
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