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Memo from Uncle Bill: Motorized Rigging
The inspiration for this article came from my most recent trip to London where I had the opportunity to see three shows. Incurable technician that I am, I couldn't help noting that two of the shows used motorized rigging and that the third one probably should have. So I'm going to share some thoughts with you about motorized rigging. (This will be a lot more fun if you imagine we're talking over a Newcastle Brown at that little pub just down the street from the Gatwick Theatre stage door.)
The most common type of motorized rigging is the cable drum winch. A cable drum winch is a machine that runs on electricity, hydraulics (sometimes) or pneumatics (rarely) and moves scenery by winding up or paying out steel cable. The movement of scenery can be up and down, across the stage or a combination of the two. The machine doesn't care. (Hold that last thought. We'll get back to it in a little bit.) To get a clear picture of what a winch looks like, here are the major components:
(The following is sung to the tune of "The shinbone is connected to the thighbone". Okay? Everybody join in.) "The motor is connected to the gearbox and the gearbox is connected to the cable drum. The cable drum is connected to the cable and the cable lifts the piece." (Mitch Miller, eat your heart out)
It all starts with the motor. There is a metal rod, called a "shaft", running through the motor and sticking out on the end. When the motor is switched on, the shaft starts rotating. Most motors will turn that shaft at about 1,750 revolutions per minute (RPMs). The problem with the motor, however, is that it's not very strong. Fast, yes. Strong, no. Lots of scenery weighs in at well over 1,000 pounds these days and a motor shaft spinning at 1750 RPMs is not going to do a lift by itself. To add strength to the speed of the motor, a transmission is installed.
The transmission, or gearbox, is really a metal housing enclosing a series of toothed gears. The gearbox takes the speed of the turning shaft and converts that speed into strength (called "torque" in engineerspeak). This is achieved by developing a mechanical advantage. In this case, the mechanical advantage is created by all the gears of different sizes working together. That shaft comes into the gearbox spinning at 1,750 RPMS but is not able to lift any real weight. Through the mechanical advantage the shaft coming out the other side of the gearbox will be spinning much slower and able to lift a considerable load. How much slower and how large a weight is determined by the size of the motor and type of gearbox.
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