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And finally, there are the limit switches. Not only does a motor need to be told when to start, it also needs to be told when to stop. This is where the limit switches come in. These are electromechanical devices that, when engaged, stop the winch from moving by turning the power off. There are several different types but the most common is the rotary switch. This is a device that has four gears on a vertical shaft. Each gear has a specific function. There's one to stop the unit when it's going up and one to stop the unit when it's going down. There's also a safety backups for both the up and down functions, usually called the ultimate limits. In addition to the usual compliment of teeth on each gear is a cam. This cam is a small nub protruding out from the rim of the gear. The gear rotates and the cam swings around and hits a button. That's the switch. What makes the limit switch gears rotate is the really important part. It's all about more gears, shafts and chains. (Once engineers find something they like it's hard for them to let go, ya know?)
The switches live in a small squarish box on the winch frame. The shaft that has the four limit switch gears on it also has a sprocket. The shaft sticking out of the cable drum also has a sprocket on it. Connect the two sprockets with a chain and you now have everybody turning at the same time. The drum shaft rotates and the sprocket rotates causing the chain to move which in turn spins the sprocket on the limit switch shaft. When that sprocket turns the gears on the shaft turn and, eventually, one of those cams will hit a switch. Now, if you make those cams programmable, you can set them to engage the switch when you want them to, and you can get the winch to stop whenever you want. Pretty cool, huh?
The most important part of this whole switch assembly is the chain. This is normally a dinky little roller chain. Smaller, even, than your bicycle chain. If it breaks or falls off, for whatever reason, the limit switch gears will not turn. And if those gears don't turn the winch will keep on running until something else makes it stop. That "something else" could be the grid or a wall or a piece of the set. Or possibly the winch frame itself. Whatever it is, it isn't good.
Remember that comment I made about 1600 words ago that a winch doesn't care? Well, now is when I finally get back to discussing it. It's also means that I'm getting close to the end of this article. (Insert wild applause here).
Winches tend to be humorless little drones that don't care that they're sitting in a dark room all by themselves with no one to play with. They just want to do their job and that job is to make the scenery move. Send electricity to the winches and they will do their job and continue doing their job until you tell them to stop by shutting off the power. If something goes wrong, a piece of scenery gets caught on something for example, the winch will simply continue to run. If that little ol' limit switch chain breaks or falls off, the winch will continue to run. And run and run. Meanwhile, the piece of scenery it is moving is now being dragged around the stage pulling everything, and everyone in it's path along with it. This is not a good thing.
The best way to avoid scenarios like the one above is to follow a stringent inspection and maintenance schedule. If you check the machine for wear or problems on a regular basis then chances are you won't have any exciting (albeit gory) stories to tell about your motorized rigging system.
- Check oil levels in the gearbox
- Make sure the drive chains and the limit switch chains have all
their parts and are lubricated properly.
- Tighten all those set screws, bolts and nuts regularly.
If you do these things then you should enjoy years of problem-free service from the winch. Ignore these things, allow the winch to sit off in a corner somewhere with no attention and...well...I'm sure you know that lack of attention can ruin a perfectly good relationship!
So the moral of the story is this…ignorance is not bliss. Ignoring your equipment will only lead to disaster. And disasters always cut into your personal time. Make sure your winches get all the attention they require and you'll have time to finish that rousing game of Monopoly in the green room.
Bill
For more Uncle Bill visit the Uncle Bill Archives and catch up on those veritable pearls o' wisdom from our favorite rigging master.
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