December 19, 2008

Mouse Trap Drop Dog

If I ever formed a band, I'd name it Mouse Trap Drop Dog. Complete non sequitur, right? No, just a cool name for a emergency braking system on older chairlifts.

I worked as a lift operator during the 1988/89 ski season. The first order of business was to learn the nuts and bolts of the mechanics of the chairlifts. Sheaves, cables and bullwheels, among other items.

The worst thing that can happen is a rollback, when the cable starts going backwards. The momentum of the weighted rightside will quickly accelerate backwards, and the chairs are whipsawed around the bottom bullwheel.

An emergency brake technique is to use a heavy steel bar (the "drop dog") that is dropped down between the spokes of the bullwheel. An abrupt stop but effective. Some bullwheels are designed with a series of notches on the spokes. As the bullwheel turns in the correct direction, a small metal latch ("mouse trap") drops into the notch, which is cut to allow the metal latch to slide out and then drop into the next one. The sound effect is a constant clicking. Think of the old playing cards in the bicyle spokes trick we did as kids.

However, should the bullwheel turn in the opposite direction the notches are cut in a manner that causes the metal latch to catch and yank on it which will then trigger the drop dog to fall into the spokes.

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