How Does a Winch Work?

[Adam Krajewski]

Originally Posted by Raul

The van door motor can be back driven unless the bearing screw (at the end of the worm gear) is tightened quite a bit. The problems with doing this are:
1) You lose power output by tightening it
2) It wears out and you have to keep tightening it to keep it from backdriving

Plus it might still back drive if it sees excessive force.

And when it wears out, no amount of tightening will do you any good.
During the summer of 2000, I did a study on the brakes. What they are exactly is a set screw (did someone inhale?) with a spring mounted 'brake pad' on the end. When you tighten it, you force the 'brake pad' onto the side of a plastic (final drive?) gear in the motor. This pad will eventually wear down and break. At that point, tightening it will do no good, as it is just a tapered end of a set screw with a hole in the middle against hard plastic. I have pictures somewhere of various van door motor brakes, but I can't seem to find them right now.

However, the bottom line is the brake should really only be used to get rid of very minor problems with backdrive. In '99, we used a van door as a winch motor to raise the lift carrying floppies over 8 feet and friction plus the brake kept the lift in the air without a hitch. In 2000 we tried to do the same thing, burning up brakes in the process on the motor that drove the shoulder of our arm. The final solution I came up with was to use a PD control system for the arm, so that 'sag' during a match didn't matter because the motor always ran to keep it in the right position, something it could do easily for 2+ minutes.

I would suggest something along the lines of what generalbrando said, and using a pin to stop the drum from rotating or perhaps some type of one-way action with a release like what rachet straps and come-alongs use.