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Originally Posted by BurningQuestion
Last year, our team used a telescoping arm/ winch system for our pull up arm.
We used a latch that would prevent it from back driving. The latch would engage/disengage from a notch in the winch drum whenever we wanted it not to back drive (the operator would press a button to latch the winch). It was spring loaded to pop into the notch, but we used a servo to hold it out of the way until we wanted it to go in.
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Simbotics used one of these last year on the telescoping arm; it worked by inserting a pawl into a groove, in order to keep the shoulder joint from unfolding when hanging.
With regard to the worm gear idea, you might want to think carefully about the disadvantages inherent to a reasonably unbackdrivable worm gear system. The amount of effort to backdrive a worm gear is a function of its efficiency; a 40%-efficient worm gear won't backdrive
easily, a 75%-efficient one will require much less effort. Since you don't necessarily want to dump most of your power in an inefficient reduction stage, a worm might not be such a great idea. Some worm gear systems, like in power winches, are designed to be more efficient in one direction than the other--this would be ideal, but I would think that this would be difficult to engineer, without customizing the gear profiles.
I would, instead, try to use a ratchet & pawl system, maybe off of a hand winch, or something from
SDP/SI (look under "Ratchets & Pawls", appropriately enough). Attach it to a low-torque section of your reduction gearing, if possible, to avoid stripping teeth. Disengaging and engaging the pawl is probably best accomplished with a servo.
Alternatively, any of the methods used for dog-shifting transmissions (hex dog, square dog, dowel pins) could be employed, if you don't need a wide range of locked positions.