Disengaging a motor on a spring powered hurdler

[Kevin Sevcik]

So to answer the actual question posed, 57 had a mechanism like this last year. You'll find that the vast majority of devices out there simply won't release under load. If you're winding back and then using the latches to control the final release then it's not as much a problem and you could use an andymark shifter and an andymark planetary adapt an FP to the tranny. That would work fine as long as you back the motor up so the springs take the load before you try disengaging.

That said, our solution was a little different. We used a worm-worm gear combo to drive the retracting pulley. An FP through a BB tranny drove the worm, and that whole assembly was mounted on a pivot and gravity biased to the worm wanted to pull away from the gear. So to retract, we'd use a cam on a servo to push the worm into the gear and retract. To fire, we'd move the cam the other way and then we'd have to reverse the motor slightly. The worm was arranged such that while retracting, it was actually pushed into the gear thanks to the location of the pivot point. Reversing the motor broke changed thing s enough that the worm broke free of the gear and out went the spring.

[MrForbes]

That sounds interesting Kevin, do you know if there are any pictures of it on the web somewhere? where they might be?

[Kevin Sevcik]

I don't think there are any. At least, I didn't take any, and it's buried inside the robot, so it's unlikely to be visible in beauty shots. As a caveat, it took build season and most of two regionals to get the working combination of software and hardware. And then an unkind robot picked out the one weak point and broke a delrin flange that attached the pushing plate to the pushing rod. Much of our time during build trying to get it to work was fruitlessly attempting to adapt various pre-made release devices to the system, like a heavy duty fishing reel, etc. That would be how we discovered that many such mechanisms lock under load. I imagine a dog shifting transmission isn't likely to be much better.

The beauty of the worm gear design was that an appropriate worm meant the system wouldn't backdrive, and yet the arrangement of it all meant it would get out of the way when firing time came. Were I to apply it to retracting a heavier spring like you would need for this task, I think I'd look very carefully at a small piston to engage and disengage the worm instead of depending on gravity to release it and an admittedly not entirely sturdy cam on a servo to keep it engaged.

Actually, come to think of it, the biggest single advantage of this system is the it's much more likely to be safe. Using the latches to hold the punch while under load means that if your latches fail, then the motor either has to hold the load itself or slowly release it, and either one takes a lot of amps. You'd have a similar problem if low volts or mechanical problems prevented the punch from fully retracting. With the worm system, the motor can latch or release the punch at any point, so the only dangerous point of failure would be the mechanism that holds the worm on the gear. And if that can only engage the worm and you depend on a spring or gravity for release, then that's pretty good too...

At any rate, as I said above, I don't have a picture available. I'll try to snap one tomorrow or this weekend, but I'll also try to cad up a schematic of the arrangement and how the forces end up making it work.

[Alex.Norton]

Why not use a winch to pull it back to a latch and then unwind the witch letting the cable go slack or mabey using a really weak spring that pulled the cable in some other direction just to keep it from floping around. This way you wouldn't need anything more than your winch and the latch on the spring.

[Peter Matteson]

Originally Posted by Alex.Norton

Why not use a winch to pull it back to a latch and then unwind the witch letting the cable go slack or mabey using a really weak spring that pulled the cable in some other direction just to keep it from floping around. This way you wouldn't need anything more than your winch and the latch on the spring.

Or you just buy an automotive winch that free wheels and adapt a kit motor to the planetary it comes with. Just fire a pnuematic to lock the winch in and release the free wheel to retract or release.