Quote:
Originally Posted by Ned Shelton
... We would use a 64:1 gear box and a belt ratio on the joint of the arm, and a 326:1 or 682:1 gearbox for the "hand". So my question is, will these gearboxes be driven backwards by the weight of the arm? (~7lbs at 50in) ...Oh also, how reasonable is Bane Bot's 35 ft-lb torque limit for the 64:1 or 326:1 gearbox? Will I just instantly strip the gears with what I'm doing? Sorry, I know that was a wall of questions, but I'm our team's mechanical captain and have almost no experience with these gearboxes.
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I'm a bit hazy on your description of the mechanism, but it seems you have some of your answers already, if you use a bit of math.
You've got something on the order of 30 ft lb of torque from the arm, based on your statement I've bolded above. That's the torque of the arm, and it is reduced by the belt and the gearbox. Unless you get hit, you probably won't exceed that torque rating.
When it comes to "back driving" the mechanism, it's possible. But with a sufficiently large gear reduction and the speed controller in brake mode, that won't happen. The question is what your ratio needs to be, and just how well can the motor hold. You'll need to test to find that out for sure. The good news is you can do that without wiring up a full robot. If you build your arm (or a mock up for it), all you need to do to test the braking capacity is to short the motor terminals (while NOT hooked up to anything else) and bingo!...you're simulating the brake mode of the speed controller.
About holding that arm in position...If you want to be sure, make a brake for yourself. I'm not sure if you're planning on pneumatics, but a modifiled bicycle break attached to a pneumatic cylinder makes a good force brake to hold everything in place. You can apply that to a belt sproket or other round device to grab it and make it stay put.
You could also use some simple feedback control to use the motor to actively maintain the position. There's more than a few references on how do to that on the internet.
I've got to ask...why such a high reduction on the "hand"? That's going to be awfully slow moving, and I doubt you'll need a lot of torque to pick up a game piece.