BB gearbox for arm/claw

[Ned Shelton]

Our team plans on using the RS-775 to power our (simple) arm, and possibly a 395 for the manipulator on the end of the arm. 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) We can't find a good worm drive to use on the motors, so I was hoping the high ratio planetary BB gearboxes wouldn't move backwards, but I have no experience with the hardware.
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.

[Kevin Sevcik]

You could work out the output torque of the trannies yourself to see. BB gives you the stall torque for each of them. The RS-775 has a stall torque of 0.588 ft-lb. The RS-395 has a stall torque of 0.0867 ft-lb. Multiply it out and see.

Also, don't count on the trannies to not backdrive. The new P60s from BB are pretty smooth, and the BEST you could hope for is for it to stick in place statically... right up until anything bumps it. So looks for other options to keep your arm stationary. Counterweight and counterbalance mechanisms work well here. Or put a sensor on it and tell your programmers to write a PID loop for it.

[jee7s]

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.

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.

[Ned Shelton]

Originally Posted by jee7s

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.

Assuming the 395 runs at 11,000 rpm, which is 25% of it's stall torque, the 326:1 gearbox makes the output speed 33rpm. The hand has to rotate 90 degrees to pick up a piece, and that would take .5 seconds at 30rpm.