pic: Dual FP gearbox

[s_forbes]

Originally Posted by NickE

The gearbox looks great overall!

Just a few questions:

• What is the overall reduction?
• Have you thought of using thinner plate than 1/4?

My main point of feedback for this is that you should really consider putting some holes in the plate around the motor mount location for the FP to vent.

Forgot to mention; the overall reduction is about 190:1. This gives the output shaft a stall torque of about 126 foot-pounds. With a 12/60 chain reduction to the arm, the stall torque at the arm is 630 foot-pounds (to which we all said "wow!") and the tension in the chain is about 2000 lbs (again, wow!). Ideally, though, the robot will never see these kinds of loads. The purpose of running two fisher price motors is to keep them well under their stall torques, so they hopefully never stall and release precious smoke.

We were considering the possibility of thinner plates, or possibly pocketing out the extra material or using delrin, but I didn't get around to it in this rendering. The total weight of both aluminum sideplates is 1.29 lbs, so the weight really isn't that bad (it could be reduced if necessary though). Total weight of the gearbox with motors and bolts is given by inventor to be about 5.3 lbs. This can be dropped to 4.6 by using aluminum AM cluster gears, and could probably get below 4 if we felt like pocketing sideplates and gears.

I also thought about breathing holes for the motors and clearance holes for their screws, but didn't get around to putting them in this rendering. I doubt we would try to build a fisher price gearbox without them.

Originally Posted by JVN

I always design with an additional 0.003" between axle centers. This has absolutely nothing to do with machine tolerances, but instead with gearbox efficiency.

They run a lot better if you run them a little loose...

I think designing them at "pitch circle + .003" is a pretty good bench mark. Should be fine.

-John

Thanks, this is the answer I was hoping for.

Our current plan is to try machining gearbox plates on a manual mill. We have access to a big one at our robot building facility, and there are a few high end ones in the AME department machine shop at my college. Hopefully we can keep all of the center to center distances within a thousandth, so it will probably work fine if we aim for 3 thousandths over nominal distance (I hope). If we get around to making something like this, we'll see how it turns out.