[michaelt]

Well, it probably wasn't that much effort. When you're 3D printing you're going to end up printing everything 2-3 times anyway, so why not dial in the ratio while you're at it?

And you can't really declare your design is "high precision" and present yourself as someone others should take transmission design advice from if you aimed for a gear ratio of 8 and achieved "somewhere around 7.9 to 8.2"

[LeifCarrotson]

It probably doesn't matter so much whether it's 7.913 or 8.186, but it would be important to know the exact value for kinematics. One way to do that is to build an object very accurately, the other is to build inaccurately and then measure the result after the fact.

It's also interesting because competing actuators with strain-wave, cycloidal, or planetary gearboxes will state exactly what the ratio is. The actual gear teeth may not be spaced out perfectly around the circumference, but the number of teeth is an integer with an infinite number of zeros.

[tonyarkles]

Yeah, I think one of the nice things about making it a "clean" number (either an integer or a rational with a small integer denominator) is that you can easily validate it without needing high-precision measurement equipment: put a mark on both gears (maybe even embedded in the 3D print), line up the marks, rotate the large gear 1 full rotation, and count the number of rotations the smaller gear makes. Check to see if the marks line up perfectly after those rotations.

[nullc]

His capstan reduction can't go all the way around even once.

[hinkley]

It could though. I don’t think the thought has occurred to him yet. He could make the stack twice as high and go around twice as far. If you moved the worm gear you could go farther, but I don’t know how he would do that with his drive.

He could also go with narrower rope, and spread the load over more windings, which would give him more throw.

[mlhpdx]

What are the loads on a drive in this use case? 35lbs robot bouncing on one foot would be 350lbs-ish of dynamic load? That can be handled with 2mm Dyneema.

[hinkley]

At 2:02 he shows a working model with a smaller diameter cord than his earlier ones. Looked like he’s already using 1/8 or smaller.

Notice that the fastener bearing the weight of the cord is wrapped around two bends. If I recall my physics properly, each of those direction changes behaves like a pulley, halving the strain on the bight (dyneema is difficult to fasten though. Slippery bugger.)

No wait, he is already using 2mm: https://www.aaedmusa.com/projects/cara but it’s 2 ropes per leg, so maybe he could use 1/16” rope (1.6mm).

[ErigmolCt]

Getting that ratio nailed down also makes future designs more predictable I think