How is there no backlash? I can’t imagine a rope drive without backlash.

Improved materials. Kevlar and Dyneema.[1] Dyneema is about 15x stronger than steel per unit weight. Kevlar toothed belts have been available for many years, and can be used for many of the same applications as this capstan setup. Neither material has much elasticity.

The advantage over gears is that overloads are distributed over much more material. You don't snap gear teeth. This is good for leg landing shocks.

[1] https://www.impact-fibers.com/info/unveiling-the-strength-ke...

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kragen 2 days ago | parent [-]

The relevant feature of Dyneema is specifically that its Young's modulus is much higher than most other organic fibers, about half that of steel, which is probably what you mean by "neither material has much elasticity".

Steel cables would work just as well if weight isn't a consideration, but I think Dyneema is likely to be more resistant to abrasion. However, heat produced by any significant dynamic friction will ruin it immediately, as I found to my sorrow. Kevlar is much more heat-resistant, and of course steel is more heat-resistant than Kevlar.

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darksaints 2 days ago | parent | next [-]

He actually used steel cables for his initial prototypes, but steel cables have a problem with rolling fatigue that actually gets worse the larger the diameter...so if a tiny steel cable is not strong enough, you might actually have to change the gear dimensions (and therefore ratios) in order to not fatigue them.

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Animats 2 days ago | parent [-]

Right. Steel cables have to be oiled internally if flexed frequently. This was a major cause of cable failure in gym exercise machines for decades. Newer exercise machines tend to use synthetic flat straps over pulleys.

	▲	kragen a day ago \| parent [-]
		The Sandia paper also claimed that steel cables need a larger pulley radius.

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regularfry 2 days ago | parent | prev [-]

Dyneema is much easier to work with, too.

I'm quite fond of bowstring as a material for this sort of thing. It usually has other fibres mixed in with it so it's a little more bouncy than raw dyneema but that's minimised if you get thread that's intended for crossbows. It's usually waxed so there's some friction to it when you're manipulating it, and it's also easy to source.

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scotty79 2 days ago | parent | prev [-]

It's very non-stretchy rope material. I imagine with enough force you can bend it away from the set position a little bit but it comes back when you remove the force. Nothing like traditional backlash with gears where you can move it with very little force between two adjacent gear teeth.