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by Animats 330 days ago
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...
1 comments
kragen 330 days ago
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 329 days ago
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 329 days ago
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.
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kragen 329 days ago
The Sandia paper also claimed that steel cables need a larger pulley radius.
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regularfry 329 days ago
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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