What is meant by "reaction surfaces slide"? Is this referring to joints?

> Also mechanical advantage seems to be a major tradeoff. There is a reason biology tugs on ropes and lets reaction surfaces slide around instead.

I'm not sure I understand. Do you think that there's a biological benefit to ropes and sliding (compared to this)? Or is it simply that ropes/sliding are easier to evolve? You probably know this, but (in case others are unaware) there are rotary "engines" in biology (e.g., bacterial flagellum and F0F1 ATPase), but, obviously, they are not quite like this spherical gear.

I believe the parent comment is talking about scale at human sized.

Thank you. I wasn't catching that.

Imagine two sliding surfaces held agains each other and pulled by ropes.

You can shape the surfaces the way you want for the movement you need. Make them as big or small depending on how much compressive load you need to transfer. You can place as many ropes, controlled ropes or just stretch rows to have as much flexibily, control and ability to survive tension as you need.

You can do all of this with pretty crappy materials and high tolerances.

When compared to that, gears that transfer all the loads through individual small teeth that must be made from incredible materials and struggle if you need multiple degrees of freedom there's really no competition.

Unless you need to deal with fast spinning things or simple 1dof fixed axis rotations gears are terrible technology and they rarely ever evolved not because they are hard but because they have huge weeknesses and barely any advantages.

The only gears that ever evolved that I know of are used for syncing movement.

Also, "ropes" have the benefit of driving wide ranges of rotation while requiring relatively small muscle contraction distance, by attaching close to a joint's pivot point, which meshes well with muscle fiber physical characteristics and limits.

E.g. a 135° forearm range driven by a 4-5cm tri/bicep contraction

Example of 1D gear used in planthopper used for syncing the movement of the legs [1]. Even there the gears are only used for young planthoppers, and adults use another syncing mechanism after their skin molts away.

[1]: https://www.smithsonianmag.com/science-nature/this-insect-ha...

> There is a reason biology tugs on ropes and lets reaction surfaces slide around instead.

And that reason is that it's very difficult to have a rolling joint or muscle because you can't get blood into it. It's largely fundamentally inaccessible to an organism that must grow itself. There are massive advantages that are unexplored by evolution.

Even further, gears are inherently asymmetric, meaning they are even harder to evolve. Nevertheless nature DOES use gears[1] when it can.

> You can shape the surfaces the way you want for the movement you need.

You can actually do the same with gears and screws: https://www.youtube.com/watch?v=100is1XpXcE

[1]: https://www.smithsonianmag.com/science-nature/this-insect-ha...

Isn't that the only known instance of meshing gears in nature?

Agreed that there are plenty of geometries that would be very hard to evolve but are very useful. Wheels are another example.

its actually like a hydraulic cushion , with a self renewable matrix between the bones. we are close to mimicking tissue renewal with some of the recent materials advances.

https://en.wikipedia.org/wiki/Shoulder_joint

https://en.wikipedia.org/wiki/Self-healing_material

the ball gear might be a good candidate for metal 3d printing

The paper mentions that the rough surface of sintered powdered metal 3D printing won’t work (presumably way to much sliding friction). It also suggests the possibility of using that printing technique and surface machining it after printing.

there are some emerging metal 3d printing techniques that might overcome that, like rapidia

By far the easiest way to deal with this would be to just print them slightly oversized, add some abrasive paste, and then 'run in' a set using some balanced pattern of movement, tightening the clearance as the parts wear in until they're sufficiently smooth.