Simon MerrettHere's a stronger version of the first design revision which uses a lazy susan bearing (the video thumbnail might show the proof of concept - watch the video to see the new design).
There are promising signs this could be useful in a compact arrangement, e.g. a small robot arm base. It felt like there was room to reduce the friction but you can't ask too much too soon! Where should we go next with this?
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I tried such a three ring gear sandwich with gears but could not quite make it work. But that may have been due to too tight tolerances (and me trying to get away without a planet carrier). I decided to make simple planetary gears work first before going back to the fancy stuff.
The three ring layout has the benefit of the planets acting as a bearing for the output gear. But is encloses the driven gear (as you need to connect the two outer ring gears on the outside). This is fine for some things like driving a belt or another gear but cannot be used for something like an endless rotation robot arm joint.
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"But is encloses the driven gear" is exactly why I haven't tried it yet. I think there may be a few ways to at least make a more compact/integrated gear than many of the robot arm bases use with the stepper outside. Lots of half-ideas in my head but just takes time to try them and you always have the doubt that if it worked someone would already have popularised it.
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As much as I would love this to work I have the creeping suspicion that this is a dead end. The torque is twisting the belt at the contact point and it has little to resist. The obvious solution to this would be gluing two tiny belts onto the rollers in a way they cannot be distorted. And then you are only one sun gear away from a planetary gear with split ring gear. And if you do not want to spend the effort for the sun gear this also works while keep driving the planet carrier. So all you need to do is ditching the belt and add involute teeth on the ring gears and the planets/rollers. Then you have the twisting force at a place you can actually control and do something about the lack of stiffness by improving the strength of the planets and the planet carrier.
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It may well be but it is interesting finding out! There is also the option of sandwiching three gear rings together and the outer ones having the same tooth count and acting as a support for either side of the belt tooth. Was wondering about a belt-linked planetary arrangement in the car earlier...
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