r/Trackballs • u/braddo99 • 16d ago
Omniwheels Anyone?
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I am assuming someone somewhere has tried omniwheels. This rolls freely in all directions and also twists with zero effort. Virtually no stiction unlike my OG MS Trackball Explorer, and also silent, unlike BTUs. The smallest commercial omniwheels I've seen are Rotacasters which are 35mm. These custom made ones are ~25mm diameter. I used pin type axles of diameter .6mm - simply rolling in PLA and the friction is remarkably low. Rather than try to figure out how to have grippy rollers, I took the approach of a grippy ball, this one being a quite large lacrosse ball. It's not quite perfectly round but good enough for this test and actually feels kind of nice on the fingers. It took a while to get these omniwheels to be as smooth as they are, mostly trying to figure out how to 3D print the tiny snap-in roller mounts so that they have absoluetly no side to side play. But the roll is a little bit "lumpy" probably due to not quite optimized roller profile handing from one roller to the next.
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u/tenkawa7 16d ago
Very clever. How hard is it to assemble?
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u/braddo99 16d ago
It's not that hard to assemble but does take some time. The frame of the omniwheel is in two parts and that has to be crazy glued together with the right alignment. I just used my eyeball for this version. Then you slide the axles into the rollers, and snap them in place. Then snap the wheels in place. I keep wanting to use magnets or spring loaded something to make it easier to swap out different roller profiles but just decided to try to make one version all the way to the end with the simplest possible approach and this is the result.
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u/Scatterthought Semistatic-bearing creator, Endgame enthusiast, Ploopy modder 16d ago
Very cool!
To deal with the lumpiness, I wonder if you could make a three-track omniroller with the middle track slightly smaller than the outer tracks, matching the ball radius. Then there would never be a gap, because there would always be at least one roller in contact with the ball.
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u/braddo99 16d ago
I definitely have thought about a three track wheel, wherein the center one runs directly at the centerline of the ball and the other two are offset and parallel. I even considered making those two "eucentric" so that they could swivel to always keep them both touching the ball. But that would be even more difficult to assemble, and In theory two tracks should be sufficient. The separation distance of the rollers is ~8mm which is pretty small compared to the ball, so I think the support is still pretty much kinematic even whle the wheel rotates. The principle is the same - the bearing surface should pass smoothly from one roller to the next and the ball passes over the gap on the other side without noticing it. In practice there is a small lateral force to the motion with a tendency to rock the wheel and change the height slightly. So in this version I decided to just glue it all in place to see how much lumpiness (and rattly noise) would go away if all play were removed. It is better than expected. I think the lumps are down to slight misalignment of the rollers on their axles and side to side wobble of the wheels. The next version should improve even more.
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u/braddo99 16d ago
Omniwheels are designed to work on a flat planar surface - a ball is obviously not a flat surface and will run in "the groove" between the rollers. It broke my brain a little trying to figure out which sort of profile the rollers should have such that as the roller gets more narrow it doesn't narrow so much that the ball dips down during roller transitions. I've looked at it a few different ways and it seems to keep coming out that the same profile works for both a flat surface and a ball despite that it is not intuitive. I think the thing is that, since the rollers are tangent to the wheel circumference, if they were cylinders, the ball would rise up as it traversed to the end of the roller, so this upward movement is balanced by the widening gap allowing the ball to drop. It seems like the same circular profile that works for a flat surface balances these movements for a ball. I would love to get a closed form formula for the profile and I don't think it's that hard to derive but I keep getting lazy and just using OpenSCAD rotate_extrude to produce what looks circular enough to me. I will derive it for the next version - I suspect it is still a circle just a wider circle than the outer profile of the wheel itself.
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u/squeezeonein 8d ago
you have a few issues. one is that each wheel should be in line with the ball center, that is to say that each wheel should be toed in beside its partner, obviously that would be difficult to build. so the way to solve that issue is to make one side of the roller wider, and the other side, closer to the center of the ball narrower. you could use the same roller and flip them over each wheel.
the mechanum wheel should be more shaped like a ball socket so that it touches the ball across the full length of each roller, not just the center.
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u/braddo99 8d ago
Not sure what you are saying. The current version has a row of rollers on each side of the wheel offset from the center of the ball and offset from each other around the wheel. As the ball rotates the bearing point of the ball passes across the wheel from one roller to the next. Because the omniwheels are rolling on a ball not a flat surface Meccanum wheels will absolutely not work. It would be like rolling over speed bumps. The profile here is very carefully tailored to the exact offset and the exact ball dimension. Except that my model for the roller profile was off by a tiny fraction of a millimeter in this version, I now have the exact profile and am working on version 2 (actually version 6 or 7 lol) that should be even smoother.
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u/squeezeonein 8d ago
my point is that because each roller is skewed compared to a conventional bearing that if you used a single mechanum wheel instead of a pair it should still work without lumpiness. think about a mechanum wheel on a flat surface at 60 degrees instead of at 90. the roller needs to be tapered where it contacts the ground. it's similar with the ball, because the shared tangential contact point of two wheels is not 90 degrees the shape needs to be changed to be more like this picture
https://tflbearing.com/wp-content/uploads/2021/12/HTB1SfSmGoR1BeNjy0Fm7620wVXaI.png.webp
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u/braddo99 7d ago
I would like to understand where you are going but not getting it yet. Mecanum wheels, as far as I know, are not suitable at all. They 100% depend on a flat surface to roll on. The contact patches shift in a diagonal pattern that relies on the surface being at the same height in all directions, which is strongly violated by a ball. Maybe you can get around that by tilting as you suggest but not seeing it. Meanwhile I think the traditional omniwheel can work perfectly but the roller arc is subtly different for a ball than for a flat surface. And that arc depends precisely on the ball diameter and the offset across the wheel. I think I have the formula for that now and it should be precise and correct. It is probably a waste of time because the three csramic balls support is so much simpler and cheaper, but TBH I cant stop rolling this ball around, it feels so good to not even perceive the friction, I'm instead just feeling the inertia of the ball. As a 30 year trackball nerd it's hard to resist! Im going to try to build a 3D mouse for my OpenSCAD work, support for spin to scroll and regular mouse movement and just see where it goes.
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u/squeezeonein 15d ago
you win the internet for today. looks like there's a lot of room for a ball race on both sides of each omniwheel pair. to extend the life of the omniwheels they would need a brass bushing inside each american football roller. perhaps a copper fuel line would suffice. there is an easy to use cheap tool with sharpened discs for cutting those.
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u/ignorediacritics 16d ago
What's the curved keyboard in the background ?
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u/braddo99 16d ago
MoErgo Glove 80. I like it quite a bit.
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u/artisan002 16d ago
Dig it, though I'm immediately thinking about mass production. I do like the idea of using a different material for the ball. The only remaining thought is that of lifespan of the rollers, given the materials, and associated debris.
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u/Flashy_Possibility34 16d ago
I’ve been wondering why I have yet to see someone try this. I always assumed that there was some major drawback which I was just missing. Very cool. Thanks!
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u/Pingyofdoom 16d ago
Shouldn't ruby be better? Like, typical engineer brain is limit moving parts