pic: Why aren't omni wheels like this used?

[BJC]

Another primary reason these types of Omni Wheels will not see use in FRC is due to the roller thickness. Both the original and the "improved" wheels have parts of the rollers with near zero thickness. This comes with a number of problems including that the rubber rollers preferred for traction won't support a robot's weight on near zero thickness (bumpy ride) and as the wheels wear they will very quickly become non-round (very bumpy ride.)

[Ryan_Todd]

Originally Posted by BJC

Both the original and the "improved" wheels have parts of the rollers with near zero thickness. This comes with a number of problems including that the rubber rollers preferred for traction won't support a robot's weight on near zero thickness (bumpy ride) and as the wheels wear they will very quickly become non-round (very bumpy ride.)

That's for sure! Neither of these designs could be expected to produce a perfectly smooth ride in real life, even after they were tweaked for durability and manufacturability. Even dualies can only be perfectly smooth if the wheel axle is held perfectly parallel to the ground (flat ground AND infinitely stiff robot frame), but with a good enough roller design, they can indeed usually be expected to produce a smoother ride than these two wheel designs would.

Engineering is all about trade-offs, however, and a design like these two would seem to strike a potentially useful balance between a traditional single-omni design and a dual-omni design: smoother than a single-omni, and lighter/more compact than a dual-omni. It still remains to be determined whether they could be produced at a low enough price point to compete with the current market leaders, of course!

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EDIT:
Now there's an odd idea. I was considering refining my design and printing it in pieces just to evaluate the concept, but then I remembered that many "impossible-to-make" designs are trivial when additive manufacturing is employed. What if a multi-nozzle 3D printer were employed: nylon for the structure, TPE for the rollers, and a soluble support material to maintain clearance where necessary?

[Greg Needel]

you mean like this one?

[Adrian Clark]

Originally Posted by Ether

When there's an axial load, what does that white nylon rub up against (material and shape) ?

The white nylon rubs against the polycarbonte body of the wheel. Looking at the AM pdf it appears the roller contacts a flat face. I could be wrong on this, I'll leave it to someone with the time to open the cad to confirm.

-Adrian

[Ether]

Originally Posted by Adrian Clark

The white nylon rubs against the polycarbonte body of the wheel.

Polycarbonate. Hmm. Is there enough room between them to install a lower-friction (perhaps nylon or Teflon) shim?

[GeeTwo]

Originally Posted by Adrian Clark

The white nylon rubs against the polycarbonte body of the wheel.

Originally Posted by Ether

Polycarbonate. Hmm. Is there enough room between them to install a lower-friction (perhaps nylon or Teflon) shim?

It would have to more like a film than a shim; the free axial motion of the roller is barely sensible. If you were willing to trim down (and presumably polish) the roller you could put a thin shim in place.

[Ether]

I couldn't find any data for polycarb-on-nylon friction. I suspect it's not pretty.

[GeeTwo]

Originally Posted by Ether

I couldn't find any data for polycarb-on-nylon friction. I suspect it's not pretty.

I concur. However, in many cases where omnis are used, the goal is either to allow a long wheelbase robot to turn (tank with 2 or 4 omnis), or to allow fine strafing adjustments so that a robot can line up on a game piece or a target after forward motion has completed or nearly completed (H/slide drive). In both of these cases, "not pretty" is still a whole lot better than "solid wheels".

I can certainly see roller friction while applying driving torque being a real problem with kiwi (three omni wheels in a triangle) or Killough (four wheels in a diamond configuration) drives. I almost suggested kiwi (implicitly using duraomnis due to our game's KoP) to one of our intramural design teams today; I'm glad I held back.

[T^2]

Originally Posted by Ether

I couldn't find any data for polycarb-on-nylon friction. I suspect it's not pretty.

Nationwide Plastics has polycarb-polycarb at 0.52 (unspecified, but I'd guess static) and Engineering Toolbox has nylon-nylon at 0.15-0.25 (static). Polycarb-nylon might be somewhere in between. Not too bad, really.

[pmckinnon]

The concept of these wheels are quite good as they provide close to a full concentric circumference, however they are complex to manufacture. A greater choice of dual and triple row omni-wheels are more readily available such as those by Vex, Andymark and Rotacaster Wheel which can provide better traction and better or equivalent ride quality.

[nate12345678]

These are both fantastic omni ideas. I personally think any omni other than the standard "strait sized" omni would be a huge improvement. We are using the practice bot's drive train from last year, and replaced traction with omni wheels. Only downside, the bot can't be driven in the huge back lot outside.