4" Heavy Duty Mecanum Wheels

[AdamHeard]

Somewhat off topic here, but this is an awful lot of work, effort and resource going into a single item on the robot.

Would it be more beneficial to the overall performance to purchase wheels, and dedicate these resources to other items on the robot?

What is more likely to result in a more competitive robot?

[Brandon Holley]

Originally Posted by craigboez

I'm especially interested in more details on getting a dual-hardness roller. Since a hard roller doesn't provide much traction and a soft roller is too compliant, the ideal setup is a hard core and soft outer surface for the rollers. From what I read team 357 uses two different casting materials and from their pictures I can see team 2865 just used a hard PVC-looking tube as their core. Anyone care to comment on how to best accomplish this?

I'm also interested in making the inner surface as slippery as possible, to act as a bearing against the axle. Does anyone have experience either a) machining Nylatron or similar material (something filled with molybdenum disulphide), or b) buying Moly powder and adding it to a urethane casting?

It's funny seeing your thought process throughout this thread. It's the exact same thought process I had when I was working on my little wheel. It all depends on how extreme you want to go on reducing that friction. Using Teflon or Delrin will be effective, but the urethane will not adhere to it, so doing some kind of insert mold is out. I looked at even purchasing tiny ball bearings to press into the rollers.

Ethers point of friction along the face should also be noted. I imagine this friction would be the harder one to solve elegantly.

Good luck!

-Brando

[craigboez]

Originally Posted by AdamHeard

Somewhat off topic here, but this is an awful lot of work, effort and resource going into a single item on the robot.

I agree.

Would it be more beneficial to the overall performance to purchase wheels, and dedicate these resources to other items on the robot?

Probably.

What is more likely to result in a more competitive robot?

The goal of this project is not to create a more competitive robot, but rather to learn new stuff, build something cool, and contribute something back to the FIRST community.

[craigboez]

Originally Posted by Brandon Holley

It's funny seeing your thought process throughout this thread. It's the exact same thought process I had when I was working on my little wheel. It all depends on how extreme you want to go on reducing that friction. Using Teflon or Delrin will be effective, but the urethane will not adhere to it, so doing some kind of insert mold is out. I looked at even purchasing tiny ball bearings to press into the rollers.

Ethers point of friction along the face should also be noted. I imagine this friction would be the harder one to solve elegantly.

Good luck!

-Brando

That's funny, I too have investigated tiny ball bearings. While not one of my primary goals, keeping this thing cost effective is at least in the back of my mind. With 14 rollers per wheel and 4 wheels per robot, any additional components can raise the cost fairly quickly. Bearings at a few bucks each are almost out of the question.

Smooth-On customer service indicated that a dual-durometer pour is possible as long as the second material is poured while the first material is still tacky. They recommended PMC-780 DRY as the core and Vytaflex 40 as the outer material, or at least said that it is something they know has worked in the past for other customers. They also mentioned TASK 2 or 3, but noted that getting rubber to adhere to plastic is harder then getting urethane to adhere to another urethane.

A 40 durometer outer material sounds a little soft, as does an 80 durometer inner material. I'm fairly sold on at least attempting the dual-durometer casting, as it uses the fewest parts and creates an integrated solution. Any suggestions on which Smooth On products to start with, or at least which durometers to use?

[Ether]

Originally Posted by craigboez

Smooth-On customer service indicated that a dual-durometer pour is possible as long as the second material is poured while the first material is still tacky.

Since these rollers are so small, why not just pour the high-traction material right over the Teflon tube?

Find some Teflon tubing with an ID just slightly larger than the OD of your axle bolt, so the bolt slides inside without binding or excessive radial free play.

See sketch.

[techtiger1]

I think everyone hit on the machining intricaies of the hub so I am not going into that. The machining on those is a good offseason project anyway. However, I think if you check out 357's customs wheels, which are by far and away the best FRC mechanums out there becuase they are sponsored by airtraxx (No offense Andy and Mark), I'm pretty sure they use exactely 40 and 80 durometer.

[Akash Rastogi]

Next time I'm at the 357 shop I'll ask to take some photos of their process, I'm sure they've changed some things since the 2006 book was published. Its really awesome seeing all the parts in person and seeing how they are all made.

Also, @Jeff801, how heavy did that billet hub turn out to be? Looks great.

[Jared Russell]

Originally Posted by Ether

Since these rollers are so small, why not just pour the high-traction material right over the Teflon tube?

Find some Teflon tubing with an ID just slightly larger than the OD of your axle bolt, so the bolt slides inside without binding or excessive radial free play.

See sketch.

I believe the issue is getting the tractive substance to adhere to the teflon tube.

[DMetalKong]

Originally Posted by Jared341

I believe the issue is getting the tractive substance to adhere to the teflon tube.

What if you instead used a bronze sleeve and knurled the outside surface that would contact the urethane?

Edit: Something approximately like this.

[Ether]

Originally Posted by Jared341

I believe the issue is getting the tractive substance to adhere to the teflon tube.

It does not need to adhere.

[Jeff 801]

Originally Posted by Akash Rastogi

Also, @Jeff801, how heavy did that billet hub turn out to be? Looks great.

Its 0.844lb if I was concerned about weight there is definitely more that could have been taken out but for where they were/ are used it does not really matter.

[craigboez]

Originally Posted by Ether

It does not need to adhere.

I don't think they need to adhere, but I think the roller will perform better if they do adhere.

Intuitively, it seems like the roller could more effectively deal with axial loads if the soft material adhered to the hard material. If the two materials are allowed to slip relative to one another the hard inner core can't transmit the axial load and the soft outer material would be forced to deal with it. Given it's soft durometer, it would deform under load. This would result in a less efficient non-round wheel and would probably also reduce the lifespan of the roller.

[Ether]

Originally Posted by craigboez

I don't think they need to adhere, but I think the roller will perform better if they do adhere.

Intuitively, it seems like the roller could more effectively deal with axial loads if the soft material adhered to the hard material. If the two materials are allowed to slip relative to one another the hard inner core can't transmit the axial load and the soft outer material would be forced to deal with it. Given it's soft durometer, it would deform under load. This would result in a less efficient non-round wheel and would probably also reduce the lifespan of the roller.

Well you've got a valid concern there, but it depends on how soft the "soft material" is, and what the cross-sectional area is of the roller end. The soft material has to be hard enough not to deform excessively under radial load, and if so I would think a thrust washer (with Teflon shims) at each end of the roller would work nicely to handle the axial load and friction.

If you do a "double pour" (with a harder inner core) that does not preclude the use of an embedded Teflon sleeve for lowest friction.

[Aren Siekmeier]

If you're going the custom route, why not make them even smaller, say 3 inches? Obviously you can't get much smaller for it to be driven really at all, but decreasing the footprint makes room for other stuff. Although 4 inches is probably good enough and it already looks to be a tough scale for some of the finer details.

I would agree that these are Heavy Duty. I wouldn't know, but those nice fat mounting tabs don't look ready to bend any time soon, particularly in comparison to AndyMark's sheet metal versions. It definitely comes at the cost of manufacturing ease, though.

You should definitely pursue this as a way to explore your team's abilities and gain experience with the detailed manufacturing process. However, in general, I don't really see a competitive advantage to these things, when four 4" omnis at 90º to each other offers the exact same functionality with approximately the same footprint, far less loss due to the rollers, and much simpler (and cheaper) wheels.

[Brandon Holley]

I think a 40 durometer (Shore A, I'm assuming) is right on hairy edge of being too soft for a FIRST wheel. Especially in something like a mecanum wheel where the contour of the roller is so important, I think you'd want something a little bit harder. The Vyta Flex comes in several durometers I believe.

I too was considering a dual pour, but it does get tricky. You are definitely going to want the outer rubber to adhere to whatever is on the inside. Otherwise the roller is just going to shred itself off. I also believe you are going to want your outer rubber to have a pretty uniform thickness along the entire roller to get the best performance (this way its actually like a "tread" on a standard wheel).

Getting the plastic to adhere to the urethane is definitely not something to overlook. I know there are certain urethane based sprays that are designed to get materials to adhere to a urethane more easily. I'll try to look them up and report back to you.

-Brando