4" Heavy Duty Mecanum Wheels

It seems that nobody produces a COTS “FRC grade” mecanum wheel in the 4" size range. The recent 4" vs 6" vs 8" size discussion (here) has made a lot of good points about the benefits of smaller wheels. I’m looking for an offseason project and some custom built mecanums sound like a good challenge.

I’ve managed to pull together some information from a few very helpful sources. Notably:

  • Jester Drive article in the Aim High Behind the Design book
  • Pics from team 2865 (here)
  • Brandon Holley’s post on urethane casting (here)
  • Ether’s derivation for roller contours (here, direct download link)

After a bit of time in Inventor I was able to come up with a rough draft of a design:

My plan is to machine the hub from a blank of 6061 aluminum and mold all the rollers using the Smooth-On brand of products. I’ll first 3D print some rollers, use those to create a silicone mold, and then use the molds to pour urethane rollers.

Before getting started I wanted to post here and solicit thoughts and criticisms from the group. I’ll be happy to answer questions on my design, as its really the product of the work of lots of others.

Not sure what resources you have as far as machining goes but there was a thread about 4" mecanums a while back. http://www.chiefdelphi.com/forums/showthread.php?t=91726&highlight=4+inch+mecanum There it is. May be of interest to you.

I’ve got a fairly large 3-axis CNC mill and a hand operated lathe at my disposal. I did see the post you linked to and probably should have mentioned it, but left it out since it was a wood project and he didn’t get into the processes I’m interested in (aluminum milling, urethane casting).

The reason I brought it up is that machining the base will be a pain in the butt. If you have a way of indexing around you could put a slot in the main base and put the roller mounting tabs on that. Would probably save some money and machining time.

As for how to mount the tabs should you go that route… I haven’t quite figured that out. I believe 1640 heated up a portion of their drive system to expand the hole then put a piece in. Perhaps that method would work.

The central hub could be a large Hex, with tapped holes on each of the faces. Blocks/channel could bolt to these holes, and could be bolted in any orientation or angle desired.

Such a hub could be made on manual machines even.

If you have a way of indexing around you could put a slot in the main base and put the roller mounting tabs on that.

I considered this, but didn’t like the idea of having a separate hub and fins for a few reasons. Strength and additional fastener weight were two, but primarily it was because the “holes” in the hub would actually be slots because of the endmill geometry, and I didn’t want to put a rectangular fin into a rounded-end slot.

With a few well designed fixtures the “hub with spokes” should be machinable on a 3 axis machine, all from one piece of aluminum. I think. CAM analysis hasn’t begun yet.

I’m actually considering machining one and using it as a master to create a mold from. Depending on how strong that urethane is, especially if fiber reinforced, it might just be strong enough.

If you want to get fancy, you could always round off the edges of the fins. (There’s a router tool whose name escapes me that does this.) You’d probably need to use a corner-rounding end mill, and pass each edge through individually—so it’s not particularly efficient. This would actually go a lot quicker if you used one of those tabletop chamfering machines, and loaded it with that quarter-round end mill. Then it would be relatively straightforward to make multiple passes quickly. (Either way, you’d probably want to machine the edges of the raw stock, then cut it into pieces.)

Another option might be to cut crosswise slots in the main slot, so that the fin can have a straight-side tab protruding from the bottom, and engaging the sides of the (slightly-deeper) crosswise slot. You don’t have much material to work with on such a small hub, but it might work.

Finally, you might want to prototype a version that relies on two or three high-strength bolts to bear the sideways load, and hence resist sliding in the slots (alloy steel socket head cap screws should be perfect).

If you mean casting the hub with spokes from urethane, you’ll additionally have to worry about stiffness, and then about friction (against the rollers).

Also, such a complicated mold shape will require some creativity to get the part to release. Much more so if you want to avoid secondary machining to get rid of draft (i.e. angled surfaces which make it easier to remove the part).

When machining the aluminium hub your going to run into issues in that with 7 fins and a 6 hole lightning patterns in the middle your going to have a hard time indexing it on your mill.

I did a short write up on how I made a set of hubs here: http://www.chiefdelphi.com/forums/showpost.php?p=1061486&postcount=12

I started a project (never finished) a couple of years ago that is exactly what you are attempting here. I designed a 2.5" mecanum wheel and had plans to produce them in a moderate quantity using silicone molds and urethane. The plan never fully materialized, but I do have the “masters” for the molds I was going to make. I hope you get farther than I did! Between school and work, it just completely fell off my plate.

The urethane is quite stiff actually, and can also be bought in several different durometers.

Also, keep in mind the beauty of a silicone mold is the ability to bend, push and pop a piece out of the mold because of how compliant the silicone is. The Mold Max 30 or 40 from Smooth On holds a very good geometric tolerance.

I think this will definitely be a fun project! Definitely look at team 357’s designs in the Behind the Design books. They have already got this process nailed so borrowing some of their expertise would definitely be helpful.

Keep us posted!

-Brando

That’s a great point. I’ll definitely want 7 lightening/fixturing holes to go along with the 7 fin design. That will simplify things a great deal. Thanks for replying. I had also originally read your post and looked at your pictures, but couldn’t find it again when I was linking to my sources. Your fixture pictures were very helpful.

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?

Why not just cast the bore larger, and use a Teflon tube insert?

Also, be aware that the bore friction is not the only concern. There are substantial axial loads on a mecanum roller, so you have to address the end-loading friction. A stack of thin Teflon washers at each end might do the trick. They could also serve the dual purpose as shims to reduce axial free play (which affects mecanum performance).

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I guess I was thinking it was important that the two roller materials were very thoroughly bonded together, though now I wonder if that is a good assumption. Would the outer roller material necessarily need to be well secured to an insert?

No, as long as it’s constrained so that there’s minimal free play both radially and axially. After all, you want it to roll, right?

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You could do two different durometers (hardnesses) of polyurethane, but why not do what’s done with roller blade and scooter wheels? A plastic hub gives the wheel its rigidity, and then urethane is poured around the wheel in a way such that it adheres rigidly to the hub. If you need more stiffness in your mecanum rollers without sacrificing grip, I’m sure you could do something similar on a small scale for rollers.

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?

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

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.

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?

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.

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