Mecanum Wheels
 

Does anyone have instructions on making mecanum wheels? Or a source of cheap mecanum wheels? The ones from Andymark seem really expensive...

Re: Mecanum Wheels
 

i cadded a set of mechanums that i calculated would cost less than 50 dollars a piece, but they require a 5 axis mill, and some casting work, i could dig out the cad files if you like. if you want to know all the math involved in making them, here we go:

the most important thing to do is to understand that the profile of the roller is a partial ellipse, where the short dimension is the diameter of the wheel, and the long dimension is the diameter of the wheel divided by the sine of whatever angle the rollers are at (usually 45) after that bit of math there is really not much to designing fully custom mechanums.

http://chiefdelphi.com/media/photos/35738

http://chiefdelphi.com/media/photos/35738

here are the renders of the mechanum wheels that i did, the blue one should cost around 50 dollars, (depending on what casting compound is used for the rollers.

Re: Mecanum Wheels
 

No sorry, but i'm curious why are you looking at mecanums?

If you want to do an off-season project mecanum might not be the best system for you and your team.

Re: Mecanum Wheels
 

There aren't exactly a lot of sources of mecanum wheels in the FRC size range. AndyMark is the biggest (and to some extent, the only) one.

You may want to get in touch with 357 (or, the Behind the Design books from the 2006 and 2007 FRC seasons); they've got casting their own down to a fine art.

Or, if you can find someone who can build them directly with a direct manufacturing machine, that might do it. Just watch out for FRC cost rules--the material can get pretty expensive.

Matt, offseason is the best time to do mecanum. Not during the season. 330 spent 2 offseasons prototyping a set before giving them a chance to make a competition robot in the build season.

Re: Mecanum Wheels
 

It's for a shifting drivetrain project. It will have mecanum for speed and agility and tank for brute strength (with traction wheels) and the ability to switch between the two.

Re: Mecanum Wheels
 

357 has a pretty impressive setup. Michael Crane is probably a good contact person from that team to ask about how they make their own.

Re: Mecanum Wheels
 

Nice!!! that sounds sweet!

well i did some searching and the robot shop has some for about 85 for a 4 pack (for the alum. hubs and 75 for plastic hubs) but the O.D. is 2.125. You might be able to do some prototyping with them using vex parts then decide if you want to drop 400 for the AM ones. Our team has done a pure mecanum drive for the last 2 years (we will not go back to it, but the AM heavy duty ones work really well).

Heres the link for the robot shop ones

PS if i did come off as negative, i just don't like a pure mecanum drive.

Re: Mecanum Wheels
 

http://www.thingiverse.com/thing:2473

http://hackaday.com/2010/04/28/print...um%C2%A0wheel/

if you have a 3d printer you could always scale a set of these.

Re: Mecanum Wheels
 

I've seen 357's casting method up close and it's pretty awesome, though I don't see most teams being able to do it. 357's wheel body is also it's own work of art - which again is pretty difficult to make, imo.

I 3D printed a set of Mecanum wheels back in '07/'08 and there's no way they'd stand up to the abuse of an FRC robot.

Not really sure why the OP doesn't want to use AM Mecanums, at $222 a set they seem like a steal.

Re: Mecanum Wheels
 

There is that. I'm pretty happy with the AM HD Mecanum wheels.

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I was assuming they were expensive for mecanum wheels; they seem expensive for the materials that go into it. But if they're the best option available, I'll take it.

Also I don't think the printed wheels would survive if we were to use them.

Re: Mecanum Wheels
 

I have made a set of mecanum wheels that were not for FIRST robotics but used in a high capacity application and the machining of them does not take too much work but more a good deal of time. I can speak more on the machining of the hubs than how to make the rollers because I used a neoprene rubber for the rollers with a nylon core and the neoprene was just too soft. As well as there are much better ways to make the rollers than how I did (CNC Lathe).

As for the machining of the hub I did it in a 3 step process where I started with a 6x6x1in block of aluminium and the first operation I cut out the profile of the wheel hub, the center bore for a FR8 bearing and a 6 hole pattern at 1.875". Then using the 6 hole pattern I used it to mount it to a fixture which stood the hub up vertically and I then used that to cut the fins and drill a hole that I would a #10-32 set screw in. I did 6 fins so that they would line up with the hole pattern and it just made life simpler. I rotated the part to cut each fin one at a time. Then I had another fixture which held the part at a 45 so that I could fillet the corners of the fins and drill a 1/4" hole for the axle of the rollers. The part was also rotated on this fixture for each fin.

I have also uploaded some pictures of the manufacturing process here:
https://picasaweb.google.com/j3ffr3y...eat=directlink

Hope that helps.

Re: Mecanum Wheels
 

nice photo.

Did you use ball, roller, or journal bearings for the rollers? Can't tell from the photo.

Re: Mecanum Wheels
 

The bearing surface is nylon on a 1/4-20 bolt (the nylon rolls on the shank of the bolt and not the threads) And between the neoprene roller and the fin there is a Hard Fiber washer from mcmaster

Re: Mecanum Wheels
 

When the roller is axially loaded toward the fin, is the neoprene rubbing against the Hard Fiber washer ?

Re: Mecanum Wheels
 

Yes but with that said the coefficient of friction between the neoprene and the hard fiber washer is greater than that of the hard fiber washer and the aluminium hub so there is just about no rotation of the washer relative to the roller. I hope that make sense. If I was to remake them i would have done it more like how 357 did it this past year with a nylon core and a molded urethane rubber outside.

Re: Mecanum Wheels
 

Yeah, I realized that after I hit the "send" button.

The axial loading forces on the rollers can be quite high, so anything you can do to reduce end-loading friction would be beneficial. Perhaps three or four very thin Teflon washers, for example. Is the bolt nickel plated?

Minimizing axial free-play also minimizes losses.

How did you determine the roller profile?

Re: Mecanum Wheels
 

If I was going to use this in more of an industrial application I would use R4 bearings a polycarbonate core urethane outside. But for where these are being used nylon is good enough and they are zinc plated bolts.

As for determining roller profile I made a spline having reference points every 1/8 inch. I have attached 2 pictures showing the sketch in CAD and the finished assembly.

Re: Mecanum Wheels
 

This looks great! Nice circular contour. If you're interested in the math, I posted a couple of short papers here showing two different ways to derive the formula for the roller profile.

You said it was used in a "high capacity application". What was that?

Re: Mecanum Wheels
 

Oh? Really?

330 3D printed a set around the 2005 season through one of our sponsors at the time--as a matter of fact, every mecanum 330's played with to my knowledge has been 3D printed. The VEX-scale ones (which were metal) had some problems with fins bending before the roller pins were installed. The resin one had problems with cracking (and staying together--it had a 2-piece hub).

The pure nylon ones held up pretty darn well in our testing. They'd take a full season, I'm certain of it--they took everything we threw at them in testing with zero issues (other than a lack of traction, resulting in some cool tricks we tried with the rollers). 8 rollers, outer rims like the AM ones, not inner ones like 357's, one-piece hub, steel roller shafts (screw-threaded on one end to act like a custom screw, and sprocket attachment points on one side. Sorry, I don't have any pictures--maybe some of the other members of 330 at the time have some, or can get them.

Were the wheels used in competition? No. We considered that a 6WD better fit our objectives that year--and for every year since. As such, to this day, 330 has not used mecanums in competition. They're there, though--waiting to be used if the game calls for them.

Re: Mecanum Wheels
 

What kind of 3D printer did you have access to? I used a Dimension for my Vex prototypes and have played with some parts Made in an FDM machine and I wouldn't trust either machine to make a competition mecanum wheel unless some sort of reenforcement metal was laid into the plastic. (Possible on an FDM machine)

Re: Mecanum Wheels
 

Ask ChrisH--he worked with the sponsoring company for work. I want to say that it was a 3D Systems machine, or something like that, but I could be wrong. Wasn't a Dimension, though. (I'm also guessing that there was some experimental stuff involved--I'm not entirely certain about that--or what was being played with--by any means.)

Nope, no metal reinforcement. Even the steel axles weren't reinforcement. As a matter of fact, the metal VEX wheels has some problems with their "fins" bending during assembly. Though that design would be a REALLY interesting one to take full-scale--it provided a very smooth ride due to the sheer number of rollers. It combined an "internal" hub (like most custom wheels, say 357) with an "external" hub (like the AM wheels), and when assembled was strong enough for VEX-scale use (EDUBot at the time).

The machine type was definitely one that could be used for industrial production.

Re: Mecanum Wheels
 

I can see how they would've held up then.

The issue I have with telling people to 3D print mecanum wheels is that '3D Printing' is a very, very general term. Most people who have access to 3D printers can only print in ABS or similar materials (Think Dimension Machines, which seem to be the most common), which from what I've seen in my own testing is no where near strong enough to make a wheel from. (Partially due to ABS, partially due to how the machine lays out the layers)

If you're talking about a higher end machine like an FDM or something even better than that that can print in something like Polycarb, Polycarb ABS or Ultem (A plastic used in Fortus FDM machines) then I could see printing a Mecanum as a viable (but somewhat expensive) option. Considering that most teams don't have access to a $100,000 + 3D Printer, I still stand by my recommendation to NOT 3D print wheels, unless you've got the right machine and use the right material.

Re: Mecanum Wheels
 

In this case, I specified nylon, not ABS. Process was SLS (Selective Laser Sintering). Again, I think there was some form of special tweaking to the process for strength.

We did do a test in some form of resin (again, ask ChrisH what type); that broke after a while. Similar process, though. But the nylon wheels are still working, AFAIK.

The reason for saying that 3D printing was a good option--if you could get access to the equipment--is that to do a similar hub with a machining process would need either a 5-axis CNC or multiple steps on a 3-axis. Not including rollers or axles, of course. Rollers are an entirely different operation; axles could theoretically be done almost entirely by hand. (I should know--I made all 32+ axles for the set 330 used on a practice robot.)

Re: Mecanum Wheels
 

We used ruber stoppers to build our off season mecanum wheels. We used a paddle drill bit to drill the angles than drilled 1/4 inch hole through for axles. The HDPE we cut out with a band saw.

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I can't quite tell from the camera angle of that picture, but it looks like those mec wheels are not independently driven??

Re: Mecanum Wheels
 

Ether, they are independently driven.

There's a ball differential in the middle of the omni wheel sprockets in the middle of their drive, allowing the mecanums to drive independently. I'll see if I can dig up the original pictures and threads in CD-Media.

Edit: http://www.chiefdelphi.com/media/photos/31755
http://www.chiefdelphi.com/media/photos/34202
http://www.chiefdelphi.com/media/photos/34594

Those should get the technical specs going for you; they represent 3 iterations of the drive.

Re: Mecanum Wheels
 

i have seen this before, there is a differential in the middle omni, not axial inputs are the chains from each mechanum wheel, this allows for good push in one direction, and it eliminates a TON of slop in the system, i have never built one full scale, but i built a lego prototype, the omni wheel spins the average of the two mechanum wheels.

Re: Mecanum Wheels
 

The picture is the protype for the 2010 robot that we won design awards for. It had a suspension and six wheel drive that was very effective going over the bumps. The only trouble we had was the ball diferentials wore out and have to be tightened. A gear diferential would make it perfect. We were able to hold our own in pushing matches.

Re: Mecanum Wheels
 

I would argue that mecanum wheels are actualy slower and less agile than ,for instance, a 6wd. Mecanum wheels are usualy heavier than standard traction wheels making it slower to accerlate. And being that mecanum wheels have rollers and no real hard pushing point, the robots momentum would dicide how quick it can stop/change direction(basic agility).

When was the last time you saw a mecanum robot going 18ft/s(?), and then stop in less than 5ft?

Re: Mecanum Wheels
 

http://www.fingertechrobotics.com/pr...mecanum-set-hg

what if instead of using 4 6 inch mechs, you used 8 or even 12 of those. that would greatly decrease the amount of rotational inertia in the wheels, those little wheels are also higher quality.

Re: Mecanum Wheels
 

I suppose it would get pretty heavy and costly to have four shifting gearboxes, but it would be possible I guess.


Doing a little back-of-the envelope math...

assuming constant deceleration

a = (V*V)/(2*x) = (18*18)/(2*5) ~ 32 f/s/s ~ 1g

... so you'd need an effective CoF of 1.

To get that with mec, you'd need about 1.4 CoF between the tread material and the carpet. So you'd have to make your own custom rollers with high-traction tread.

Re: Mecanum Wheels
 

The software would get interesting, unless you plan to mount 3 coaxially at each of 4 locations and drive all three 3 wheels within each coaxial set at the same speed (which would be approximately correct but not quite, resulting in friction).

Re: Mecanum Wheels
 

i was actualy thinking of mounting all 3 right next to each other on a standard frame setup. i would probably use #25 chain directly driven from a cim ( no gearbox)

Re: Mecanum Wheels
 

By "right next to each other" I assume you mean "in the same plane" and not "coaxially".

Such being the case, a correct inverse kinematic translation would require that each of the 3 wheels in a set be driven at slightly different speeds.

If you just chain them together and drive all 3 at the same speed, you'll get some scrubbing of the contact patch which means additional loss of power and traction.

Re: Mecanum Wheels
 

i think that the same thing would happen if the were mounted co-axially. while it is true that while using all 3 degrees of freedom there would be some scrub, during normal translation there would be none, conceptually it is 3 entirely separate mechanum bases, driving in the same space, at the same speed.

Re: Mecanum Wheels
 

I believe that is correct.

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Looking through the links on this thread, I see that the number of rollers per wheel seems to vary based on who is designing them. Is there an optimum number?

Re: Mecanum Wheels
 

It depends on what you are trying to optimize.

Re: Mecanum Wheels
 

Ether is right. If you want to optimize a smooth ride, lots of rollers are better (less "flat" distance between crowns of rollers). If you want to optimize manufacturability, fewer rollers are better (to a point--4 rollers to a wheel will most likely really, really make life miserable for the drivers). And it will depend on your design.

The center-hub designs typically carry about 6-8 rollers per wheel from what I've seen; the AM outer-hub design carries something like 32. I've seen an outer-hub design carry 8, and a combination of outer and inner carry something like 20. It varies depending on design and wheel designer.

Re: Mecanum Wheels
 

Fewer rollers also means fatter/wider wheels, assuming diameter and angle stay constant. The rollers have to be longer to cover a sector of the wheel, and since they're at a 45 deg angle, that means they'll stick out farther to the sides as well.

Also, can everyone in the thread agree that the Mecanum vs. 6WD discussions were already done this season in numerous other threads? If someone thinks that horse isn't dead enough, I implore them to revive one of those threads or at least give us a break until next season.