Mecanum Wheels

[Jeff 801]

Originally Posted by Ether

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

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.

[Ether]

Originally Posted by Jeff 801

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.

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

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.

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?

[Jeff 801]

Originally Posted by Ether

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?

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.

[Ether]

Originally Posted by Jeff 801

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 sweep having reference points every 1/8 inch. I have attached 2 pictures showing the sketch in CAD and the finished assembly.

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?

[EricH]

Originally Posted by thefro526

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.

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.

[thefro526]

Originally Posted by EricH

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.

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)

[EricH]

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.

[thefro526]

Originally Posted by EricH

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.

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.

[EricH]

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.)

[joeweber]

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.

[Ether]

Originally Posted by joeweber

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.

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

[EricH]

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.

[Hawiian Cadder]

Originally Posted by Ether

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

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.

[joeweber]

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.

[Marc S.]

Originally Posted by dudefise

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.

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?