Problem with Mecanum wheels sticking/seizing

The kids got some new mecanum wheels and, after mounting them on a chassis, they have been complaining about them periodically sticking or seizing up when they manually push the robot along the floor to test their axles for friction.

After examining the mecanum wheels, I notice that the green rubber rollers sometimes rub up against the grey hard plastic part to which the green rubber rollers are mounted, a situation that seems to depend on how the robot is pushed along the floor. In my opinion, perhaps a little silicone grease might solve the problem, but the kids tell me they don’t trust the wheels and they are ready to abandon mecanum wheels altogether, so I thought I’d ask if this sort of stiction might be common and if there’s a simple solution to it.

The wheels don’t look dirty or anything, and if I pull the green rubber rollers away from the grey parts, everything works smoothly… at least for a while. It seems to me that there ought to be a plastic washer or something between the hard grey plastic and the rubber roller to keep this from happening.

http://www.vexrobotics.com/media/catalog/product/cache/11/image/5e06319eda06f020e43594a9c230972d/m/e/mecanum-wheel-front.jpg

We don’t have any mecanums and haven’t used them, so I can’t comment on any specifics. However, I would avoid using any grease, as it may make a mess of the field. And the wheels would probably be considered to contact to playing surface.

Is it possible to insert some sort of washer in between the rubber and plastic? Like cut a slit in it and push it onto the metal shaft?

FullMetal,

I got out one of our Mec-wheels and I see what your saying. I’m not sure if that would work. If you would place a washer on each side, and it doesn’t look like there is enough room/tolerance. The green tread sticks out past the gray rim and is rubbing the each mount. I think a better solution would be to trim the green tread portion of the wheel back slightly beyond the gray rim. You would need to find a very uniform way to trim though to keep the tread same diameter. I’m thinking 0.015"-0.020"

Yes, I can see how that could cause a problem.

I wondered about that, too, maybe cut a delrin disk or something, but I’m afraid it could slip out and jam the entire wheel.

I’m also wondering if my kids aren’t over-reacting to something. There’s a certain whining that goes on sometimes about new things. Or maybe perhaps the wheels behave differently when they are pushed around vs. the motors actually driving them around. Maybe I’ll try some symbolic dabs of grease and then have them hook up the motors and see if they detect the sticking any more. I was just wondering if other people had encountered similar problems.

Is the the rollers on the wheels that are seizing up or the wheels stop rolling themselves? What type of motors are you using?

Occasionally, with motors that have IME’s, I’d get a motor that siezes up and stops. I’m not sure as to what the IME’s have to do with it, but some will cause motors to jam when not being powered. Change out an IME and it fixes it.

Don’t add any grease, try the wheels with the motors. I suspect everything will be fine. The rubber used for these wheels did change at some point, the last set I bought were different to those I bought a couple of years ago. Let me check mine tomorrow and see if they have the same behavior as yours.

The kids assembled their chassis with the mecanum wheels and then spun the wheels to check for friction. No problem. But then they set the chassis on the flat floor and started pushing it around and that’s when, once in a while, a wheel would seize up. I was called in to investigate and found that when their wheel seized up, one of the green rubber rollers on a wheel would shift too close to the hard grey plastic and the rubber suddenly would act like a brake. They have not yet put any motors on their system because they were freaking out about this stiction phenomenon. So the problem is not with the wheel’s axle but happens because of the roller rubbing on its own grey mount.

People seem to have so many issues with IMEs, I’ve told my kids not to use them. Recently jpearman informed us that the I2C protocol that the IMEs use wasn’t even meant for distances exceeding those seen on a typical printed circuit board (like, we’re talking about maximums of no more than a few feet or so from the Cortex.) Based on that information, I’d say the only place an IME might be good for is on the chassis motors, but then I’ve heard people complain that the IMEs are sensitive to electrostatic discharge, the very kind of thing you might see near the wheels during the winter. :frowning:

Okay. I’ll hold off the kids for a while. I’m hoping the problem disappears when they power them rather than push them around. Thanks for taking a look at this.

Thanks, everyone! :slight_smile:

The next time they seize up like that, mark which roller got stuck with a small dot with a marker. After a couple times, you might notice that it could just be one or two rollers, or it might be something happening randomly with all of them. I’d be interested to find out whether it might just be a bad roller.

We are not big fans of mecanum wheels or IMEs after last year. We had a mecanum base on our Toss Up robot for a long time and had a lot of problems with the base over heating, especially after strafing. We don’t know for sure if it was from the mecanum wheels or other problems, but we largely blamed the wheels. We don’t think they are as efficient as other wheels.
We also had a LOT of problems with IMEs and static last year. I think it has been said that the mecs cause a lot more static to build up than other wheels.

So this is what I see with my own wheels.

No binding on any plastic or rubber parts.

Here is a shot of one of my wheels showing maximum and minimum clearance (click on them for larger images).

Here is a shot with the rubber part removed.

Neither the rubber or plastic inner parts touch the support.

Do all of your wheels have the same problem?

Does the rubber outer part or inner plastic part touch the support?

Yes on our Mec-wheels the rubber sticks out maybe 0.010-0.015" past the gray wheel.:frowning:

I’m fairly sure not all of the wheels have this problem but I’m not sure if there are any more than just the one I identified as having a problem. In other words, I tested a few of the other rollers but not all of them to see if they had the problem. The kids told me that more than one wheel would sometimes seize, so I’m not sure. I’ll try to check them all tomorrow.

It seems to me that the green rubber is the part touching the hard grey plastic. It looks that way and there is the characteristic rubbery grippy feel when it catches, too, and not a plastic-on-plastic feel to it.

Do those green rubber rollers simply pop off and on for replacement? Perhaps I could just pop off the roller and Dremel it a little then press it back on???

They come off fairly easily, pull and twist, a little patience and they come off.