Which mecanum wheels to buy?

[cad321]

I'm looking at getting some mecanum wheels to play around with in the off season but want to ensure I'm buying a quality product. I was just about ready to commit to some of the 6in vex pro ones but after hearing about the issues with the 4in wheels, I am now wary. What have you had best experience with/ recommend? The reason I was going to go for the vex ones over andymarks was due to both the weight, price and I also like the versa keys. However if the rollers/wheels are going to break or wear out quickly, id rather invest a little extra more and get the andymarks. Thoughts, Concerns? Other recommendations?

[Ether]

Originally Posted by cad321

I am now weary.

Wary ?

http://www.chiefdelphi.com/forums/sh...19&postcount=6

[1629GaCo]

This was 1629's first year using the Mecanum wheels. We used the 6" from AndyMark, and they have been phenomenal. We have only used one set all year, during build and during competitions, and have seen no noticeable flaws with them. Yes, they are expensive, but IMHO they are worth it.

[jvriezen]

We've used 6" AM Mecs on three bots, 2011, 2012 and 2014. 2012 went through two regionals -- into elims in both (finals in one), two off season 1 day events (elims in both) and the MN state tournament (1 day, also to elims) and haven't had any problems. Two of those events we went to finals. 2012 bot also was in lots of demos. 2011 wheels were re-used for 2014 -- also no problems. I saw several lost rollers on Vex wheels in the bot barf box at 10K Lakes this year but other than that have no first hand experience with them.

[Max Boord]

1523 used andymark mecanum wheels for the past 4 years and discovered a few differences between them:

6 inch wheels:

• smooth
• break rollers frequently
• easy to replace rollers
• more rollers

8 inch wheels

• less grip
• VERY loud
• easey to replace rollers
• rollers hardly ever break
• vibrate the whole robot

both feature unbreakable hubs
We have experimented with gear ratios, motor choice, and code as well.
2011: CIM drive 12.75:1 gear ratio on 8 inch wheels PID controlled
2012: CIM drive 8.45:1 gear ratio on 8 inch wheels PID controlled
2013: Mini-CIMs 8.45:1 gear ratio on 6 inch wheels PID at first event no PID at second- fifth
2014: Mini-CIMs 5.95:1 gear ratio on 6 inch wheels no PID loops

We are happy with how our 2014 drive is performing although it was probably not a good choice for this years game. All 4 drivetrains have never suffered an on field mechanical failure beyond carpet/ field tape getting caught in the wheels and the occasional roller damage.

[jbsmithtx]

Our team has used both sets. In 2013 we used the 6in Andymark ones. This year, we moved to the 4in VEXpro ones.

Frankly, the 4in VEXpro set was rushed a little bit, and they had several problems. But I have discovered all the causes of these various problems and IMHO, despite all the problems we had with VEX, I think I prefer them. They have several problems which are being resolved, but these little things are nice and are very strong. VEX personnel were much easier to work with too. They sold us replacement roller at an extremely low cost for us. THEY WANT THESE THINGS TO WORK. The andymark rollers collected carpet really bad and had a lot of sticky rollers. The VEXpro ones handled these speed much better.

But also, I haven't heard of a single team having problems with the 6in variation from VEXpro. I personally prefer the VEXpro variation, because the andymark wheels are just bulky and harder to work with IMO. Feel free to PM me any questions

[MrRiedemanJACC]

Originally Posted by jbsmithtx

Our team has used both sets. In 2013 we used the 6in Andymark ones. This year, we moved to the 4in VEXpro ones.

Frankly, the 4in VEXpro set was rushed a little bit, and they had several problems. But I have discovered all the causes of these various problems and IMHO, despite all the problems we had with VEX, I think I prefer them. They have several problems which are being resolved, but these little things are nice and are very strong. VEX personnel were much easier to work with too. They sold us replacement roller at an extremely low cost for us. THEY WANT THESE THINGS TO WORK. The andymark rollers collected carpet really bad and had a lot of sticky rollers. The VEXpro ones handled these speed much better.

But also, I haven't heard of a single team having problems with the 6in variation from VEXpro. I personally prefer the VEXpro variation, because the andymark wheels are just bulky and harder to work with IMO. Feel free to PM me any questions

There were many teams with problems with the 6" Mechanum...

https://twitter.com/Team2611/status/...096000/photo/1

Everyone who had them at the tournaments we went to had a problem, especially their 2nd and 3rd events. But VEXpro was great with their support, just be ready to change them.

[Ben Wolsieffer]

On our 2010, 2011 and 2012 robots we used the original 8in AndyMark mecanums (w/o the side outer rollers). These worked well, but after a couple of seasons we had lots of bent axles (which can be easily replaced) and the rollers were pretty worn.

This year we decided to upgrade to the new 8in AndyMark wheels which are a lot beefier than our old ones. They can support up to 500 lbs per wheel, compared to 180 with the old ones. This helps prevent axle bending when if the robot ever slams into the ground (we accidentally drove our robot off the cart once and the wheels were fine). The main functional difference between the original AndyMark wheels and the new ones is the addition of a set of roller outside of the wheel plates. These greatly reduce the vibration of the robot as it rolls from roller to roller.

My main complaint with the AndyMark wheels is that they are difficult to assemble. The roller bolts that were provided were just barely long enough to reach the nylon part of the nut on the other side and we had to get them just right to prevent the nuts from falling off or the rollers from binding. We had problems with rollers binding when the wheels were brand new, but after a lot of practice and a couple competitions they now roll perfectly and we haven't had any problems with them at all.

Lastly, I would not recommend wheels that use plastic to attach the rollers simply because I have seen snapped off rollers too many times to feel comfortable with them.

[Ether]

Originally Posted by Max Boord

8 inch wheels
vibrate the whole robot

Why do you suppose this is ?

[Max Boord]

Originally Posted by Ether

Why do you suppose this is ?

This is due to the 8 inch wheels having less (12 vs 16) and larger rollers (~ 2x the size) therefore causing the wheel to act in a slightly less round way.

Electronica1, the wheel upgrades where not out in 2012 which was the last time we used them. I would think the problem would still exist although slightly reduced with the newer wheels.

[Ether]

Originally Posted by Max Boord

This is due to the 8 inch wheels having less (12 vs 16) and larger rollers (~ 2x the size) therefore causing the wheel to act in a slightly less round way.

How does fewer rollers cause the wheel to act in a slightly less round way?

[buchanan]

Originally Posted by Ether

How does fewer rollers cause the wheel to act in a slightly less round way?

There isn't any reason they must necessarily, but it does seem in practice that the 8" AMs do act that way. Some possibilities:

• The curvature of the rollers may simply be incorrect for the wheel geometry. Has anyone confirmed that it's exact?
• There may be significant bending deflection of the rollers across their width, changing the effective wheel radius depending on where on the rollers the load is.
• Since the radius, and hence the thickness of the rubber, is different in the middle vs the end of the rollers, the compression of the rubber itself under the same load will be different at different points. This can be compensated for, but only at one load value, right?

I have not investigated any of them to see which is/are of practical importance, but I can confirm the observed phenomenon, and it surely does have an explanation somewhere in theory.

[Ether]

Originally Posted by buchanan

There isn't any reason they must necessarily,

I concur.

but it does seem in practice that the 8" AMs do act that way.

That's been my observation

Some possibilities:
The curvature of the rollers may simply be incorrect for the wheel geometry.

That is probably a contributing factor.

But my guess would be that the main factor is one not on your list: the transition from one roller to the next is not smooth. It's a not clean handoff. You can clearly see this on a shadowgraph. You can do this experiment at home:

In a dark room (a basement is ideal) place a sheet of paper on the floor.

Put a small coffee or tea cup in the middle of the paper.

Balance the mec wheel on the cup so the plane of the wheel is parallel to the floor, and less than 4" from the floor.

Mount a spotlight on the ceiling facing down directly above the wheel, at least 8 feet above the wheel.

Darken the room and turn the spotlight on.

Observe the circumference of the shadow of the wheel on the paper.

You can clearly see the "identations" where one roller ends and the next begins.

Here's a mec wheel roller design calculator if anyone's interested:

http://www.chiefdelphi.com/media/papers/2390

... scroll down the list of attachments and download the one named
"2011-09-30a mecanum roller design calculator"

[Ben Wolsieffer]

Originally Posted by Ether

But my guess would be that the main factor is one not on your list: the transition from one roller to the next is not smooth.

I have noticed the same thing. The outer rollers on the new AndyMark wheels touch the ground during the transition from one roller to the next. When we compared our old wheels to the new ones we could feel the big difference that the extra rollers had simply by rolling them across a table.

[adciv]

Were those these 8" wheels or a different set?

We used the 6" VexPro ones this year and our bot was shaking quite a bit.