Problems with Vex Pro mecanum wheels

[omsahmad]

Originally Posted by Ether

Glue the rollers to their axles and forget about strafing.

Or this? http://www.chiefdelphi.com/media/photos/36998

LOL... Maybe we will try switching the wheels on Wednesday back to the Andymark one? We have to take the transmissions off to swap gears anyways.

Originally Posted by omsahmad

LOL... Maybe we will try switching the wheels on Wednesday back to the Andymark one? We have to take the transmissions off to swap gears anyways.

I don't think he's joking around. I see no reason why a standard traction tank drive wouldn't be more advantageous for you to use instead of mecanums.

[Sam_Mills]

Originally Posted by Andrew Lawrence

I don't think he's joking around. I see no reason why a standard traction tank drive wouldn't be more advantageous for you to use instead of mecanums.

Let the battle begin!

[Mark Sheridan]

Make sure to check every roller, last year we had a strafing issue and it was because one roller was stuck. Our issue looked more like an intermittent speed issue is strafing.

I would also check if each wheel can rotate forward and backwards at full speed with the robot off the ground. Hopefully you have the encoders hooked up to each gear box to see the counts per second. At least this can sort out programing versus mechanical issues.

Just to be clear how loose our rollers are on our 2012 robot, the rollers will spin a couple time after flicking them.

[omsahmad]

Originally Posted by Andrew Lawrence

I don't think he's joking around. I see no reason why a standard traction tank drive wouldn't be more advantageous for you to use instead of mecanums.

That's fine, but we could just put real traction wheels on if we are taking the transmissions of anyways.

[omsahmad]

Originally Posted by Mark Sheridan

Make sure to check every roller, last year we had a strafing issue and it was because one roller was stuck. Our issue looked more like an intermittent speed issue is strafing.

I would also check if each wheel can rotate forward and backwards at full speed with the robot off the ground. Hopefully you have the encoders hooked up to each gear box to see the counts per second. At least this can sort out programing versus mechanical issues.

Just to be clear how loose our rollers are on our 2012 robot, the rollers will spin a couple time after flicking them.

Ok thanks, I'll test all of this out Wednesday. I would prefer to keep mecanum for this year.

[Ether]

Originally Posted by Mark Sheridan

Just to be clear how loose our rollers are on our 2012 robot, the rollers will spin a couple time after flicking them.

Problem is, the rollers can appear to be free-spinning when not under load, and then bind up when they are loaded.

There is a lot of axial force on the roller under real-world operating conditions. It is prohibitively expensive to put high-quality thrust ball bearings on FRC mecanum wheel rollers. I've never seen the VEX mecanum wheels. What provision is in the design to react the high axial forces without creating excessive friction? IOW, what two materials are scrubbing against each other when the roller is axially loaded?

[Tom Ore]

Another potential problem with staffing is if your frame isn't square - ie, loads on the wheels have too much difference. We had that problem this year - we shimmed one gearbox and the staffing was fine after that.

[Mark Sheridan]

Originally Posted by Ether

Problem is, the rollers can appear to be free-spinning when not under load, and then bind up when they are loaded.

There is a lot of axial force on the roller under real-world operating conditions. It is prohibitively expensive to put high-quality thrust ball bearings on FRC mecanum wheel rollers. I've never seen the VEX mecanum wheels. What provision is in the design to react the high axial forces without creating excessive friction? IOW, what two materials are scrubbing against each other when the roller is axially loaded?

Yeah I was going to caution that a wheel unloaded will be different from a loaded wheel. My team never came up with a good test method for this.

I am sure you know this Ether, but so others can follow, I will be a bit more detailed. On the andy mark wheels, there is a bushing in the wheel. This rotates on a bolt. Between the wheel and hub plates are washers. If one is too zealous, one can tighten the bolts too much compressing the washers and wheels until the wont move. We had to play with this a bit because once the wheels are on the ground, the deflection of parts can create binding too. I think if its too loose, the hub plates could touch the roller. I forget how much it took to cause the failure mode, but gut tells me that situation required the bolts of be barely tighten.

I am guessing the Vex wheels are similar with a shoulder bolt or screw that supports the rollers. Thus i may be possible to over tighten the screw until the rollers bind. The roller would be compressed by the head of the screw and the hub ( plus a few washers in there).

[pfreivald]

We switched to VEX mecanum wheels for our octocanum drive this year, and had no issues whatsoever with their performance. (And the added maneuverability was very useful in lining up to the feeder slot!)

[omsahmad]

Originally Posted by Tom Ore

Another potential problem with staffing is if your frame isn't square - ie, loads on the wheels have too much difference. We had that problem this year - we shimmed one gearbox and the staffing was fine after that.

So I was thinking along the same lines as Ether as well,

Problem is, the rollers can appear to be free-spinning when not under load, and then bind up when they are loaded.

There is a lot of axial force on the roller under real-world operating conditions. It is prohibitively expensive to put high-quality thrust ball bearings on FRC mecanum wheel rollers. I've never seen the VEX mecanum wheels. What provision is in the design to react the high axial forces without creating excessive friction? IOW, what two materials are scrubbing against each other when the roller is axially loaded?

But we have a 28"x28" robot with a center of mass right in the middle... So my thought was maybe the wheels just aren't as smooth, but apparently other are having good luck with them.

[Tom Ore]

Originally Posted by omsahmad

But we have a 28"x28" robot with a center of mass right in the middle...

For a quick check, pick up on each corner of the robot and see if you can detect a difference. If the frame isn't square you should be able to tell.

[omsahmad]

Originally Posted by Tom Ore

For a quick check, pick up on each corner of the robot and see if you can detect a difference. If the frame isn't square you should be able to tell.

Oh sweet! I'll give that shot too. Thanks

[BigJ]

Also, your robot being 28"x28" doesn't necessarily mean your wheelbase is a square (but it may very well be or "close enough").

[faust1706]

We are also using these wheels. We used them to make us always be pointed at the target when we have vision solutions if we choose to be. I have noticed that strafeing is rather slow too...