Octocanum 101
 

We are attempting octocanum this year. We have working field centric & shifting control.

For more details about our design , check out this page.
http://2014.discobots.org/node/94

As of late, with our practice robot,
We have broken three VEXpro mecanum 4" rollers and
here are a couple questions we are facing at the moment.

How to reduce stress on the mecanum rollers ?
a. proper amount of frame flexiblity ?
b. slop in the axles (using churro vs VEXpro black hex)
c. limiting power of wheels during strafing to avoid stress on rollers

Re: Octocanum 101
 

Hey Andrew,

How long had you driven the robot? We have found reduced life but failure typically occurred after ~4hrs of heavy driving (far more than a regional).
You can keep an eye on the rollers by checking how easy it is to flex them to the max extents.

We are currently working on a fix but plastic part revisions have a significant lead time, due to the shortened life span we have highly discounted the replacement roller kits to keep teams rolling.

The VEXpro Hex stock is 7075 Aluminum vs the "Churro" which is 6063, very different materials for different jobs.

-Aren

Re: Octocanum 101
 

Did you take into account that an octocanum will essentially drop the robot on the mecanum wheels when you switch, I think that would shorten the life span a little?

Also, this is another case where a 8 motor (4 cim, 4 minicim) drive mess up the vex mecanum wheels. So far 8 motor drives have been the one thing that is the same with all these cases.

Re: Octocanum 101
 

I'm pretty sure those are just the public ones that are breaking, I know of at least one more team that has broken them and I assume VEX knows of others or they wouldn't be completely remaking the rollers.

Re: Octocanum 101
 

One thing that could be happening is that when you are shifting, the mecanum wheel isn't landing level. If I recall correctly, when the mecanum wheels had the wrong size roller, it would break when all the load was on the end of the cantilevered roller. Could it be that when you shift you sometime land on the end of one of those cantilevered rollers?

Re: Octocanum 101
 

All,

Just to stop the speculating on this one, we know exactly what is happening because of all the great feedback we have received from our customers via our support e-mail channel.

This failure mode has almost nothing to do with the transmitted torque to the wheel. The way these wheels are designed to eliminate the "mecanum bounce" flexes each individual roller from tip to opposite tip. This deflection causes cyclical stress across the roller hub. We have a stress concentration at the base of the roller boss (where the shaft bushing is located) that sees a fatigue stress at around 4 hours of driving time for the 4" mecanum.

This failure is caused by two things:

1. Our resin manufacturer did not use the specified brand of resin for the injection molding pellets. They used the correct material, but a brand that had a lower fatigue strength.

2. Our hub design had a stress concentration at the exact wrong location on the rib. This stress concentration was due to too small of a fillet on the roller boss.


We have redesigned the roller hub and put in process checks in place to guarantee the correct material in production. We are currently modifying the injection mold for the roller hubs (the 6", 8" and 4" all use the same hubs, just different rollers).

With all of that said, this problem should only show itself on your extended hour practice robots. Competition robots will need to participate in approximately 96 matches (including practice matches) to equal 4 hours of run time. We have reduced the price of the replacement roller kits to below our cost in order to temporarily assist teams while we await the improved roller assemblies.

My plan is to send the new roller kits to all customers of the 4" mecanum wheels free of charge once they are in stock.

I hope this was helpful in understanding the problem.

Paul

Re: Octocanum 101
 

Although my team is not affected by this, i truly appreciate when a company is honest with its customers.

Thanks
Paul

Re: Octocanum 101
 

Thanks, that explains a lot.

Re: Octocanum 101
 

That makes sense and matches from what we have seen in testing.

Thanks Paul !

Re: Octocanum 101
 

That is a nice octomecanum drivetrain! It is quite flush to the ground and I think you guys will do well with this, this year. Are those parts waterjetted? The manufacturing is your parts is quite seamless. How fast is it's reaction time? Is it powered by pneumatics or motors? Also, what size are those mecanum wheels?

Re: Octocanum 101
 

Thanks!! The drivetrain is the VEXPro 2014 Drive-In-A-Day Chassis. I want to say about 80% of our parts are from the VEXPro VersaFrame. The shifting is powered by pneumatics and those are 4 inch mecanum wheels.

This has been a fun drivetrain to work on and test this year. It was a huge learning process for us because we haven't done anything like this before. We're still learning and we hope to be very competitive this year!!

Re: Octocanum 101
 

Paul,

I can't agree that it takes 4 hours to break the wheels. We had no practice on the robot before it was bagged, so the wheels were fresh when we got to the regional. We ran 4 practice matches on Thursday, then in our fifth match on Friday one of the roller stands broke off. We did not have enough time to change it before the next match, so we ran the match with one roller missing on the wheel. During that match we broke two more roller stands off the same wheel. It was the last match of the day, so we had time to change all of the rollers on the wheel. By my calculations, we did not have half an hour on the wheel. Unfortunately, we were at our only regional on the first weekend, so we learned about the breaking wheel before this was posted.

Jack

Re: Octocanum 101
 

I feel your pain Jack !

We still like the idea of octocanum drive, and might go back to testing in the summer.
However, We switched to butterfly (omni-wheels instead of mecanum)

It took quite a bit of time to change at the event on Thursday, no regrets, butterfly drive is quite nice.