Re: Chain driven vs Direct drive mechanum
 

Seriously?

Re: Chain driven vs Direct drive mechanum
 

the fact that they were grade 8 was probably the problem, hard bolts don't bend, they snap, and aren't very forgiving of shock forces, grade 6 is probably a better option and 8 more holes would weaken the sprocket pretty severely.

i've never heard of a team who have had issues with bolts snapping before, we never have that problem, but then again, we've never used traction drive, only mechanum and a couple tank drives.

Re: Chain driven vs Direct drive mechanum
 

Grade 6? As far as I know there are grades 2, 5, and 8. Grade 8 bolts are marked with 6 lines though, it's all very logical :rolleyes:

Fasteners do very poorly when they're loose, regardless of grade. I would recommend using ny-lock nuts at the very least, and maybe add a split or star lock-washer. It might be an appropriate place to use a smaller fastener with a higher torque so that the fastener does not lose clamping force during operation.

Re: Chain driven vs Direct drive mechanum
 

We started with grade 2's. The only type of nut we use is nyloc. Those worked for 1 competition, and started breaking and falling onto the field.

We went to grade 8 with nylocs. Those worked for 1 district, State Championships, then started snapping on the Newton field during the finals match.

We've replaced the grade 8's that had broken at Championships and are running that way. I don't expect a problem: we'll check it after the Tardec competition this Monday, after Marc later this month, and then after IRI next month.

However, I think that this may be a larger problem for high-torque drives that are used a lot. The side force from the fasteners simply isn't enough to keep the sprockets from working, and that eventially snaps bolts off or causes them to loosen (nyloc or not).

That's why I'm suggesting some sort of mating system between the sprocket and the hub of the plaction wheel.

Admittedly, our robot got a lot of use this year at 65+ matches plus practice field time. In addition, we had a high amount of skid-steer drag during turning. We went with the longest wheel base that the motors could turn so that it would be difficult for other robots to spin us. It was ALWAYS the bolts in the double-sprocketed wheel that failed first (the rear wheel was chain driven on the inside and had an outside sprocket that drove the next wheel forward). We had one sprocket on the inside and one on the outside and were using laser-cut spacers that were essentially copies of the Andy-Mark ones.

Re: Chain driven vs Direct drive mechanum
 

It sounds as if the bolts are directly transmitting drive-line loads. To the best of my knowledge drive-line fasteners (or any fastener for that matter) does its job best when the clamping force it provides transmits loads through the friction produced from said clamping force. Are the bolts being re-tightened after every match? Are you using a torque wrench to ensure a minimum clamping force or just "TFT" (till it feels tight)?

An FSAE car I worked on used a single lug nut in the middle of the wheel and 6 aluminum studs on the wheel hub. There was no way the aluminum studs could handle braking or engine forces on their own. We did the math and determined that our center-lock lug nuts would need to be torqued to around 110ft-lbs to give enough clamping force to transmit engine and braking torque without relying on the studs, which were there mainly for aligning the wheel. Every time before the car went out, the lugs were all re-torqued, and we never had a problem.

Re: Chain driven vs Direct drive mechanum
 

We're definitely not using a torque wrench, but we're also making those bolts pretty tight. (I generally have a good feeling for pretty tight - I work in an engine plant and deal with a lot of bolt torque testing). I'll check on the torque next we tighten them and compare it with the recommended torque for a grade 8 fastener of that size.

The more I think about it the more I think you're right about the fasteners taking all the shear rather than just acting in tensile. Thanks for the suggestion.

Re: Chain driven vs Direct drive mechanum
 

Do you have room to increase the diameter of the standoffs? Stiffer standoff with a bigger footprint would likely reduce the fatigue failure of the bolts.

Re: Chain driven vs Direct drive mechanum
 

Let us know how it turns out!

A second thought occurs: is there any abnormal wear around the bolt head or nut clamping area? (I think this is called "fretting") it could be that the bolts are losing tension from part wear.