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Unread 31-10-2011, 16:05
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FRC #1718 (The Fighting Pi)
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Re: High Tensile Sprocket Bolts

Quote:
Originally Posted by Brandon Holley View Post
Tom-

Can you just clarify what setup you had on your plactions. Was it a full combination of AM plactions, AM (plastic) spacers, and AM sprockets?

Was there generally a period of time before you would shear the bolts? For example, would you replace the bolts, get ~20 matches out of them, and then end up having to replace those bolts again?


I'm just hypothesizing about the nature of plastic parts in these situations. I'm wondering if the plastic parts creep slightly and therefore gradually lose the pre-load of the bolt. The parts may still appear tightly attached together, but losing some of that pre-load would then amplify any shear stress on the bolt and eventually cause a failure.

Just a thought I had.

-Brando
We started seeing bolts shear about midway through our second district. We played through the semis of the first district, so that was 20-24 matches, + about half of the next competition - 10 or so. So call it 32 matches.

What we routinely see is that the locknut shears off, then the bolt head works its way out until it starts hitting the frame. It's always the nut side of the bolt that shears (probably due to the smaller minor diameter where the bolt is threaded).

The components are mostly Andy Mark. The plaction and sprocket is AM, the spacer is a copy of their design made out of ABS, so it's solid rather than having the pockets theirs has.

After the first failures, we put new hardware in and over-tightened them - the chain actually started skipping teeth because we bent the sprockets they were so tight. After that we taught the kids to tighten them but stop if they saw any deflection - essentially as 'tight' as they could be without causing damage.

We ran the same gear ratio in '11 but with no spacers and never had to replace one bit of the hardware. Perhaps the plastic to plastic interface is too slippery and allows relative motion.