SPI Wheel coupling cracking like crazy!

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Posted by Eric Reed at 2/15/2001 7:36 PM EST


Coach on team #481, NASA Ames / De Anza High School, from De Anza High School and It could be you!.



We are using the wheel couplings from Small Parts. We have been following our robot around (No, it is not done, thanks for asking) for about two weeks, picking up nuts from the coupling as they fall off. Yesterday we put lock-tite on them, tightened them hand-tight with a cheap little screwdriver. Today the couplings are completely falling apart. The plastic is cracking and pieces are falling off. Anybody else had this experience? Is there some kind of reaction with the lock-tite, or are the couplings just really cheap?

Very Frustrated,

Eric
Team 481

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Posted by Matt Leese at 2/15/2001 7:59 PM EST


Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Fiber Technologies & RIT.


In Reply to: SPI Wheel coupling cracking like crazy!
Posted by Eric Reed on 2/15/2001 7:36 PM EST:



Plastics and locktight don't tend to mix well. I think someone already mentioned this problem. I would definately recommend using your own hardware for the wheel hubs. I also recommend making your own spacer (polycarbonate works well for that). Otherwise, just be careful with them.

Matt

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Posted by Kevin Sevcik at 2/16/2001 9:34 AM EST


Other on team #57, Leopards, from BT Washington and the High School for Engineering Professions and Exxon, Kellog Brown & Root, Powell Electrical.


In Reply to: SPI Wheel coupling cracking like crazy!
Posted by Eric Reed on 2/15/2001 7:36 PM EST:



Seeing as how there next to no time left for making new hubs and spacers, and assuming you've still got some of those SPI hubs left.... Why don't you just use nylon lock nuts? they rarely come loose in my experience..

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Posted by Eric Reed at 2/16/2001 10:40 AM EST


Coach on team #481, NASA Ames / De Anza High School, from De Anza High School and It could be you!.


In Reply to: Re: SPI Wheel coupling cracking like crazy!
Posted by Kevin Sevcik on 2/16/2001 9:34 AM EST:



: Seeing as how there next to no time left for making new hubs and spacers, and assuming you've still got some of those SPI hubs left.... Why don't you just use nylon lock nuts? they rarely come loose in my experience..


Thanks! I'm going to order more hubs...we only lost two. Hopefully SPI has some more.

Eric
Team 481

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Posted by Erik Justen at 2/19/2001 2:12 PM EST


Engineer on team none from none sponsored by The Ohio State University/looking for one.


In Reply to: SPI Wheel coupling cracking like crazy!
Posted by Eric Reed on 2/15/2001 7:36 PM EST:



We too had some problems with these hubs cracking. (for reference, on 8" wheels, with locktite) The conclusion we came to was that there is quite a large gap between the two pieces right were you bolt them together. The cracks we started to see were stress factures, perpendicular to the screws, near the middle of the piece, and always where the screws are. To me this seems to shy away from any degrading issues with the locktite, and more towards stress, as they were away from the surface the locktite was applied to. (but we did have one failure that I would chalk up to the "locktite theory". So... we put some polycarb. spacers in between the two pieces. They weren't hard to make, just time consuming (we used several thin pieces - not ideal, but the sheet was free...). I think this does two things: 1) helps relief stress by filling in that gap, and 2) prevents someone from over-torquing the screws, and pulling the two halves too close together. For the second reason along I think they were worth the effort.

Just our thoughts
Erik
(disclaimer - I am not, nor do I pretend to be, and ME or materials science person - I'm just some lowly EE college student....)