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Or you could just do GT2 belts, C-C +.003-.006 and not worry about it? :P

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The advantage of pressing both bearings into the same bore is that they're definitely lined up straight. You don't need them to be perfectly in line, but then you're going to wear out your bearings much more quickly, you're going to sap efficiency, and if its bad enough, the whole thing will seize up. You'll be able to see and feel the improvement of having the bearings in the same bore, but if its just impossible for you to do it that way, it can still work, just not was well

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Just to note, 696 has always done two separate bearing blocks machined from 1/4" x 2" flat bar held inline by nothing more than the axle through bearings and 4 bolts and we've never had a problem. Much less machine time and material cost. Although, we may go for the fitted-tubes style next time, just to try it out.

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Alternatively, you could make your bearing block out of a single piece of 1.25" wide by 2" high at 1/8" thickness tube, available at onlinemetals:

http://www.onlinemetals.com/merchant...269&top_cat=60

I know it's 6063 aluminum; our drivetrain was made with the stuff last year, and it was just fine to machine.

Bore your bearing hole and a couple of screw holes, and then lop off the top of your tube to make a C-channel like piece. Slide it onto the robot, tighten down your screws, and you're good. Our team used the system on previous robots before we moved away from the cantilevered drivetrain design; while I cannot personally attest to its effectiveness, as it was before my time, our mentors recall the solution as doing a fantastic job. Since we intend to return to cantilevered drive this year, we have of course revamped the design; a screenshot is here:

https://plus.google.com/u/0/photos/1...06823682841202

All the advantages of imbedding your bearings in a single piece, without having to make three different bearing block pieces. I thought it was genius when I first saw it, and the material is readily available.

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Seems like a pretty fantastic idea altogether. :D Have you considered using rod ends instead eye bolts for tensioning? I might be misunderstanding how your system works, but the eye bolts in your CAD seem excessively large compared to the bolts being pulled.

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This is genius :D

One question, so how do you keep overzealous tightening of the two black bolts from crushing your bearing block? (or am I just missing something :o )

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How about the idea of no bearing blocks at all?

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This is what we did with our off season robot. We used the correct Center to Center for 25 chain, the did loosen a bit but over 3 off season competitions but we never lost a chain.

We are looking into going to belts however for our new iteration of the drive.

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I, for one, think this is really the best solution, but it takes too much guts for me to try out. We didn't prototype a exact c-c drive in the offseason, and there's too much potential for failure for my comfort level. I would need to know that we had this design in the bag (like you guys do) to use it during the season.

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Jose,

I clearly haven't searched McMaster closely enough. The rod ends seem like a great idea; I used the eyebolt mostly because it was the first thing that came to mind, and will work just fine, as I only need to pull on the bearing block to achieve correct chain tension. If the rod ends become necessary, I will be indebted to you for having brought them to my attention :)

Garrett,

Again, I was not a member of Team 846 when we first used this design, so I do not know what ferocious punishment was threatened upon those who tightened the bearing block too tight ;) I imagine that if you put some sort of spacer inside (or stack a bunch of those cool little VEXPro washers inside), just a touch smaller than the width of the tube you are clamping with the bearing block, you would prevent the bearing block from ever being tightened tight enough to cause failure.

All you fixed C-C proponents:

Yes, I wish we could go fixed C-C . . . with belts! That would be amazing. I do not think we would ever go fixed C-C with chain, we've just had too many issues with sprockets wearing down and consequently changing the tension on the chain . . . never mind just the stretching with time. As we get a little more experience with using 7075 for our sprockets or even implementing our off-season belt drive design, we may reconsider that stance. If any of you guys try it out (or in 973's case, have), we'd love to hear how that went.

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I'd be scared with #25 chain and/or indirect drive.

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All I am going to say we did it: http://www.chiefdelphi.com/media/photos/38147
Ran 3 off seasons, not a single chain lost.

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If you really need to that would probably be fine(but only if you're direct driving your centers). I just see so limited downside in going without bearing blocks. If you have the resources to spend on completely custom gearboxes then it's not going to take very long to make 8-12 973/1323 bearing blocks. Especially now with the new WCP gearboxes you can shift the resources used to make the gearboxes over to making bearing blocks that improve your consistency and efficiency.

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But the point is that if you simply don't need them, don't bother with the weight, complexity, cost, or manufacturing time, and the resources you would have shifted from gearboxes to bearing blocks can now be focused on something more relevant to the challenge. The question is whether you need them, and there is some solid evidence coming forward that you don't (with the usual caveat: if you do it right).

And I'm not sure what, if anything, direct driving the center wheel would have to do with an exact c-c?

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He is concerned about losing mobility (chain popping off) when using #25 chain without bearing blocks.