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pic: Offseason 8W Tank Drive
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Seems like it would be tough to work with in the pit if it would need to be worked on. I'm basing this by the number of gears.
Can you post some specs? looks cool! |
Re: pic: Offseason 8W Tank Drive
Sure!
The drive channels are two pieces of 1"x3" C-channel, with the two sides connected by a single 0.090" bent sheet metal piece. You could also replace this bent sheet metal piece with the front/back rails of the 2014 VEXPro Drive in a Day or the 2015 kitbot end rails (at least as much as I can tell from the drawings). The gearbox is mostly just a repackaged Ball Shifter. The only trick is that two of the CIMs are on a separate first stage reduction and then connected to the rest of the gearbox with a 9mm HTD belt. These then drive 8 4" Colsons on dead axles. Here are the drive numbers: ![]() Weight seems to be about ~40 lbs, including motors and chains. |
Re: pic: Offseason 8W Tank Drive
What program are you using for those drive numbers? AFAIK, it's pretty much impossible to pull 90 amps per CIM on a 6CIM drive under any circumstance with the batteries we use in FRC.
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FWIW, we found that the 1.3 CoF was a little on the high side for normal traction wheels. |
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http://www.chiefdelphi.com/media/papers/2755 |
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I know the calculator on the WCP website does take battery voltage drop into account, which is nice, but their "max pushing force" calculation uses the static COF even for a traction-limited drive, which is not-so-nice. |
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I'd tend to think that the bent sheet metal would cause flexing within the frame, causing and uneven chassis. Then all the problems evolve. Could you use 3x1 box? it would have more strength |
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Some things to keep in mind for sheet metal: bends and gussets increase strength, if properly applied. This design appears to be proper application, and there's space in the outer rail for bumper supports/backing which will also increase strength. |
But bending two pieces exactly the same could be difficult if a team doesn't have access to the necessary resources to make an EXACT copy, that why I'd opt for box or channel all around
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Our primary choice would be to find a sheet metal sponsor to fabricate the piece for us. We do have access to a waterjet and a break, however, the cutting area on the waterjet is too small to manufacture the piece and the manual break is of rather low quality.
If we can't find a waterjet sponsor, we'll split the single sheet metal piece up into 4 (two end sheets and a two piece bellypan) so that they'll fit on the waterjet and bend the c-channels ourselves. If we're unsuccessful at bending the end sheets ourselves AND finding a waterjet sponsor AND still want to use this drive during next build season, we'll probably just purchase the end sheets from the 2015 kitbot. |
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http://www.chiefdelphi.com/media/papers/3038 |
Re: pic: Offseason 8W Tank Drive
A quick question, the sheet piece as shown is made out of 0.090" Al 5052-H32. I know other teams, such as 33 and 67, have had success with thinner sheets. Would trimming down to 0.063" be a wise decision?
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![]() Here's the current revision of the drive. The only main differences are the lightened bellypan and less aggressive pocketing on the front/back rail. Any last tips? I've debated elongating the top end rail flanges and possibly putting another flange down the back as 971 does. Another change (not shown) is changing the outer gearbox bearings from round to hex. This allows the intermediate shafts to be pulled out without having to disassemble the outer drive rail. |
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Hmm, 971's bellypan extends to the outer rails. They also have plates on the top of the rails for added strength. Have you considered doing this with your design, at least in the middle of the rails?
I'm no ME, but the proximity of the 4 rivets without any other support raises an eyebrow. The likelyhood of the sheet metal shearing or holes elongating (making the drive not square) seems pretty high. |
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![]() I've often done two rivets on a diagonal in a 1" square, but not 4. That I pretty much copied from the VexPro Drive in a Day, which I believe is bolted together. I'll definitely consider that. The dead axles and gearbox shafts should help transfer the load from the outside drive rail to the inside drive rail on a hard side impact. Also, the plan for superstructure mounting is a 2"x1" box crossbar, which should help as well. I'm thinking about ways to mount the bumper that would increase the strength of the frame, as 33 does. ![]() |
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Just one comment: Your "combined motor stall current" figure seems to be purely determined by motor selection, as it doesn't vary with changes to internal battery resistance or circuit resistance. The current displayed on the "stall conditions" graph, on the other hand, does seem to be calculated from the relevant values. Is there any reason for this? |
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http://www.fightingpi.org/Resources/...culations.xlsm |
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