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Unread 07-11-2014, 10:19
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Re: pic: Offseason 8W Tank Drive

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Originally Posted by Oblarg View Post
This seems to be reasonably close to the experimental data I have, and all the behavior seems correct. Thanks for sharing, I'll be sure to keep this around.

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?
That stall current is the "spec" current at 12V. This is just like the motor performance data we are given, but instead for a single motor, it is an equivalent for multiple motors. It is based on Paul Copioli's calculation in his "Useful Calculations" spreadsheet.

http://www.fightingpi.org/Resources/...culations.xlsm
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Unread 26-10-2014, 21:24
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Re: pic: Offseason 8W Tank Drive

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Originally Posted by Knufire View Post
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.
Thanks!
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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Unread 26-10-2014, 21:35
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Re: pic: Offseason 8W Tank Drive

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Originally Posted by 75vs1885 View Post
Thanks!
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
That would depend on the bends of the sheet metal. There's a reason a lot of teams like to use sheet metal; if it's handled right, it can be as strong as channel and box. And, it's a lot lighter than box extrusion.

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.
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Unread 26-10-2014, 23:55
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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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Unread 27-10-2014, 00:12
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Re: pic: Offseason 8W Tank Drive

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Originally Posted by 75vs1885 View Post
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
And those resources are a break, a shear, and a good set of measuring instruments, particularly when you're just bending yourself a C-channel. If a team is designing in sheet at all, for a component as major as the drivetrain, and doesn't have those resources at their disposal, they really need to be rethinking their design. But if they DO have those, which they probably do, then they could bend two, three, four, as many pieces as they needed to in order to build their robot and its twin if they were building one of those.
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Unread 27-10-2014, 01:14
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Re: pic: Offseason 8W Tank Drive

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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Unread 30-10-2014, 10:44
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Re: pic: Offseason 8W Tank Drive

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Originally Posted by Knufire View Post
Did you look at 2337's DT? They split a piece of 2x4 in half and use that for the outer and inner rails for one side. It would be a good reference.
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