pic: Offseason 8W Tank Drive
 

Re: pic: Offseason 8W Tank Drive
 

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.

Re: pic: Offseason 8W Tank Drive
 

The visual style looks different, but it is just like JVN's Calculator

Re: pic: Offseason 8W Tank Drive
 

He's using JVN's design calculator. It just calculates out how much torque you'd need to slip the wheels with the gear reduction, then, from the motor curve, it figures out the current draw per motor. For high gear, voltage drop will play a huge role, and I don't think he'd be able to get the wheels to slip at all. The current/motor number is more important in low gear.

FWIW, we found that the 1.3 CoF was a little on the high side for normal traction wheels.

Re: pic: Offseason 8W Tank Drive
 

It's the updated version of JVN's calculator:
http://www.chiefdelphi.com/media/papers/2755

Re: pic: Offseason 8W Tank Drive
 

Ah, ok. I guess he decided to scrap the florescent colors for something more soothing :P

Re: pic: Offseason 8W Tank Drive
 

Thanks. Not taking battery voltage drop into account would explain that pretty well.

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.

Re: pic: Offseason 8W Tank Drive
 

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

Re: pic: Offseason 8W Tank Drive
 

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.

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

Re: pic: Offseason 8W Tank Drive
 

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.

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.

Re: pic: Offseason 8W Tank Drive
 

Try my calculator out.

http://www.chiefdelphi.com/media/papers/3038