pic: Offseason 8W Tank Drive

[Knufire]

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[75vs1885]

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!

[Knufire]

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.

[Oblarg]

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.

[BBray_T1296]

Originally Posted by Oblarg

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.

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

[Knufire]

Originally Posted by BBray_T1296

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

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

[BBray_T1296]

Originally Posted by Knufire

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

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

[Jared]

Originally Posted by Oblarg

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.

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.

[Oblarg]

Originally Posted by Jared

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.

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.

[Michael Hill]

Originally Posted by Oblarg

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.

Try my calculator out.

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

[Knufire]

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?

[Michael Hill]

Originally Posted by Knufire

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?

You can go even thinner. We're experimenting with 0.050" 2024-T3 this offseason, and it should actually be stronger than .090" 5052-H32. We get this added strength by not drilling lightening holes. Your robot is really only as strong as the smallest cross-sectional area (in tension and compression). So by eliminating lightening holes, the smallest cross-sectional area ends up where the axle holes are. It ends up almost the same weight, but almost twice as strong (ballparking, of course).

[asid61]

Originally Posted by Michael Hill

You can go even thinner. We're experimenting with 0.050" 2024-T3 this offseason, and it should actually be stronger than .090" 5052-H32. We get this added strength by not drilling lightening holes. Your robot is really only as strong as the smallest cross-sectional area (in tension and compression). So by eliminating lightening holes, the smallest cross-sectional area ends up where the axle holes are. It ends up almost the same weight, but almost twice as strong (ballparking, of course).

Is 2024 easy to bend?

[Michael Hill]

Originally Posted by asid61

Is 2024 easy to bend?

We use a 0.125" bend radius. It's a bit tighter than aviation requires, but it's good enough for FRC, especially if you bend against the grain. We've reached the limit of our brake, but fortunately it's big enough for a robot.

[Oblarg]

Originally Posted by Michael Hill

Try my calculator out.

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

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?