25-10-2014 21:04

75vs1885

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!

25-10-2014 21:43

Knufire

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.

25-10-2014 21:46

Oblarg

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.

25-10-2014 21:53

BBray_T1296

Re: pic: Offseason 8W Tank Drive

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

25-10-2014 21:57

Jared

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.

25-10-2014 22:00

Knufire

Re: pic: Offseason 8W Tank Drive

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

25-10-2014 22:02

BBray_T1296

Re: pic: Offseason 8W Tank Drive

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

25-10-2014 22:05

Oblarg

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.

26-10-2014 21:24

75vs1885

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

26-10-2014 21:35

EricH

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.

26-10-2014 23:55

75vs1885

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

27-10-2014 00:12

EricH

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.

27-10-2014 01:14

Knufire

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.

27-10-2014 16:05

Michael Hill

Re: pic: Offseason 8W Tank Drive

Try my calculator out.

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

30-10-2014 10:38

Knufire

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?

30-10-2014 10:44

RonnieS

Re: pic: Offseason 8W Tank Drive

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.

30-10-2014 17:21

Michael Hill

Re: pic: Offseason 8W Tank Drive

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).

30-10-2014 20:46

asid61

Re: pic: Offseason 8W Tank Drive

Is 2024 easy to bend?

30-10-2014 21:18

Michael Hill

Re: pic: Offseason 8W Tank Drive

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.

30-10-2014 23:44

asid61

Re: pic: Offseason 8W Tank Drive

Oh, very good then. I was worried the stiffness would make the bend radius unusable. I'll have to ask our sheet metal guy if he can do 2024.

03-11-2014 02:18

Knufire

Re: pic: Offseason 8W Tank Drive

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.

03-11-2014 09:08

JesseK

Re: pic: Offseason 8W Tank Drive

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.

03-11-2014 09:39

Knufire

Re: pic: Offseason 8W Tank Drive

I had a version where the bellypan extended to the frame perimeter. Since the wheels pairs are so close together and the chain hangs below the button of the frame, by the time you have wheel and chain clearance cutouts, you only have the middle section underneath the gearbox left, which makes it significantly harder to access the gearbox.



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.

06-11-2014 21:15

Oblarg

Re: pic: Offseason 8W Tank Drive

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?

07-11-2014 10:19

Michael Hill

Re: pic: Offseason 8W Tank Drive

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