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-   -   pic: Offseason 8W Tank Drive (http://www.chiefdelphi.com/forums/showthread.php?t=130914)

Knufire 30-10-2014 10:38

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?

RonnieS 30-10-2014 10:44

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by Knufire (Post 1405751)

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.

Michael Hill 30-10-2014 17:21

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by Knufire (Post 1406451)
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 30-10-2014 20:46

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by Michael Hill (Post 1406507)
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 30-10-2014 21:18

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by asid61 (Post 1406526)
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.

asid61 30-10-2014 23:44

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by Michael Hill (Post 1406530)
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.

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.

Knufire 03-11-2014 02:18

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.

JesseK 03-11-2014 09:08

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.

Knufire 03-11-2014 09:39

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by JesseK (Post 1407011)
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.

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.


Oblarg 06-11-2014 21:15

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by Michael Hill (Post 1405975)

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?

Michael Hill 07-11-2014 10:19

Re: pic: Offseason 8W Tank Drive
 
Quote:

Originally Posted by Oblarg (Post 1407714)
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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