907 4 inch IFI Sheet Metal Drivetrain Feedback

[Matthew.Mc]

Hey CD,

So recently our team has been delegating with potential sheet metal fabrication sponsors. We have come to an agreement that our team would like to pursue a sheet metal construction for next year. (Well at least Drivetrain)

I posted on CD about a month and a half ago on my first sheet metal drivetrain that I made in Pro Engineer. Since then, I have taken the information that some members of the site gave me, and created roughly 6 drivetrains. All better each time. I haven’t posted one since the first, so I believe that this is another that can be criticized. I believe that I have much room for improvement, and hopefully you guys can help me address the issues I have.

The Drivetrain will consist of the following:

  -6 Wheel Design (Dropped Centre (0.125)
  -4 inch IFI Wheels
  -Super Shifter's (OR Custom Shifters)
  -Lightweight Design
  -Rigid design
  -Belly Pan With Electronics
  -0.090" 5052 H32 Aluminium  

Please don’t mind the massive amount of rivets. They are simply to demonstrate where all the “possible” holes are to rivet. Not going to rivet everywhere their are rivets in the assembly.

Things that we would like answered even after hours of searching for answers are:

-What is the best method of securing the two outside plates?
-Direct Drive vs No direct Drive. One of our alumni said that a few years ago one of our pinion gears on the cim motor had shattered because of the sheer amount of force that was struck upon the gear when we would stop the drivetrain or hit something hard. Basically he was saying that when you have chain driving from a sprocket off the output shaft of the gearbox, that takes the load, and not the cim gear.
-Belly pan thickness (0.125? 0.090?)

-“Reinforcements” on the axle points. I obtained the idea from 1114’s robots from the past couple years (thanks guys). Im sure that other team’s use the same idea but more or less it provides that extra strength to dissipate the load more equally when you get hit. I will post a link to their website to the picture with the idea.

-I want to make circular lightening holes because they are easier to make meaning less time. But my team-mates, want to use the “118 iso-grid”, because they say it obviously better but in reality it takes longer to CAD and I’m not sure that its a whole lot better in terms of a drivetrain. Maybe though?

“118 iso-grid”](http://www.chiefdelphi.com/media/photos/38095)

http://www.simbotics.org/files/imagecache/node-gallery-display/038.jpg

Included is the render done on CAD. This is about the fifth revision.

Thanks, CD. I look forward to the feedback as I’m looking for lots!

[BJC]

Hey there, nice drivetrain. I’ll put my comments in red.

Hey CD,

So recently our team has been delegating with potential sheet metal fabrication sponsors. We have come to an agreement that our team would like to pursue a sheet metal construction for next year. (Well at least Drivetrain)
This is a really good step, sheet metal is a very powerful construction method in FRC. Congrats to your team on finding a sponsor.

I posted on CD about a month and a half ago on my first sheet metal drivetrain that I made in Pro Engineer. Since then, I have taken the information that some members of the site gave me, and created roughly 6 drivetrains. All better each time. I haven’t posted one since the first, so I believe that this is another that can be criticized. I believe that I have much room for improvement, and hopefully you guys can help me address the issues I have.

The Drivetrain will consist of the following:

  -6 Wheel Design (Dropped Centre (0.125)
  -4 inch IFI Wheels
  -Super Shifter's (OR Custom Shifters)
  -Lightweight Design

Do you know the aproximate frame weight?
-Rigid design
-Belly Pan With Electronics
-0.090" 5052 H32 Aluminium
.09 is a pretty good sheet thickness for 5052. My team used .06 last year but also used either 6061 or 7075 which is a bit stronger (can’t recall off the top of my head.)

Please don’t mind the massive amount of rivets. They are simply to demonstrate where all the “possible” holes are to rivet. Not going to rivet everywhere their are rivets in the assembly.

Things that we would like answered even after hours of searching for answers are:

-What is the best method of securing the two outside plates?
There are a lot of good ways to do this. It seems you’ve taken some inspiration from 1114. They make stellar drivetrains. Your connection methods seem solid, I would suggest placing some “Simbot” box plates inbetween your driveline plates. That would stiffen them up considerably.
-Direct Drive vs No direct Drive. One of our alumni said that a few years ago one of our pinion gears on the cim motor had shattered because of the sheer amount of force that was struck upon the gear when we would stop the drivetrain or hit something hard. Basically he was saying that when you have chain driving from a sprocket off the output shaft of the gearbox, that takes the load, and not the cim gear.
Both ways have been proven very effective. I personally would go with an indirect drive for this up-comming year while your still just getting into sheet metal.
-Belly pan thickness (0.125? 0.090?)
.09 will be plenty strong provided that you support everything correctly.
-“Reinforcements” on the axle points. I obtained the idea from 1114’s robots from the past couple years (thanks guys). Im sure that other team’s use the same idea but more or less it provides that extra strength to dissipate the load more equally when you get hit. I will post a link to their website to the picture with the idea.
If I’m thinking what your thinking then that is actually a method of holding a dead axle and providing structural support. Our team does this to great effect as well.

-I want to make circular lightening holes because they are easier to make meaning less time. But my team-mates, want to use the “118 iso-grid”, because they say it obviously better but in reality it takes longer to CAD and I’m not sure that its a whole lot better in terms of a drivetrain. Maybe though?
Circles are theoretically the best because they don’t have corners providing stress points that you get with with triangles. That said, either can and have been used to great effect. I would suggest, however, that you try to make your holes 3" or bigger so they can be used as access holes when trying to reach inside the robot. You’ll thank yourself later

“118 iso-grid”](http://www.chiefdelphi.com/media/photos/38095)

http://www.simbotics.org/files/imagecache/node-gallery-display/038.jpg

Included is the render done on CAD. This is about the fifth revision.

Thanks, CD. I look forward to the feedback as I’m looking for lots!

Also, something that really stuck out with me was the cutting away the top flange to make room for the gearbox. In my experience you don’t want to do that. That flange is a key structural piece and by comprimizing that one spot you basically comprimize the tortional stiffness of the piece. The second thing I noticed was that I saw nothing in the way of chain tensioning. Lastly, .125" drop on the middle wheel gives you .25" of rock in the chassis. If you want .125" of rock lift an outer wheel .125" or the middle .063".

Good Luck!
Bryan

[Gregor]

Also, something that really stuck out with me was the cutting away the top flange to make room for the gearbox. In my experience you don’t want to do that. That flange is a key structural piece and by comprimizing that one spot you basically comprimize the tortional stiffness of the piece.

What if the inner flange faced outward, and the gearbox was mounted to the flat face?

The second thing I noticed was that I saw nothing in the way of chain tensioning. Lastly, .125" drop on the middle wheel gives you .25" of rock in the chassis. If you want .125" of rock lift an outer wheel .125" or the middle .063".

What do you recommend to be the optimum rock?

[Dr_Theta]

Looks pretty solid. I don’t know how much I can add to what has been stated above. I do have a general question though about drivetrains designed in this manner that I’ve been hoping to get answered. How do you guys attach your bumpers to the sides of the robot? We have only ever used bolts and T nuts set into the wood, and I have been hoping we can take our drivetrains more in the direction of one like this.

[system]

Needs more Colson…(Talk to Gregor about that. He’ll tell you what I told him)

[ttldomination]

I’ve often wondered this, but what’s the benefit of having the outer rail face flange outwards?

From my understanding, it would only complicate bumper mounting and other minor things, but the benefit is lost to me.

• Sunny G.

[Matthew.Mc]

I’ve often wondered this, but what’s the benefit of having the outer rail face flange outwards?

From my understanding, it would only complicate bumper mounting and other minor things, but the benefit is lost to me.

• Sunny G.

I believe that having this flange face outwards, makes it easier to access the wheel assembly. If anything were to go wrong out on the field, it would be faster to fix. From what I have researched it may also have a benefit strength wise compared to bending the flange inwards. But don’t quote me on that. Perhaps someone could shed some light on this?

[BJC]

What if the inner flange faced outward, and the gearbox was mounted to the flat face?
That would work perfectly well. You could also make your chassis 5” tall to accommodate the gearbox between the upper and lower flanges. I’ve also designed a couple of drivetrains with the gearbox actually inside the wheel well and only the motors sticking out into the machine. There are lots of ways to mount a gearbox without cutting through any flanges.

What do you recommend to be the optimum rock?

This directly relates to how torsionally stiff your chassis is. The older Andymark kitbots were very flexible which in turn meant that more rock was needed to turn (so opposite outside wheels couldn’t touch the ground.) In general, the more tortionally stiff you can make your chassis the less drop you can safely have. 1/8” is a general rule of thumb although if you’re using wide wheels 3/16th may be a better choice. There are also ways to design in methods to change your drop so you can best adjust it to what you need in season. If you’re interested in something like this let me know and I’ll try to explain what we do more in depth.

Looks pretty solid. I don’t know how much I can add to what has been stated above. I do have a general question though about drivetrains designed in this manner that I’ve been hoping to get answered. How do you guys attach your bumpers to the sides of the robot? We have only ever used bolts and T nuts set into the wood, and I have been hoping we can take our drivetrains more in the direction of one like this.

The fun and cool thing about bumpers is that they are free weight! If you do your bumper mounts right you can significantly strengthen your chassis for essentially no weight. A really strong way to do this is to mount two 1/8” think 1” L angle along the length of your bumpers. Then you can pick up that angle along the top and side of your chassis. To really strengthen your chassis though, it’s best to make one or two really solid bumper pieces that just fit around your robot. Making the corners of the bumpers strong is the key here. Again, there are many ways to do this, it’s more a team preference thing.

I’ve often wondered this, but what’s the benefit of having the outer rail face flange outwards?
From my understanding, it would only complicate bumper mounting and other minor things, but the benefit is lost to me.

• Sunny G.

I guess another question you could ask is what is the benefit of having it face inwards? Having the rail face outwards allows for us to put stuff including dead axle mounts, decorative body panels, and our lower bumper mounts within that outer 1” cavity without worrying about being outside the bumper zone. Additionally you decrease the length of your axles by one inch reducing the chance of it being bent by a hard impact. All in all though, it comes down to preference and the way your team like to do things.

Regards, Bryan

[roystur44]

Looks good.

Keep in mind you need to get your chain breaker in there if you go the chain route.

Needs a battery holder which can be made integrated to the rear panel. Keep in mind you want a short cable run to the PDB

Needs mounting points for your bumpers. We have had success with using locking compression latches.

You could make the chassis thinner to save weight.

Looking at the drive train from the top you could improve access to the front and back wheels by removing some material in the corners.

Think of a cool way to mount your encoders to the drive train away from the motors.

Check with your sheet metal sponsor and ask about using csk head rivets on the bottom to make your bottom smooth. They may have a tool that can csk the holes.

Keep in mind with your squared profile on the front and back of the chassis if there is a ramp the chassis gets lifted and the wheels could come off the ground and free spin.

Lastly access to electronics, cables and pneumatics can come via the bottom of the robot via a removable panel or access cut outs.

[Gary_Dillard]

I want to make circular lightening holes because they are easier to make meaning less time. But my team-mates, want to use the “118 iso-grid”, because they say it obviously better but in reality it takes longer to CAD and I’m not sure that its a whole lot better in terms of a drivetrain. Maybe though?

Nice design Matthew. “Obviously better”? I guess my answer is that it could be but I wouldn’t say it’s obvious. I’ve done a little research in isogrid myself, but the details and the heavy math on how and why it works are found in the NASA Isogrid Design Handbook. Chapter 4.8 discusses the open shear webs that you have drawn. A few things to point out without getting too technical:

1. The belly pan isn’t truly isogrid - it’s skewed plate versus equilateral triangle so it transforms the loads and stresses differently. Regardless, it’s function is to transfer loads normal to the plate and not in-plane shear so the advantages of the isogrid don’t really help you that much. I would consider perforated plate stock or a simple pattern of smaller holes to give you more flexibility in mounting components to the pan.
2. The advantages to isogrid come from tailored thicknesses relative to in-plane compressive stresses and buckling limits. Consider it this way: a .063 thick solid plate wall has pretty much the same theoretical stiffness in plane and the same weight as a .125 thick 50% open area plate because it has the same average shear flow top to bottom. However, the thinner plate has 1/2 the buckling capability (1/2 the radius of gyration) so it deflects more out of plane (even without buckling) and is effectively much less stiff.
3. Weight advantages of isogrid are usually quoted relative to the equivalent thickness solid plate, with both optimized for buckling. In your case, you are pretty much selecting a common thickness and then taking additional material out of it either by an isogrid pattern or a hole pattern, so you are reducing weight at some rate and stiffness at some other (hopefully lower) rate and the question is where is it optimum and is it anywhere near buckling limited? With so many load variables and unknowns that you have to estimate, my gut would say leave in more material and make the simpler cutout.
4. You gotta admit, isogrid looks more “aerospacy”; that must count for something.