pic: 6wd Chassis Design

[Alex.q]

loading photo ...

Nice CAD! Just curious, why did you put the chain sprockets for the wheels on the outside? Wouldn't they be safer and more effective on the inside?

[thefro526]

Your drive is going to be way too fast ( > 16.5 fps) with a cimple box direct driving a 6" wheel. You'll probably have issues with current draw as well due to under-gearing.

[Akash Rastogi]

Aside from what Dustin said, I can't quite tell, but how are your drive axles supported?

[Aren Siekmeier]

You will want to take another look at your gearing.

Also, going along with Akash's remark, with the C-channel it looks like you are only supporting your wheel axles at one point. You will want at least two. For this and other structural reasons I would suggest box tube in place of the c-channel. It comes in various sizes (1"x2", 1"x1", etc.) and wall thicknesses (no less than 1/8" for a drive base usually) and can be found at (perhaps donated by) your local metal shop. Quite a few teams (including my own) use it structurally, and you can either weld or rivet (with gusset plates) the joints.

[Chris is me]

Originally Posted by thefro526

Your drive is going to be way too fast ( > 16.5 fps) with a cimple box direct driving a 6" wheel. You'll probably have issues with current draw as well due to under-gearing.

It looks like a Toughbox Mini.

[thefro526]

Originally Posted by thefro526

Your drive is going to be way too fast ( > 16.5 fps) with a cimple box direct driving a 6" wheel. You'll probably have issues with current draw as well due to under-gearing.

(Ignore this if you are using a Toughbox Mini as Chris said in post #6.)

[Alex.q]

It's a toughbox mini, the max speed is just under 9 fps I think.

Hopefully these two pictures shows how the axles are supported a little better. There is a piece of c-channel nested in the main one and slotted on the top and bottom so we can slide it to move the chain. The axle is supported in two places.





We used c-channel instead of 2x1 box because it seemed to us easier to bolt things on to c-channel than box. With a box frame, you wouldn't be able to get a nut on a bolt. Do other teams usually use rivets to avoid this problem?

I put chains on the outside on the middle to the back wheels, and the chains on the inside on the middle and front wheels. Maybe I'm wrong, but at the time I did it, it seemed like that would take less space and the wheel wouldn't have to be cantilevered as far. In any case, I believe the bumpers should adequately protect the chain, won't they?

*also, sorry for huge pictures, I uploaded them to Tinypic and didn't know how to make them smaller here.

[EricH]

Originally Posted by Alex.q

We used c-channel instead of 2x1 box because it seemed to us easier to bolt things on to c-channel than box. With a box frame, you wouldn't be able to get a nut on a bolt. Do other teams usually use rivets to avoid this problem?

Most teams that use box tubing just go all the way through both sides of the tubing and put a nut on the bottom.

I put chains on the outside on the middle to the back wheels, and the chains on the inside on the middle and front wheels. Maybe I'm wrong, but at the time I did it, it seemed like that would take less space and the wheel wouldn't have to be cantilevered as far. In any case, I believe the bumpers should adequately protect the chain, won't they?

I would want the chain inside of the wheels. Less cantilever to the sprocket means less chance for the chain to jump the sprocket.

I wouldn't worry about the bumpers protecting the chain so much as the chain rubbing the bumpers...

You could also try a trick that a number of teams using WCD pull--they run their chains inside the frame outline.

[Alex.q]

Originally Posted by EricH

Most teams that use box tubing just go all the way through both sides of the tubing and put a nut on the bottom.

That would take pretty long bolts though, so I don't know if it is the best option.

I would want the chain inside of the wheels. Less cantilever to the sprocket means less chance for the chain to jump the sprocket.

I wouldn't worry about the bumpers protecting the chain so much as the chain rubbing the bumpers...

You could also try a trick that a number of teams using WCD pull--they run their chains inside the frame outline.

I didn't think of some of those issues with the chain. I might move that chain to the other side of the wheel if we have room.

I'm not sure what you meant by unning the chains inside the frame outline. If you meant using live axles and having the sprockets on the inside, we are using dead axes so that wouldn't work for us.

[EricH]

Originally Posted by Alex.q

That would take pretty long bolts though, so I don't know if it is the best option.

Other teams use welding to secure their frame.

[Aren Siekmeier]

Originally Posted by Alex.q

That would take pretty long bolts though, so I don't know if it is the best option.

Why are 2" or 3" bolts hard to come by? And with box tube, you would no longer need the extra piece of c-channel to support the axle in a second spot, so you don't need those particular bolts any more anyway, AND you've gotten rid of 30 components (1 bracket, 2 bolts, 2 nuts per wheel) in the entire drive base, making the whole thing much simpler and less prone to failure.

Also, I would probably only position nuts/bolt heads in tight spaces like that as a last resort. That's harder to get at than you might think, and you want that sort of thing to go fast.

Other than that, CAD looks good, and with Toughbox Nanos you should be fine (the orthogonal render made them look more like CIMple boxes...). I might prefer a bit faster, but 9fps is safe, and you'll have little trouble pushing people around. You might want to see if you can get it to the traction limit.

[theprgramerdude]

Originally Posted by thefro526

Your drive is going to be way too fast ( > 16.5 fps) with a cimple box direct driving a 6" wheel. You'll probably have issues with current draw as well due to under-gearing.

If they are frugal with weight and keep it at about 100 pounds w/ battery and bumpers like we did in '11, the Cimple boxes aren't a bad choice. The reduced weight makes it much easier to accelerate, and thus easier on the motors. The speed can be harder to control in tight spaces (probably not the best choice for most of LogoMotion), but it really helps when cruising down the field. Cross-field runs occur in about 4-5 seconds with it from stop, which can really save time

[Madison]

Do you have a capable engineering mentor on your team or in your area that can sit with you and review your work? You'll get a lot more from that than you will from this forum, unfortunately.

There are a lot of problems here -- and that's okay. We all learn by trying new things and making mistakes. In drive design, the devil is in the details a lot of the time and your modeling isn't very detailed, so forgive me if I've made incorrect assumptions in the advice I offer.

1 -- It looks like you're using Pro/E. You have my sympathy.

2 -- Do not use C-channel to build your frame or otherwise expect to add a VERY rigid (i.e. thick) piece of wood or aluminum plate to the inside to add rigidity. You'll otherwise have a lot of flex in the frame -- I know this from experience. The problems you're anticipating with using rectangular or square tubing aren't real.

3 -- The Toughbox Mini with long output shaft driving the center wheel is supported in two places in the Toughbox housing. That puts the load on that shaft at 1" (channel leg height) + spacer + sprocket + 1/2 wheel width away from the bearing. That's probably 2.5" -- too far. There's all sorts of detail missing with respect to how you're going to position the wheel on the shaft -- is it retaining clips? spacers? Retaining clips weaken the shaft. Spacers will exert an axial load from the wheel onto whatever surface (presumably the channel) the spacer works off of. How will you handle that thrust loading?

4 -- You've said the C-channel is 2x1 -- presumably by 1/8" thick. That means the blue parts you show as your dead-axle support are 2.25 x 1.125 x .125". That size isn't commercially available to my knowledge. How will you manufacture those parts?

5 -- It doesn't matter because they won't work. You've got a hole in blue channel and a slot in the frame channel along side slots for mounting the blue channel. I presume those mounting slots are for chain tensioning. Think about what happens when you slide the blue channel away from the center when the chain is under tension. Your axles will not remain parallel to the center axle/toughbox output and you will throw chain and add a lot of drag to the drive.

6 -- Is each Toughbox attached with only two screws? That's going to exacerbate the flex in your frame. Follow the loading -- weight of the robot passes through center wheels, bending toughbox output upward. Flex in output shaft tries to rotate the entire gearbox housing about its mounting point. This is hard to describe, so you have my apologies.

7 -- The location of the sprockets doesn't matter. All of the loading is passing through bearings / hubs in your wheels. The sprockets are floating around the shafts and your biggest concern isn't torque applied by the chain bending your axles in a horizontal plane, but torque applied by the weight of the robot bending them in a vertical plane.

None of us get things right the first time -- or the second, third, or fourth. After looking at a design for a long time, it's hard to be objective about its strengths and weaknesses, so asking for advice from new sets of eyes is a good thing. If some of the things I've written don't make sense, I apologize, but I'm in a bit of a rush this morning -- I've got presentations to give this morning on drive design

[Ether]

Originally Posted by theprgramerdude

If they are frugal with weight and keep it at about 100 pounds w/ battery and bumpers like we did in '11, the Cimple boxes aren't a bad choice. The reduced weight makes it much easier to accelerate, and thus easier on the motors. The speed can be harder to control in tight spaces (probably not the best choice for most of LogoMotion), but it really helps when cruising down the field. Cross-field runs occur in about 4-5 seconds with it from stop, which can really save time

Assuming you used 6" wheels (which is what the OP has), did you do an analysis to see how much faster cross-field runs from a dead stop would be* if you had more than just the CIMple box's 4.67:1 reduction?


* and less current draw, and quicker acceleration and easier control in tight spaces