pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

[Mr. Lim]

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Great! What are the drive's advantages, in your opinion?

[Chris is me]

I do have to ask: Wouldn't an 1/8 wall square extrusion weigh just as much as a quarter inch L, with a much stronger profile?

[Madison]

Is the idea here that the bolts are threaded into the frame member or are they through-bolted with a nut on each side?

My gut reaction is one of concern that the bolts will unscrew themselves, but maybe that's unwarranted.

Edit: Nevermind; missed that this is a shoulder bolt.

[craigboez]

This is very nice and simple but I'd be worried about the forces that cantilevered load is putting on a very small amount of aluminum. Id guess that over time either the hole in the aluminum would oval out or the angle itself would want to bend below 90 degrees. You may want to run some CAD analysis, but with me this doesn't pass the sniff test.

[Aren Siekmeier]

The shoulder joint will definitely hold, but it has to be supported by the frame. With a single support point in just 1/4" of aluminum, it will likely just bend the angle aluminum and not stay horizontal. A box extrusion will solve this problem by providing two support points for the bolt and not deforming under the loads we're talking about.

[Jonathan Norris]

I'm pretty sure this would hold, over time you would probably see the slots warp a bit after repeated use. My main concern would be the flex in the main angle bar. The middle wheel would likely flex a decent amount when it had weight on it, if there wasn't any extra cross support in the middle of that system. I could see it flex enough in this configuration that it could almost negate a 1/8" center drop. But that flexibility could be interesting for a mecanum drive...

[MrForbes]

Interesting design, it might work for some games, but not for others.

We built a cantilevered dead axle drivetrain on our 2008 robot, using 1/2" threaded rod for the axles, and 1/8" thick 1x4" pultruded fiberglass for the frame members. We had an adjustable chain tensioner instead of slotted holes. It worked fine for the Overdrive game, since the playing field was flat and most of the action was driving laps.

http://www.chiefdelphi.com/media/photos/29697

[Mr. Lim]

What are the drive's advantages, in your opinion?

• Simple base components (80/20, L-angle, shoulder bolts)
• Reduced machining operations (no custom shafts, non-CNC milling)
• Simple chain tensioning method
• Quick assembly time
• Easy servicing (one bolt wheel removal)
• Lightweight
• Large baseboard area for electronics
• Less gear reduction required for 4" wheels (more efficient)
  
• I could see teams with some basic machining capabilities knock two of these out within the first few days a build season, then move on to bigger and better things

Wouldn't an 1/8 wall square extrusion weigh just as much as a quarter inch L, with a much stronger profile?

Yes, but the shoulder bolt shaft needs to be bolted to a plate with sufficient thickness, or else the plate will bend under load. Anecdotally, 1/8" wall seems too thin and will flex. Also, the L-angle leaves more space for the electronics board, and provides nice mounting flange. The L-angle looks to be plenty strong(?), and the bottom flange could be milled down to 1" or less to save weight - but that's another machining operation.

Is the idea here that the bolts are threaded into the frame member or are they through-bolted with a nut on each side?

It is a shoulder bolt (through-bolted) with a single nylock nut, and two washers sandwiching the plate. The above image best shows the setup. When the shoulder bolt and nut are tightened, it causes the washers to squeeze the plate tightly. The idea is that this is enough squeeze force to a) keep the cantilevered shaft perpendicular, and b) keep the shaft from sliding in the slot (the plate is actually SLOTTED for the front and back wheels to allow you to slide the wheels back and forth to tension the chain)

My gut reaction is one of concern that the bolts will unscrew themselves, but maybe that's unwarranted.

This is where my lack of mechanical experience shows through. I am concerned about this too, but I don't know the limitations of Nylock nuts, and threadlocker.

This is very nice and simple but I'd be worried about the forces that cantilevered load is putting on a very small amount of aluminum. Id guess that over time either the hole in the aluminum would oval out or the angle itself would want to bend below 90 degrees. You may want to run some CAD analysis, but with me this doesn't pass the sniff test.

The concept is that the washers that sandwich the plate distribute the load over a larger area of the plate. If this happens, there should be very little load inside the slot itself. CAD analysis is a good suggestion to see whether this works.

[Tristan Lall]

You might want to try heavy-duty all-metal locknuts (the ones with the elastically-deformable bore), or high-strength steel nuts and threadlocker. Torque them to the limit specified by the manufacturer. Given the shoulder bolts, you'll probably need an unusually-strong Allen wrench to make this happen. That's the only way I could see this holding tightly enough in a slotted hole.

Consider http://www.mcmaster.com/#92501A030 or http://www.mcmaster.com/#93591A200, for example.

[MrForbes]

Stover lock nuts, you mean?

I suggest you might want to find or figure out how to make a more substantial washer. Our design used large hex flanged nuts. You might be able to do this too, and use 1/2" fine thread threaded rod for the axle as we did. We used two jam nuts to retain the wheels, but this required that whoever was working on the robot needed to know not to tighten the nut too much!

[Mr. Lim]

Originally Posted by squirrel

Stover lock nuts, you mean?

I suggest you might want to find or figure out how to make a more substantial washer. Our design used large hex flanged nuts. You might be able to do this too, and use 1/2" fine thread threaded rod for the axle as we did. We used two jam nuts to retain the wheels, but this required that whoever was working on the robot needed to know not to tighten the nut too much!

I saw the CAD of 1726's 2008 robot way back in the day, but back then I didn't realize it was 1/8" C-channel for the side rails. I thought it was box, and the cantilevered shafts were supported at two points. Missed that one big time.

And here I thought this was an original idea .

The flanged Stover lock nuts look pretty nice, and I think they would go a long way to improving the design. Placing one on either side of the plate instead of the washers, upsizing to a 1/2" shaft of fine threaded rod and two jam nuts to keep the wheels from falling off seems to be the direction I'd like to go in... and then an FEA.

Questions:
Did your wheel bearings rest on the threads of your threaded rod? If so, any concerns with this? Excessive play? Still worth the trade-off IMO, but curious nonetheless.

Did you have frame flex issues at competition weight? Did you have to add more support to your C-channel side rails to prevent them from twisting, resulting in negative camber?

Was that a typo, or was your fibreglass C-channel really only 1/8" thick?

Thanks all who posted in this thread who shared their comments and concerns (yes, that includes you Dr. Joe!).

Really, as a programming and electronics guy, if I can't get the mechanical guys to cringe at least once or twice a day, I'm not doing my job right .

I believe that we humans utilize our emotional brains to sort through complex space of possible solutions. Our emotional brains are just really good at searching through complex system interactions.

P.S. If you believe in the quote above as much as I do, you need to read Malcolm Gladwell's book "Blink." Not just a blatant plug for a fellow Canadian, but his work really expounds on why some people are so good at what they do. There is so much more behind the "emotion" and "gut feelings" we get when working in our fields of expertise. The vast amount of experience and expertise we subconsciously put into honing those "feelings" allows us to correctly decipher really complex problems in the blink of en eye. Often without even realizing the vast amount of information we actually considered in the process.

[MrForbes]

Originally Posted by Mr. Lim

Did your wheel bearings rest on the threads of your threaded rod? If so, any concerns with this? Excessive play? Still worth the trade-off IMO, but curious nonetheless.

It's not a tight fit, there's a bit of slop, but it's only a few thousandths of an inch. And the reduced load bearing area of the threaded rod vs a solid shoulder bolt means that after decades of use, it might wear the threads enough that you'd want to think about replacing the axles. It's really not an issue at all. I've worked on enough old cars to have a pretty good feel for how long something like this will last.

Did you have frame flex issues at competition weight? Did you have to add more support to your C-channel side rails to prevent them from twisting, resulting in negative camber?

The frame is surprisingly stout for how little there is to it. The 1/16" aluminum bellypan and top corner gussets really make a strong structure when combined with the channel. Did I mention it's light? Notice that there really is no material where it doesn't need to be.

Was that a typo, or was your fibreglass C-channel really only 1/8" thick?

Yeah, only 1/8" thick, that's plenty strong. As far as weight, the fiberglass has roughly half the density of aluminum.

Again, this robot was designed for a specific game, and worked well for that game. The 2009 robot we made of wood was even more unconventional, and also worked very well for the game it had to play. The steel robot we made the following year didn't work as well, it had cantilevered axles, but because of the bumps it had to traverse, the axles bent significantly during the competitions.

[Rob Stehlik]

Actually, my original concept called for 1/2" shoulder bolts in a slightly different configuration. I intended to have the shoulder portion of the bolt pass through the frame, and tighten the entire wheel assembly against the frame with very thick washers.

The thread of a 3/8" shoulder bolt is 5/16"-18, which in my opinion is too small to be supporting this kind of cantilevered load. Thicker washers would help some, but the thread length is only 1/2", so you don't have much to work with, probably not even enough for a heavy lock nut.

I think what you have shown could work with a larger shoulder bolt, but it looks like squirrel's method with 1/2" threaded rod would be simpler and stronger. And fine thread really helps to get things tight.

Another issue is bending of the frame itself. Putting a cross bar at the top of the angle next to each wheel would help a lot. It could be a thin wall tube with a small threaded rod running all the way through.

[Joe Johnson]

There is a lot there to love.

A lot that is cringe worthy as well, but overall, a nice effort.

Got me thinking... ...How can I use this? Maybe there is something there...

Thanks!

Joe J.