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!

[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.

[JamesCH95]

Cantilevered shoulder bolts can take a lot of load. I used them in an FSAE car suspension rocker that saw around 5,000lbf. The trick is to get the shoulder supported and eliminate bending stresses from any threaded portions. The only serious issue I can see in your design is how the shoulder bolts are connected to the rail.



In this picture near the left-hand edge about 1/3 of the way down you can see where we used a 5/8" (1/2"?) shoulder bolt on the suspension rocker. About half of its length passed through a steel mounting point, then went through a needle bearing in the rocker, and was captured with a nut on the inside of the car. We sandwiched thrust bearings on either side of the rocker so we could clamp everything down very tightly yet still spin it by hand.

I hope you find this useful *cough* needle bearings in wheels *cough*

[Tristan Lall]

Originally Posted by 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.

Now that I think a little more about this, watch out for the stress distribution in the preloaded part of the bolt (i.e. the part from the threads to the shoulder that is under tension when tightened). Normally you preload a bolt to a signficant fraction of its tensile strength. The large bending moment from the cantilever will also add stress right around that area—meaning that you might end up overstressing a bolt tightened according to the manufacturer's directions.