Team 7461 Kitbot On Steroids -- 2020 Edition

Team 7461 is proud to release what we hope will be a valuable resource to teams through the entire FIRST Community - a collective set of modifications to the AM14UX series of chassis, that we hope will iron out many issues teams have had with it to date.


Sample Chassis using an 8WD configuration, with 4" SDS Treaded Wheels

The modifications detailed here were made with adaptability, serviceability, and robustness in mind, and most can be made with hand tools, though nicer versions can be made with access to a Lathe and 3D Printer.

The defining trait of this chassis, in my opinion, is the use of a piece of 1/2" ThunderHex stock bored out to 3/8 of an inch as a spacer between the bolt and the wheel, which allows for easy wheel swaps without having to fiddle with bolt spacers and simultaneously trying to align the bolt, as well as the use of self-aligning weld nuts riveted to the inner plate to retain the axle, which eliminates the need to reach into the robot with a wrench to remove a wheel.
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The ThunderHex axle can utilize snap rings, spacers, or shaft collars to axially retain the wheel, depending on teams’ manufacturing capabilities.

Another neat addition is the use of 1/4-20 aligning weld nuts riveted to the outer plate for bumper mounting, which is an easy yet robust way to quickly attach and remove bumpers.

A few other minor modifications were added, including putting both pulleys on the inside, to maximize the wheelbase, 3D printed nut holders for the outer plate, for quick and painless removal if you need to replace a center wheel, and NEOs on the drivetrain rather than CIMs.

This set of modifications should be adaptable to any configuration of the KOP Drivetrain, and can be used with any wheels with a 1.875" bolt pattern and 1.125" bearing bore.


Wheel configuration using 4" SDS Treaded Wheels, and snap rings for axial retention

Wheel configuration using a 6" HiGrip, and spacers for axial retention

OnShape CAD can be found at: https://cad.onshape.com/documents/e387957b63ee0962199a29d0/w/fc6095508e8127e3bf2c20ac/e/ae3ff4ebfc9109c79dbab270

Next steps on this project include creating a series of documents and/or videos detailing how to fabricate it, and creating more example configurations for teams.

Special thanks to @nuclearnerd from Team 5406 for providing suggestions and inspiration through the course of this project, as well as everyone who posted in the Interesting AM14U2 and AM14U3 upgrades thread, and the folks at AndyMark for putting together such an adaptable and robust system for teams to use.

If you have any questions, thoughts, comments, or concerns, please feel free to post them here, and we’ll try to respond. Good luck in Infinite Recharge!

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I’m loving the bumper attachments, were these sourced from McMaster or elsewhere? Any chance you have a part number for these?

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This is an awesome project, we were looking at creating something with a lot of the same ideas. Adding in a nice rigid wooden belly pan and this is the start of a great robot.

I was hesitant to recommend teams to drill out thunderhex wheel axles to 3/8" since that is best done on a lathe and many teams don’t have one. I was contemplating either using 3/8" OD, 1/4" ID bushings in the frame, to allow for 1/4"-20 bolts to be used instead of 3/8" and then just using regular VEXpro 1/2" round tube axle. Another option would be to stick with using 3/8" bolts and spacers but include a spacer between the bearings in the wheel/pulleys so that you can actually tighten the 3/8" bolts without sideloading the outer bearing races and increasing friction.

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This is great. Nice job!

We ran a similar tube axle setup in 2016 which worked really well.

The next upgrade would have been to design a little 3d printed v-guide to rivet to the inside of the plates. The guides would help center the tube axles and tension the belts (which is the hardest part during installation), and act as axial spacers (so you don’t need the snap rings). I think team 67 does something like that, am I right @Mike_Schreiber?

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Yes, we mill a V groove into our dead axle blocks to help with alignment. Because we use round axles and don’t through bolt, we also added an anti rotation flat on one of the blocks / one side of the axle, less of an issue with hex you can hold with a wrench. The V grove significantly improved our wheel swap times, it won’t ever be as fast as WCD, but with no bag that doesn’t really matter unless you break a wheel or need new tread at an event (which seems rare).

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Do you have a photo or CAD rendering that you can share to illustrate this?

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It appears that one would have to drill holes in the tabs of the self-aligning weld nuts in order to be able to rivet them on. What size rivets have you used? Would a screw-mount nut work too?

In the original AM14UX design, the 3/8" bolt is held on with a lock nut. Is there some anti-vibration feature in your modification?

What if one flips the 3/8" bolt around and fasten it permanently to the inner rail with a nut? A standard 3/8" - 16 nut is 21/64" according to McMaster which is thicker than the short spacer (0.280") so one would have to use a square nut that is 0.250" thick. On the part of the bolt that protrudes through the outside rail, one can install two 3/8" - 16 nuts that are torqued against each other to jam them (lock washer optional).

I have attached a sketch showing this arrangement.

I’m gonna be honest, I have no idea what you mean by this. Can you provide a drawing or something?

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I think that would make wheel changing much harder wouldn’t it? Fastening the bolt to the inner rail permanently that way would mean a) you’d have to take the outer rail off every time you wanted to change a wheel and b) you’d have to line up every single axle at once when installing the outer rail, even though they’re each under belt tension. Am I misunderstanding?

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Also you would have to a fully threaded 4"+ 3/8" bolt.

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The idea of the inside weld nut or other captive nut was to allow changing corner wheels without removing the outer plate nor accessing the area between the inside plates.

So, you’re promoting using hex shaft as a dead axle? It would make much more sense to me to use a round axle. Perhaps cutting lengths of tubing which already has the appropriate hole, e.g.

Yes. It’s a choice between taking more parts off and manipulating the spacers in tight spaces. We have always been frustrated by this. Either way, one would have to deal with the belt tension.

With a 6-wheel arrangement, the outer rails would have to come off to change the middle wheels. It also seems the middle wheel is the one I have seen most commonly being worn down.

I think that’s why @heatblast016 chose to recommend the 8WD configuration.

Understood. I was comparing to the stock KOP 6-wheel configuration.

If I were to go to the 8-wheel configuration, I would use the 3/8" tube axles that @AllenGregoryIV mentioned but cut them long enough to go through both side rails so the bushings are not needed.

I forgot to ask @heatblast016 if the 8-wheel configuration uses holes already existing in the side rails and if commonly available belts work.

Yes, the KoP chassis has a standard 8 wheel config which uses existing holes in the chassis and belts available from AndyMark.

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Crude ppt drawing because I don’t have CAD access at the moment.

We use shaft collars for this reason.

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I thought I remembered correctly, thanks. I am sure that this piece could be 3D printed for @heatblast016 's design, and attached to the insides of the drive rails to help tension the belts. Add a little nub to contact the bearing inner race and it could avoid the need for spacers on the tube axle too.

[edit] - except without the back wall and through hole. Just the ramp so that the tube still clamps to the rail, not the (plastic) guide.

Although if someone supplied these parts machined out of aluminum, you could reduce the bore down to 1/4". That way you could use the Vex tube axle stock (with a 1/4" through bolt). If the inner plate guide is threaded as yours were, you might not need a separate riveted nut.

You mean something like this? From 1114 Solidworks series

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You wouldn’t get the advatange of being able to preload the tube axle as a shaft spacer to make your frame more rigid, which is the most important part of going to a tube axle. The spacer advantage is only a time saver, actually being able to tighten your wheel bolts makes your frame stronger.

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