pic: FRC488 -- Fall Project

2013 was our first season using sheet metal construction in our robot. Our machine was plagued with frisbee-related issues at our first event, so we often played vigorous defense after the frisbees jammed. Ultimately, this resulted in a badly beaten up frame.

We fixed the frisbee problems for our 2nd event, but the twisted frame meant everything was misaligned and we threw chains nearly every match.

So, in a knee-jerk (6 mos. later) reaction, I’ve designed a drive that doesn’t use chain. :slight_smile:

We’ll build this in the fall.

Welcome to the “Chains are Evil” club.
Nice work.

Madison, I really like this any chance you are going to share the CAD?

Not a bad start. I assume that you are posting this here for feedback, so here I go. I see a lot of similarities to 971’s frames, which means this frame has the potential to be bullet proof and very nice.

Extend your belly pan all the way to the outer sheetmetal rail, and bend a flange off the bottom of it to make a box structure. This will make your frame a lot stronger. It looks like you might have to move the idler gears up some to make that work, but my experience says that it is well worth the pain.

Put flanges on the 2 vertical faces on both ends of the clamshell. Those will tie in nicely to the frame and add even more strength. See if you can also add a flange on the other angled surface on the outer edge to catch the bellypan.

Battery box? You can nicely integrate that into the belly pan and rear frame rail. You can find some pictures of how we did it (which worked great) if you need inspiration. While you are at it, press in some PEM nuts for all the electronics, and consider punching in some lances to use to tie the wires down with. You can bolt the electronics down with plastic bolts, and then cut them flush with flush cutters.

The row of hole + slots that you have right next to the edge of the frame look nice, but will be a pain to bend, and take a lot of strength out of the frame. I’d either skip them, or drop the slots.

Bend another flange off the inner top edge of the front and back of the frame. This adds even more strength. This frame will hit things, and you need all the strength you can get there. I would also make the flange like 1.5 - 2" long, and not neck down like you have drawn.

See if you can fit some ribs in your box structure to give it more torsional rigidity and transfer the load from the outer rail to the inner rail.

The tolerances on the C-C on the main transmission gear isn’t going to be very good. Bend tolerances are ± 0.010 in my experience, and the stackup on the two bends and the slop in the bolt holes isn’t going to be good enough. We pressed dowel pins into the frame to give us a very accurate alignment, and then kinematically constrained the gearbox from there. Consider holding the output shaft at the end as well. That is a lot of torque to be cantilevering out that far.

What thickness? 090 will be strong. If you are careful and add some ribs, I would be willing to bet that 060 or 080 would work well as well.

Any particular reason that you are using hex shaft for the drive axles? The tolerances on that aren’t so great, which will cause problems with your gear c-c’s again. Same with your idlers.

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Any particular reason that you’re using live axles? It seems like you could use dead axles with a little gear machining - drilling out the wheel gears to allow a bit more clearance for a 1/2" shaft to slide through, and boring out a bearing hole in the idlers. Using those axles as standoffs could potentially add some rigidity to your frame for “free”. Plus, no hex bearings, so it saves money too.

Edit: On a closer look, they’re definitely dead axles that are already serving this purpose, but my point still stands with regards to saving some cost + it’s far easier to find good round stock than good hex stock.

I like the concept a lot!

So if your competition frame was bent and threw chains, how does going to an all gear setup help? If you run your gear-driven frame rough and it gets bent it sounds like it will destroy gears due to incorrect mess?

The obvious solution to both is to build a frame that doesn’t bend, but after that why not just stick to chain or my personal favorite timing belt? We ran our timing belt frame pretty hard this season and never had issues with throwing belts, and in the past we never threw chains (even on the kit frame) after using tensioners/careful alignment.

I agree. 971 runs timing belt which IMO requires the most precision out of the three. Chain requires the least. I think even strengthening up the frame and switching to 35 chain(which can take a lot of slop) will be a better solution that may even weigh less.

I suppose I can, though I’ll have to find some efficient way of doing so.

Austin – I appreciate all of your feedback and will address it as well as I can tomorrow. It’s late now. :slight_smile:

Dead axles everywhere. I’m using McMaster-Carr part no. 91780A901 for the axles. I’d use round standoffs here to save a bit of money / frustration with the hex bearings if I could, but the COTS hex standoffs are more readily available and cheaper than anything round at that length.

The wheels are VexPro Versawheels with 3/8" hex bearings coupled to 64 tooth, 1/2" hex bore gears. The 1/2" hex bore clears the 3/8" dead axle hex. We’ve run this arrangement a few times.

The idler gears are 35 tooth AM shifter gears and are designed to have a bearing pressed in. Here, we’d like to use the WCP equivalent gears in aluminum.

Knee-jerk reaction. :slight_smile: Given the same conditions we had in the 2013 season, this frame would fair only a little better. The biggest problem we faced (outside of our frisbee systems being garbage) was that we couldn’t get any sheet metal parts beyond the frame made in our build window and the finished frame was missing the belly pan. I expected that’d give it a lot of rigidity and, in its absence, the entire machine was too flexible.

In the end, if it is feasible, we’re happy to avoid chain. Even when it’s working (95% of the time for us), it’s still a pain the butt to get it installed and tensioned properly.

Ah that makes sense, I did our belt drive WCD on a similar principle at first…

Yes the belly pan can be very important in terms of structural strength. And with the build season rush that’s a similar problem with had with some sponsors.

When we did WCD with chain we just C-C the chain, when after putting the chassis together we rolled the chain on and then put in the master link, clearance for doing so was designed before hand so it was an easy job. Also tensions can be pretty simple, but you have to design for them before hand; instead of trying to add them on later the fact.

At Boilermaker the back of our drive frame twisted out like you see in this picture (design failure) The gears happily kept humming along and had zero problems with the twist and worked without destroying themselves.


Gear drives are cool, and I’m reading the discussion on axle choices carefully. Cool discussion.

I’m with Chris on the axles. For the wheels, 1/2" tube axles seem like the easiest, cheapest, best option to me.

For the idler gears, I’d personally be looking at raising them higher than the line of the wheel axles for some extra resistance to bending about the front-back axis of those wheel channels. One could use the WCP 48T dog gear with round 1/2" bearings and tube axles. For fun, I sketched that in CAD using the same wheel spacing, and the bigger idler gears would end up in a line 1.31" above the line of the wheel axles. That would fundamentally alter the design, though, because then the gears stick up 1/2" above the wheels.

Personally, I think it would be really cool to build a gear drive. Chains are pretty annoying. If I added up all of the time we’ve spent messing around with chains in the past couple of years, it would be quite a few hours.

Thanks for sharing this very cool project.

Why not go with belts?

Could you add two additional flanges to the gearbox mounting piece to fasten it to another face of the channel? Would it be necessary?

I appreciate everyone’s continued feedback. I am reworking the design with it in mind and will have a revision ready later on today.

Already, it’s looking better, though there’s obviously a long way to go.


I am pretty sure this can be assembled. :slight_smile: I moved the assembled wheel clamshell into place from the outside in the model and, while it was a tight fit, it seemed doable.

Ignorance, mostly. There are lots of gears and sprockets available with the “FRC bolt pattern,” but not so with belt pulleys. There are the AM kit pulleys, but I don’t like the ratio.

Our mill and lathe are often bottlenecks in our fabrication process, so I try to design as few parts as possible that require milling or turning. So, until belt pulleys are sold with that 6-hole pattern (and versakey! …hint, hint), we’ll not head down that path.

Have you thought about how you would replace the versawheels? We have run 6 competitions this year and are on to our third set. I really like them but they do ware. Overall they are still very cheap even when you add in replacement costs. If it works out that they can just drop out the bottom even when fully assembled, that would be perfect.


We too had to replace our versa wheels typically once an event for a full set and then half-way through the event we would replace the middle ones.

We switched to AM HiGrips for IRI and while they might not start with as much traction compared to a fresh “cleat” like versa wheel they do last much longer. We are still running the same wheels (minus two that broke during our fall in Semi 1.2) after a ton of drive practice and we haven’t noticed a drop in performance whereas we could tell when our versas were worn down.

With the current design, would you have to remove an entire drive assembly to remove a wheel?

You ought to be able to remove the axle bolt and drop the wheel out of the bottom. Since the idler gears on either side of the wheel should be free-spinning, I don’t think there’ll be any issues with sliding the gear out, but I am having some trouble visualizing that.