pic: AM14U Flipped CIM Gearbox

I designed this gearbox with teams like mine in mind. Designing and building a whole custom chassis is a lot of effort and custom parts, so we always use the KoP chassis. The AM14U is great, but it has a few drawbacks. One big drawback (for some games) is the lack of space in the middle of the robot. This season AndyMark released the first AM14U drop in gearbox (other than the stock TB Mini). I figured a good project would be designing a flipped-CIM drop in replacement gearbox. This gearbox extends only 2.5" into the chassis, which leaves so much room for activities in the middle. All of the parts are COTS except for the two plates which are CNC-able, and the axles which can be done quickly on a manual mill.

Some specs
Final gear ratio: 7.67:1
Final speed w/ 4" wheel: 9.83 ft/s
Weight: 7 lbs (1.4 lbs without motors)

CAD and pictures are on GrabCAD here.

I’d love to hear people’s thoughts and suggestions.

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Nice work! In order to conform to bumper rules though, it might be a good idea to move it a bit further in, so the CIM doesn’t bleed over the edge like it appears to.

This needs more encoder… or cowbell… or maybe it’s encoder.

Excellent work!

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Looks great! The AM14U definitely has the overhang area to take advantage of a flipcim design. I’m slightly biased, but I would make a shifting transmission modeled after the WCP 2 CIM Flipped DS - that gearbox specifically because you could use it to bring your CIMs in a bit closer to prevent overhang while still taking advantage of the open space over your wheels.

Keep it up!

Looks good! The idler is a nice touch, because it lets you use longer standard belt lengths (60t and up from Vex) and tensions the belt and gives you more engagement. Because this is non-shifting, you could also use a geared first stage to save some $$. But as it is right now, I would seriously consider it for any KOP chassis or sheet metal team.
10fps is a very slow free speed, though. Is it easy to adjust the second stage gears to get something up to 16 or 18fps? I know we at 299 struggled to get higher speeds for the field this year.

Let me get this straight - this is only useful for teams with both CMC and manual mill capability who want to use the KoP chassis? Until reading this post I wasn’t aware that there were enough teams meeting these criteria to match up with the thumbs on my left hand.

To me, it looks like a great thing in search of a use case.

There are a lot of teams with either CNC or manual machining capabilities who benefit from (and use) the Kit of Parts drive system. It saves time and resources that can better be directed to other mechanisms of the robot that can’t as easily be outsourced to a COTS component.

Growing up I thought there were two types of teams: Those with large fancy machine shops and those with a hammer and a drill. After being on teams from both ends of the spectrum I’ve found that tons of teams exist in the middle. Teams have varying degrees of resources, and what works as the best solution for one team may not always be the best solution for another. :slight_smile:

My team for example has a well above average shop with a cnc router, lathe, mill, laser cutters, and 3d printers. We totally have the capability to make our own drive systems, and we have in almost every game to date, but there are many cases where a kitbot type drive will work just as well as a custom wcd, and this style gearbox is a great addition to that.

Our team made a slippery pig drive with the 2014 Vex Drive in a day, and with a predone electronics board it took under 3 hours from box opening to actually running. While we chose against it for a number of reasons for steamworks, should there come a game where we won’t need a gap in the frame and those dimensions just about work, we will likely use it again. This coming offseason we have been considering experimenting with Andymark’s setup, and I would bet on similar results.

Just cause we have tons of fancy machines and could spend a whole week making 3 drivetrains, doesnt mean we should devote time, resources, and skilled manpower to doing so when we could have newer students follow instructions and have something done faster while freeing up the above to work on manipultors. For a few teams the gearbox space is a huge issue with the AM kitbot drives, and this setup solves it nicely in a solution packaged perfectly for the AM kitbot. Still goes further towards the resources, machine time, skilled manpower, etc level, but in certain games with certain requirements etc this setup would totally be useful, and is totally the type of thing 1836 would look into should we be running a kitbot style drive.

Props to OP for taking a solid look at his teams resources and not being afraid to use a kitbot drive, and especially for really improving that kitbot system and solving a problem with the capabilities his team has without substantially compromising the whole simplicity reasoning behind using the kitbot.

Good stuff!

Thanks for all the compliments! I’ll start by mentioning that right now I’m working with an international team, so my priorities might not match those of a lot of US teams. I actually tried to use as few parts from FRC suppliers as possible because they are very expensive here. We have access to CNC and manual mills and lathes from our school’s robotics program, but the time and money it would take to design and order parts for a full custom chassis makes it infeasible. For a less-equipped team, I could imagine them asking a neighboring team with access to those tools to make the 4 custom plates and axles for them. Most of the other parts else can be bought at a hardware store, and a few parts are ordered from Vex.

To address some of the concerns:

  • The back edge of the CIM is .1" from the outside face of the chassis. With the wires zip-tied to the side of the CIM, that should be enough. If that’s cutting it too close, it would be easy to move the CIMs in a bit by making the plate spacers longer, at the expense of space in the middle of the chassis.
  • I completely forgot about an encoder. It should be possible to switch the tapped screw in the back of the output shaft for a slip ring and a hole for an encoder. If I decide to make a v2 I’ll add that in.
  • I was considering making this a shifting gearbox, but I decided against it because I thought a custom shifting gearbox was a bit above my team’s abilities (we’ve never run a COTS shifting gearbox before). It should be possible to modify the design to put two rows of gears on the second stage and put the shifting cylinder where I said the encoder would go. If you look at the AM14U chassis, there are two L brackets between the two plates right on either side of the gearbox, so pushing the CIMs apart and putting the cylinder between them (like the WCP 2 CIM flipped DS) would mean raising the CIMs even higher.
  • My goal was to only have bearings in the holes made for the TB mini, which would have made it hard to use idler gears for the first stage. I chose belts instead because it meant less holes that would need to match up with the AM14U plate and it’s easier to package. It probably could be done with idler gears, I just didn’t think it was worth the effort.
  • Yes it can definitely go faster. I tend to like slower drivetrains, so 9.8 ft/s is the slowest it can go while still using the bearing holes designed for the TB mini (and without a VersaPulley on the first stage). You can increase from there to basically any speed by changing the second stage gear ratio. There’s basically no constraints there. You could also use this same gearbox with the 6" wheels that came standard with the KoP chassis last year, and you would get a speed of 14.75 ft/s.
  • I talked a bit about my team in the top of this post, but I’ll say a few more things here. We, and I think a lot of teams, really like the strength and versatility of the KoP chassis, and knowing what chassis we’re using from the start saves us time in both designing and manufacturing. If we find that we need the space in the middle, however, switching out this gearbox would be less expensive and time consuming than designing a whole new chassis, and still gives us much of the versatility of the AM14U. And if we find that it doesn’t work for some reason, we can easily put the stock TB mini back which we know works.

New thread created here to discuss the new gearbox.

It’s not exactly v2, but I made another very similar gearbox for 3 mini CIMs instead of 2 CIMs. Same power and a bit heavier, but more clearance inside the chassis and less current per motor. This one only sticks 2" into the chassis (less if you trim the CIM shafts). Final weight ~8 lbs including 3 mini CIMs (~1.5 lbs without motors). Still mounts directly to the AM14U chassis, but this needs small notches to be cut out of the top flange to make room for the wider gearbox. Same ratios and speeds as the first version. Now including an encoder.

Some pics and animations:
Gearbox 360* Animation
Full Robot 360* Animation

Gearbox CAD
Full Robot CAD

Do you have a link to the parts used in the assembly? I visited the grabcad link you provided but its just assembly files and pictures. Great design!

I just uploaded a pack & go zip that you should be able to download and open in Solidworks. Let me know if there’s anything else I can do.

I would dig through the “Gates light power and precision” Manual and look at allowable back bend radii for those belt run idlers before you cut any metal, I think you’ll find you’ll want it at least twice as big as you have it.

Looking shiny otherwise.


You’re right, they recommend a minimum bend radius of 1.25" for 5mm HTD belts. In theory, you should be able to replace the 1/4" ID 1/2" OD bearings with a spacer and FRC standard 1/2" or 3/8" bearing (1.125" OD). That should be good enough for FRC purposes.

P.S. I didn’t actually check the CAD on that. If anyone is actually planning on making this, please make sure that the belt doesn’t hit the large pulley

That’s great. I got it thanks. I think I will mill that out and try it. We are a team that has a good amount of technical capability but lack in the people resources like mentors. So we have made our own custom drives 3 times and all were successful but we definitely use the AM14u when we can. IN the past we 3d printed gear boxes to get the motors up and over the wheels. Your design is very nice and simple. Thanks.

This may depend on your local “culture”. I am guessing that about 20% of the mid-level to upper mid-level teams I saw at Ont Prov Champs were like this and gave the same reasons. From a distance, one would see upper structures that had a lot of machine work. Up close, one could see that it was all mounted on a KOP chassis. I also saw several non-Texas teams like this at Houston Champs.