pic: AM14U Flipped Mini CIM Gearbox

It’s not exactly a v2 of the AM14U Flipped CIM Gearbox, 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). Also mounts directly to the AM14U chassis, but this may need 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

I love the integration with the original chassis. The pocketing seems a little aggressive but other than that, fantastic looking gearbox. Something I would seriously consider using if I were to find my self in a position requiring me to use the kit chassis.

If the gearbox is only attached to the frame via the bottom two standoffs, you are going to end up putting a lot of load on your output shaft and bearings, and this may lead to shortened bearing life / clicking / general inefficiency. If you could add at the very least a third mounting hole that isn’t collinear with the other two, that should help substantially.

And the pocketing is a bit aggressive for what you’re trying to do with the cantilevered plates. You don’t wanna allow too much flexibility in this plate.

It appears that there are another 2 mounting points just above the large pulley. Looking at the animation that is.

It’s hard to see from the still renders but you can see in the animation and CAD that the gearbox has attachment points at the top of the chassis plate as well. It actually uses the same mounting holes as the TB Mini.

I know the pocketing is a bit agressive. I basically copied the pocketing from my first design, but this one is bigger and I didn’t compensate by making the struts bigger. Even if strengthening it adds a few tenths of a pound, the gearbox still a very reasonable weight. I think without pocketing it was about 1.9lbs without motors so even that isn’t so bad.


On your second render, you’ve installed it on the wrong side of the inside plate; it looks like your gearbox plate passes through the flange of the chassis member.

I just had a thought this morning. If you rearranged the CIMs here, I think you could avoid idlers all together. Imagine an M shaped belt path like this:


In exchange for raising the CG of the gearbox by putting two motors up high and one lower, you eliminate your two idlers. I’m not sure the geometry totally works for this, but it might be worth looking at.

Correct me if I’m wrong. But wouldn’t this belt path also require a double sided belt? Which is not a bad thing, I just wanted to clarify.

That’s true, it would require a double sided belt. These should be about as readily available as single sided belts, but you may need to plan ahead to deal with lead times on any proprietary belt profile like GT2-3

I actually thought about that, but I chose not to do it that way for two reasons. The first is that I like having the bearing idlers because I can add layers of electrical tape around the bearings to act as a “shim” to keep the belts tight. I could still add in the idlers in that design, but it would force the motors apart even more and add “unuseful” weight.

The other reason is because I didn’t want to mess with double-sided belts. Mid-level teams that might be using this design would probably rather not have to deal with outside retailers (especially if their school has to approve a purchase orders for all new retailers). Especially for my international team, where it seems that even harder than getting FRC parts shipped here is finding compatible local non-FRC parts. This design uses a “normal” 110t 9mm wide 5mm pitch HTD belt, which you can get from Vex.