I’ve been trying to teach myself CAD, so I made a gearbox that Shaker Robotics was considering using this year. 2 CIMs and FP (modeled with 3 CIMS to ensure clearance on everything) driving a two stage reduction, single speed gearbox. The output is ~8.5:1, suitable for direct driving a 4 inch wheel. I tried a few things that I have no idea whether or not they will work but I figured it would be worth modelling so smarter people can tell me if I’m doing something wrong.
Would the gearbox be able to be held together with just the motor screws like that? I put through holes in the spacer and the sideplates, but maybe that wouldn’t hold it together… I also wonder about snap ring placement to hold all the gears and shafts and everything, whether or not I need to put clearance / dead space in between everything, etc. Any tips would be appreciated.
2.3 pounds without motors and with an arbitrarily long output shaft. I could probably take out a fair bit of weight by lightening the gears and 1/4" plate.
You can definitely use the CIM mounting holes as the transmission stand offs. This design overall looks good. You could look into using delrin plates instead of aluminum to drop a bunch of weight for the same amount of machining. Also think about how you are going to mount the transmission as right now there are no mounting holes.
We did a gearbox with delrin plates, and the gears wore about twice as quickly as our aluminum boxes.
Also, assembling the box with the motor bolts is quite doable, but makes assembling a bit of a hand gymnastics exercise.
Edit: A bit of a suggestion: If you can recess the standoffs to allow the box to ‘snap’ together, it makes assembly much easier, and makes aligning parts much quicker.
Looks really nice thus far. You may want to search cd-media for our 2008 drivetrain. I designed it to be a 6 motor (4 CIM, 2FP) single speed drive system with 4.75" wheels. Just something comparable to look at.
If I am interpreting your question correctly you were asking about clearance inside the gearbox. Be sure you give yourself some clearance for tolerance stack up. If you don’t, when you tighten everything up your gearbox could be seized up, or you may even damage your bearings.
The motor screws are completely legit to use as a method to hold the gearbox together.
If you’re gonna mill the plates, you’re gonna need some exterior fillets. If you’re gonna jet them, you’re probably fine, but I still like having the fillets there for cosmetic reasons.
Having counterbores for the standoffs certainly keeps everything more ridgid, but i would say makes assembly much more difficult. Lining up every standoff with its corresponding hole can be quite a challenge (which would probably become even harder with these being the motor mount screws). If you’re gonna include counterbores for the standoffs, do not do it on all six standoffs. Even with just four counterbored standoffs on our gearboxes this year, it was still quite a challenge to line them all up.
That’s pretty much the only reason, and I wanted two identical plates. You can’t put 3 CIMs on the gearbox with this ratio without overlapping the bearings.
The Team221 WildSwerve modules use a 1/8" bearing shim so that 1/8" plates can be used. They work pretty well. I’ll see if i can find a picture.
Also, I wouldn’t use the motor mounting holes as the gearbox plate holes, mainly because I like those motor mounts TIGHT and sometimes gearbox assembly bolts need to be loosened a little bit so the gearbox can run with less internal friction. I suppose that may be a personal bias though and not a requirement.
One other thing to think about: How would this mount onto the frame of a robot? The machined plates look cool, but how would you mount it? That’s probably too much of a robot-specific question to fully answer right now but is worth thinking about.
Since you have 2 stages there Chris, I’d evaluate the gears you have available and try to shoot for center distances between the second gear stage shafts that will allow you to swap out for a couple different gear sets. Much like the final stage on the super-shifters allows you to modify the gearing by swapping 2 gears. This is FAR easier than changing the spur gears on the 3 motor outputs, and will allow you to modify easily should you ever want to drop a FP or CIM and need a slightly different torque.
Right now the distances between gears would accommodate a standard Toughbox ratio, but the placement of the motor / spacers interferes. The use of a standard ratio was far from intentional though.
We’ve also used 1/8" shims with 1/8" sheet metal plates in our gearboxes for the past two seasons. We use a single #8-32 button head screw and PEM nut to hold them in place. Here’s a photo:
