While working on another project I had an idea for attaching 3D printed gears, pulleys, etc… to shafts in a reliable way by taking a small (and cheap) pulley and knocking off the outer flange then modeling the inverse of the pulley teeth into whatever you are trying to attach to the shaft (see picture below). Essentially what this gives you is a compact, cheap, micro-spline connection.
I had been eyeing gear drive drives for a little while and was intrigued, but they were all too rich for my blood, so naturally when I had this idea I decided to make a fully 3D printed gear drive out of it. I knew that people had been playing around with 3D printing the drivetrains, but 3D printing anything that attaches to the motor had always seemed like a fools errand to me (I know, I know, some people kind of got it to work a little bit), so it was mostly just printing the wheel gear (which is still awesome). Well, with my handy dandy new cheapskate micro-spline I decided I was duty bound to try it because science. There are certainly pros and cons to a printed gear drive, but again, science.
I picked up a 15t 3m aluminum pulley which is just barely bigger than an 8mm shaft for like $3 from Aliexpress (https://www.aliexpress.com/item/32786813888.html?spm=a2g0s.9042311.0.0.6cc34c4dmGpk8l) and some Taulman 910 Nylon filament and got to work. I did some rough calculations and got that the splined connection should have a factor of safety on the torque it can withstand of like 8 or something ridiculous like that with the motors we use. I also built a little text fixture to determine how much torque it could take before the connection failed, and the pulley to shaft connection always failed before the spline (I was just using grub screws, so I realize this could have been better). Anyway this gave me enough confidence that this spline would hold up well, and it would just be a question of if the gear teeth could take it or not.
I threw together a test setup to see if there was any hope for the printed motor gear or if it would immediately blow up. Picture to follow (sorry, new account on this forum). Please excuse the ghetto-ness of this setup, but I wanted to test it out with what I had before going whole hog.
The herringbone gears make it a little tricky to assemble and so it requires a split gearbox, so you can see one half o the gearbox in the picture.
Last night I took it for the first test run, which mostly just consisted of me hammering the throttle over and over again up a hill to see if I could break it and the drive took it like a champ. I inspected the gears when I was done and they looked like they were in great shape with no noticeable wear (only about a mile of riding so don’t get to excited). The drive felt pretty great compared to my belt drive with what feels like a lot less resistance, and of course it was basically silent because of the herringbone gears (Although I don’t run FOC so my board still sounds like a spaceship, which I actually quite like).
I’m stoked that this drive made it this far, and it gives me hope that this might actually be viable. Two big questions remain: how long will the motor gear last before needing to be replaced? And when this does fail, how will it fail? As far as the first question goes if the gear can make it a few hundred miles before needing to be replaced that is a win in my book. Yes it is a hassle, but it would probably be a worthwhile trade off for me for the massive cost savings (this whole test cost me <$10 if you pro-rate the cost of the filament for the amount that I actually used). The second question is a bigger deal in my mind, because if the drive fails by throwing a tooth, seizing up, and launching me off the front of my board, that would probably be a deal breaker for me.
Anyway, I have high hopes and am interested to see how this all pans out. If anybody else is feeling adventurous and wants to build a setup for themselves, please go for it (although I don’t really have any CAD files I can easily share because they are highly tailored to my setup).
