In our mechanical design course, we were tasked with designing a gearbox for a micro hydro generator. The gearbox was required to accept a high-torque, low-speed input from a water turbine and convert it into an output with low torque and high speed. Our group‘s solution was a 3-stage spur gear system, with a drive ratio of 18.34 (49.08RPM input, 900RPM output) producing 13 horsepower.
With a few modifications, I was able to create a 3d-printable model of the gearbox. I cut out the faces so that the system's inner workings would be visible and replaced the tapered roller bearings with skateboard bearings, but aside from that, this miniature model stays true to the original design. All parts except for the ball bearings, bolts, washers, and nuts are 3D-printed: https://youtu.be/b7kzGnCRKPg
I created this model to be compatible with 608 ball bearings when printed at 50% scale. In the video, you can see I'm driving the system in reverse, as the torque required to drive the input shaft was too much for my little drill (and likely would have ended up snapping the PLA input shaft anyway!) However, I have nevertheless included the crank I made that can be used to drive the input shaft once the system is broken in. Use with caution!
In addition to the provided STLs, you will need:
6 - 608 ball bearings (skateboard bearings)
32 - M3x16mm Allen screw
32 - M3 lock washers
16 - 1/4"x1" hex bolt
4 - 1/4"x2.5" hex bolt
24 - 1/4" flat washers
4 - 1/4" nylon lock nuts
If you strip any of the bores for the 1/4"x1" screws, you can substitute 5/16"x1" lag screws.
The banana for scale in the pictures is another design of mine: https://www.thingiverse.com/thing:3167156
Slice at 50% scale. Use supports and brim for the shafts. Use Cura's "Fuzzy Skin" on the center casing to achieve the same texture present on my model.
After assembly, drive the geartrain through the smaller output shaft with a drill for an extended period of time in order to "break in" the system.