This design is somewhat incomplete. I did not finalize the design because my alternative approach (Floating short belt tensioner https://www.youmagine.com/designs/ultimaker-short-belt-tensioner--2) was successfull while being significantly less complex.This proof of concept works surprisingly well if printed at sufficient quality to meet the strict tolerances of the gear pairings. Finalizing the design essentially would require some fine adjustments to the clearances of the screw heads, reinforcing the mounts and other minor cleanup. Maybe I will get back to this later.I put the design here because I think it is still an interesting implementation of the direct drive concept. Other concepts around include bulky external assemblies. I didn't want to mess up my UM's great aesthetic value and create a virtually invisible direct drive that does not require too much tinkering.Print at highest quality possible. The gear pairings will only work without jamming or play if printed perfectly round and smooth. Break in the gears carefully to remove edges and bumps. Gear wheel print quality is essential. The slightest imperfection will introduce jamming or backlash. Try to interchange the gears to find perfectly matching pairs. Reprint and try again before installing the device! You will not be able to print improved wheels with an imprfect assembly in place. Think before you spend hours for installing and rolling back your UM to the belts. Like I did... ;-)