i am building a small generator. the big gear can mount on a water wheel or wind mill, whatever. the little gear is mounted on a small generator. i gave you the blender files which you can modify the gears. the little gear can easily just change the center hole. the big gear is made to screw or bolt to a bigger drive unit, then drive the little wheel to spin a small generator faster. small generator could be a DC motor or other perminant magnet gen. 36.95mm is the center to center of the gears, with my printer. gives a free spin with no binding. it is the same center to center as my open sourced 3D printed bobbin winder, and the same gear pattern. the original gears are from a rep rap filament drive