A gear design for steel bead chains that you might find on a window blind or light fixture.
This gear design fits beaded chains where the diameter of the bead is 4.5mm and the link length (aka distance between beads) is (roughly) 1.5mm.
The gear's inner diameter is 5.5mm, and its outer radius is 20.1 mm. There are 21 teeth. Each tooth is a half sphere (actually, it's a hull) meant to fit at least half of the bead.
I recommend the larger zaxis hull design because it is a little more forgiving to minor deformities in the gear or beads and catches the beads better. The stl file "with sides" uses the zaxis hull design by default.
If you print this on a 3d printer, I highly recommend setting the fill pattern to concentric circles, unless you have a good printer whose y axis can vibrate back and forth very quickly, or you set the speed to something dreadfully low.
To generate this stl file yourself, you can checkout my attached openscad code. The latest version of the code exists here:
To find your own dimensions for a gear of different make, you can use this script: