I like automatic curtains, but it don't like the belt systems that usually drive it. I also want to open or close my curtains manually from time to time, without destroying the whole machine.
So I thought I'd make threadless ball screws and do some math to calculate the angles of the bearings such that each ball screw moves at R*s*(n mod 1) where R is a rotation of the rod / shaft / tube, s is a scaling factor and n is 1 to N (number of ball screws) or -1 to -N. This way the screws spread out evenly when the rod is rotated and it even works both ways at the same time!