I realized that I put too high demand on the 3D-printed plastic parts. Now I have changed the construction and use three push pull screws to move the focuser draw tube. At the same time I now don't need the extra gearbox. The screws itself is the drawbox. If I direct connect it to a 200 step stepper motor each step move the focuser 3.5 my (M4 screw). With the timing belt gears I can extend or reduce it. Less backlash with this construction.
Here is my drawings over the design:
http://www.astrofriend.eu/astronomy/...-friction.html
Lot of small details still to solve, but much easier now compare to the earlier design.
/Lars