Hi Gunter,

re:
> I created the surface by sweeping to half ellipses around the ring rails:

When you make a closed surface there is what is called a "seam edge" at the surface's starting/ending point.

In this case you are going to want the seam edge to be at the bottom area instead of the top area. You can make that happen by rotating your circle rail curves by 180 degrees around their center point before doing the sweep.

Or also there's a plugin here that you can use to relocate the seam point on a closed curve:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10125.8

The seam edge of the sweep surface will be located at the seam point of the closed rail curves.

- Michael