The DIY versions seems to run with the rollers only and the heavy duty versions have an outer "nut".
BUT, there also seems to be two different styles of production designs. One with the outer nut fixed as seen earlier in this thread, and one with the outer nut rotating, moving the axial load to the flange via roller bearings. Check the picture (or
http://www.itwspiroid.com/rolrscrw.htm).
--Sven