Dear georgeburns,
You can buy yourself an awful lot of mileage if you make each end of your screw rigidly "in-line" by using dual bearings (spaced apart) at each end. If you can tension the screw as well, you get even better performance.
Nook know all about this...
http://nook.texterity.com/nook/linea...ide/?pg=5&pm=1
The good stuff is on page six and after.
Best wishes,
Martin