Check out Machine Design. They have some great articles on ball screws and other stuff. And it looks to be free. I've read some stuff and was never pestered for registration etc....
Machine Design Magazine - Ball Screw Basics
Machine Design Magazine - Is That a Single Nut or a Double Nut Ball Screw?
Machine Design Magazine - Which Lasts Longer, Single or Double Nut Ball Screws?
Thats a very good heads up Samault.
Could you also put that into the links section.
This post will dissapear in days, unlike a link in the links section.
EDIT: It may be a good idea to post seperate links to the seperate articals too.
Being outside the square !!!