Search this site for rotating nuts, there should be quite a few threads. If you look at the critical spped charts at www.nookind.com, the list 4 screw mounting types. Having both ends fixed with a rotating nut only gives you about 20% more rpm than having both ends rigidly mounted with AC bearings. Although you would think it would be higher. So it's probably easier to mount the screw at both ends with better bearings, and possibly increase screw diameter rather than go to the trouble of fabricating a spinning nut. And yes, a lot of commercial machines use spinning nuts.