This is about the linear bearings from 8020 which uses the UHMW stuff. I posted a question on deja.com and the replies are this. They said it is subject to distortion under load, the slides will wear so you have to replace them but since they are cheap it should be ok, and does not have high accuracy. It seems that people instead use THK bearings instead of UHMW bearings. Also, you can buy shims to make the fit tighter, however the slides will move slower. See the discussion at:
http://groups.google.com/groups?hl=e...om%26rnum%3D10
Thanks