Contraptor - linear bearing

This thing is a part of Contraptor - a DIY open source construction set for building Cartesian robots: http://contraptor.org

Linear bearings ride on linear rail http://www.thingiverse.com/thing:1071, driven by belts or leadscrews. A pair of rails and at least a pair of linear bearings are needed to make a complete linear stage. There are three sizes of the linear bearing: 3.5", 2.5" and 1". The part list is for 3.5" size.

Sketchup source:
http://sketchup.google.com/3dwarehouse/search?q=contraptor+linear-bearing