This is a tubular drive mechanism. It is designed to use 1/2 inch aluminum conduit. This is a work in process, and I am already working on a newer version that will use dual rails. I think that this type of mechanism will work well as an linear axis drive mechanism when combined a stepper motor, and magnetic linear encoder would provide high accuracy large scale positioning.

Tubular Drive Test Fixed Drive
http://www.youtube.com/watch?v=k0G8B77UOJs

Tubular Drive Test Fixed Conduit
http://www.youtube.com/watch?v=70dTRN7zSpU

Print the parts with 75% infill 3 shells. Print the spacers at 100% infill.
This uses standard 608 skateboard bearings for the wheels. Brush the sides of the wheels with acetone and sandwich the bearing between them. After the wheel has setup and is hard brush the surface of the wheel with acetone, this will help to bond the abs even more.