I made an OpenSCAD script to generate designs for roller bearings.
The support must be printed with a soluble material, I used HIPS and ABS but you could use PVA.
The attached renders show the internal workings of the bearing and the support material.
The print is quite difficult and I have not yet had a completely working bearing. See more about the roller design process at http://blog.laserphile.com
The included STLs are sized to suit a spool holder with an 8mm rod and 31mm outer diameter. If you use the STLs included you must print with 0.3mm extrusion width and 0.2mm layer height, this is to do with the tight tolerances.
If your printer does not match the above print settings or you want to change the size of the bearing then download the OpenSCAD script and modify the variables to suit your printer and desired bearing size. There are detailed instructions in the script.
In order to print the soluble support alongside the solid material you must load both STLs into the same print. I used slic3r and had individual settings for the support material and solid material. You should use the following settings:
For soluble support:
-Extruder soluble material (HIPS)
-Top solid layers 1
-Solid bottom layers 0
-Solid infill threashold area 1mm^2
-Extruder solid material (ABS)
-Top solid layers 3
-Solid bottom layers 3