Summary

Simple roller bearing with 8 captured rollers. 1/4" square center post, approx. 1.38" (35mm) outer diameter, 1/2" thick.

Instructions

Update 13Oct14: loosened up the spacing between rollers and inner and outer rings; can now be broken loose w/o tools.

I printed this with my Printrbot Simple Metal out of PLA (on mine I use 75% filament flow to get better dimensional accuracy). The design uses all curved notches and bump-outs, and the rollers start flat, so no support material is required. Must be printed all at once, as the rollers are captured. I've printed both .1mm and .2mm layer thickness; print at 100% infill, and make sure the center of the rollers are solid or they may snap.
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Designed for a filament cradle for my Printrbot, but could also be used as a pulley.
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I've also included the Sketchup (8) source for tweaking and adapting.

Planetary Bearing