Summary

Taking inspiration from BiZaRrO_John's bushings, I decided to add real rolling bearings to my Printrbot Metal Plus spool holder (http://www.thingiverse.com/thing:675439) to make the filament feed really smooth.

Starting with unitconversion's parametric ball bearing design, I tweaked it, optimized it for 3/16-inch balls, and added the cap piece that turns it in to a wheel for the PVC tubes.

Instructions

Print four of these per spool (so eight if you have dual extruders). Load twenty 3/16-inch balls through the feed hole (while the inner ring is in place), and then place the cap piece over the top. The cap fit should be tight enough so that no glue is required.

[Oh, don't forget to print my original spool holder first!]

I believe you'll be able to find the ball bearings at any bike store, but I bought a pack of 144 for $5.45 from Amazon (http://www.amazon.com/gp/product/B000AO5KOW/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1). I will actually need to buy some more, as I'll need 160 for my dual-extruder set up.

I printed with 0.25mm layer height.

If you need to tweak the fit for your particular printer the parameters you may want to adjust in the OpenSCAD model are:

ball_spacing_ratio - clearance around the balls in the raceway [one of unitconversion's original variables]
cap_clearance - gap between outer wall of the bearing and the cap
bead_radius - (scroll down to find this variable) radius of the friction fit beads on the inside wall of the cap. These control the snugness of the cap press fit.

This was my first time using OpenSCAD, so my apologies for the untidiness of the code. I was learning as I went along.

Bearing for Printrbot Metal Plus Filament Spool Holder