I've added more settings to allow a greater freedom of design.

Video: https://youtu.be/wmJcy1uERZ8

24 Dec 2015:
Added a KeyChain Tab option per request from Concerwe

16 Aug 2015:
There is now a centered HONEYCOMB INFILL PATTERN built into each of the gears with customizable parameters for strength.
This should greatly reduce the material needed and still retain most structural integrity. You can disable this if you don't want it.

14 Aug 2015
There is also a new precision setting to adjust the complexity/accuracy (number of vertices).

The trouble with 3D printing a ball bearing is that spheres don't adhere to the 45 degree rule, so they tend to print poorly, or if metal balls are used instead, then the bearing requires assembly. This is a new kind of bearing that can only be manufactured by 3D printing and comes preassembled.

It is a planetary gearset and functions like a cross between a needle bearing and a thrust bearing. No cage is required to keep the rollers in place, because their gearing keeps them perfectly spaced. The gears are all herringbone, which is why it cannot be disassembled and also why it can act as a thrust bearing. If you're wondering how well a 3D printed bearing could work under load, please check out this video: http://youtu.be/ZghwSBiM0sU

This design is fully parametric and made to be adjusted in the Customizer app to whatever you need.
This is an expanded version of: http://www.thingiverse.com/thing:53451
Thanks to emmett for sharing the original version.