This repair guide explains how to replace (or clean and lubricate) the main brush casing gears on a 500 series Roomba. For details on how to first disassemble your Roomba, please see Roomba 500 Series Servicing and Repair Guide Chapter 3: How to Open Up Roomba. Some of the 500 series Roombas are pictured below: 
7 How to Replace Roomba’s Main Brush Casing Gears 7.1 How to Remove the Old Main Brush Casing Gears Roomba replacement brush casing gears can be found at RobotShop. 7.1.1 Start by flipping the robot on its back with the bottom cover removed.
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/blog/show/chapter-7-how-to-replace-roomba-500-series-main-brush-casing-gears