A very common malfunction for the iRobot Roomba is the Side Brush. There is an inherent flaw in the design of the motor where the plastic gear is prone to stripping. The gear should have been made of metal. Rather than replace the entire motor module, it is a lot cheaper to simply replace the damaged gear inside the motor. This video shows you how to do this.

There is a link below for an eBay seller who has these gears for sale. At $5 for a pair, it certainly beats buying an entire motor. And since the gears are plastic, they are going to wear out again and the second gear will come to use. Thanks for watching!

eBay link to the gear used in this video:
http://rover.ebay.com/rover/1/711-532...

If you prefer to buy a new side brush motor, here are several on sale:
http://rover.ebay.com/rover/1/711-532...

This video is applicable to the following models:
510 530 531 532 533 535 536 540 545 550 551 552 560 561 562 570 580 581 585 595
600 610 620 630 650 653 654 660 700 760 770 780 790 800 870 880 980