Why is the side brush not turning

I just fixed my Roomba 760 with the same problem. The side brush had stopped working but Roomba still operated but once in a while the TANGLE light would come on. After checking and cleaning the whole Roomba I found there were no tangles. Took me a couple days of troubleshooting. Took the motor out and applied voltage and it ran. Ran the Roomba without the side brush motor in it and Roomba operated just fine and the TANGLE light never came on. So I took the side brush motor/gear assembly apart and checked the gears. No worn or broken teeth so I greased all the gears with white lithium grease and gave the central shaft of the motor and tiny drop of 3 in 1 machine oil. Reassembled the unit and put it back into the Roomba. Put the side brush back on but was careful not to tighten the side brush central screw too much. Now it works great. The side brush functions perfectly and the tangle light does not come on. Apparently the gears in the side brush assembly were not running free enough and needed lubrication and/or the central screw on the brush was too tight. I remember I put on a new brush earlier and I may have tightened the screw too much.