I've been using a Dirt Dog (model 1100) since August, and its now having some serious trouble. The problem is the brushes have stopped spinning. If I remove the brushes, then the drive mechanicsm will spin - however any small amount of resistance will cause it to stop turning. I've looked at the gearbox, at the connection between the second drive gear and the smaller brush gear is very weak. I cleaned out the gearbox, relubed everything, but still have the same problem. In particular, the gear which drives the smaller brush seems very loose, so its easy to imagine that it can move a little and not be driven at all.
So, first, is it common for the gears to get worn down in these devices? Second, can you buy new gears for the Dirt Dog? They don't offer a CHM module or much in the way of spare parts on their web site, which is a bit dissappointing.
I also have 2 Roomba 530 modules, both with the red CHM modules. These have also been troublesome, but at least the gears haven't worn out. I have the new Aerovac bin on both Roombas, and that seems to help.
The Dirt Dog was actually may favorite, since it was very little maintenance until now. But I hope its not dead - especially if the problem is a few plastic gears.