All five of the 203123 gearboxes (with matching 118777 motors) we purchased last year have begun to lock up intermittently. The spare gearbox (#5) was installed as a replacement for the fourth gearbox after it locked up. This fifth gearbox has also begun to lock up, after less than a week of light usage.
We opened up the fourth gearbox and removed one stage at a time to see if we could determine the source of the lock-up. After removing the 1st stage (the one closest to the motor), we turned the gear's output shaft and noticed that lock-up occured when the white ceramic posts holding the planet gears in the 3rd stage (the one closest to the output shaft) lined up with holes in the 2nd (middle) stage's planet carrier. At this point it seemed obvious that the ceramic posts in the first stage's planet carrier had become loose and were getting stuck in the second stage.
Upon removing the second stage, we could see that at least one of the ceramic posts was no longer flush with the top of the gear that it held. This was confirmed in a second gearbox which had locked up (motor #1).
The 203123 gearboxes were bought with new motors to replace similar motors with the older 110404 gearboxes, for the same application. The older motors and gearboxes, used for a longer period than the 203123s and their motors, never displayed this problem.
Update[Feb. 3, 2003]: Frank Kirchner emailed me back to let me know that similar problems occured with the Scorpion robot's motors. The gearboxes they were using (with smaller motors) had loose ceramic posts, too. They eventually exchanged them for all-metal gearboxes. This seems to confirm my feeling that this is a design problem, not a usage problem.
Download the first video, http://www.cim.mcgill.ca/~jasmith/images/maxon_gearhead_problem.mov [QuickTime 15 MB], and a second video http://www.cim.mcgill.ca/~jasmith/images/MaxonClicking2.mov [Quicktime, 1.5 MB] illustrating the clicking sounds produced by contact between the ceramic posts in the output stage of the gearhead and the middle stage of the gearhead. The clicks occur every time the misaligned ceramic post lines up with a hole in the middle gearhead stage.
Both videos require that your computer have a copy of Quicktime, which can be downloaded at http://www.apple.com/quicktime/download.
