We’ve been using GM3 gearmotors for our RepRap extruder motors, but they have a few problems (most notably being underpowered and having cheap plastic gears) so I decided to shop around and try to find a motor that I could be proud of. I stumbled onto the site kysanelectronics.com and ordered a bunch of their 12v DC gear motors. I got about 10 different motors with various gear ratios, sizes, etc. I decided to sacrifice one and take it apart. Turns out I was actually (miraculously?) able to piece it back together again, in working order too!
Here is a flickr set with the teardown.
Anyway, here are a few observations about these motors:
* they are all-metal construction (gears, housing, shafts, etc) the only plastic was an insulator on the terminals
* they are very powerful (my test is attaching vice grips and letting them rotate the vice grips, then trying to stop the rotating pliers with my hands. all could easily move the pliers, and most took a decent amount of force to stop)
* they are pretty cheap! for a single motor, they are $9.07 for a single unit and at volumes of 100+ they drop to $7.34. Not as cheap as the GM3, but they have dramatically higher strength, etc.
I think I may just have to place an order for a hundred of them. =)