I had an American Flyer prewar 1680/447 type Hudson engine that ran, but had an issue with the armature overheating within a few minutes. I had changed the springs and brushes and took the engine out of the boiler several times and tried to find the issue. I wanted to swap the armature, but anyone who has worked on one of these engines knows that in order to swap the armature one has to remove one wheel to get the armature out of the motor. Not wanting to go that far and not having another armature to swap into the motor, it sat.
Finally last week I was rebuilding another motor and thought it would be a good time to swap and see. I swapped armatures and the one that had the overheating issue suddenly ran great with no overheating.
After getting the "problem" armature out of the motor, I noted that it was very dirty and greasy, including the plates that the brushes rest on. Here is a photo of it.
Note how dark the area where the brushes make contact.
I theorized that this might be the problem and a friend told me that the grease/oil residue would cause heating when electricity is applied.
I tried several means of cleaning the armature, including electrical contact cleaner, but the burned on grime would not come off. Finally, I took a Q-tip and dipped it in copper cleaner and rubbed gently. I also made sure not to put too much on or leave any cleaning residue on the armature. Here is a photo with 2/3 of the armature cleaned and one dirty contact
Note the difference between the dirty and cleaned areas.
After fully cleaning it, I installed it in the engine that I was rebuilding. The engine ran fine with no overheating.
I am not sure how the armature got so dirty or how the grease/oil burned into the brush plates, as it was in a very nice 447 engine that came to me as part of a very nice boxed set from 1939. I am just glad I found the issue.