The recent thread about Pittman motors got me interested in them, so I bought a couple of used ones off the big auction site for a song. These are both much older models from the 80s or 90s which I'd seen on there many times and thought to myself, if the gearbox on the top could be removed maybe this could be used like a normal motor. You know, back when manufacturers were still putting quality parts in these expensive "toys" of ours.
Here's what one looks like from the outside.
This particular model was made in 1982 when I was only 4 years old. You can see it has a part/model number of GM87xx-xx. I think the GM stands for "gear motor" and it's an 8000 series. I don't know what the rest of the numbers stand for, nor have I looked too deeply. This one also has a modest 6.3:1 gear ratio. I can slow the output shaft down quite a bit half amp or so at ~12-14V from my DC bench supply.
Since the 6.3:1 gear ratio seemed maybe useable in a project, I decided to take apart the other one I got. It was a 60.5:1 gear ratio! Once running, I could NOT stop the shaft or really even slow it down much with my fingers. It could probably twist the lug nuts off my Jeep it's so strong. Maybe I'll actually try this later. :-)
Before I potentially destroyed it, I measured the RPMs on the the 60.5:1 motor. At 12V I counted 39 revolutions over 30 seconds from the final output shaft, which comes out to 78 RPM. It was keyed so very easy to count. That multiplied by the gear ratio yielded about 4700 RPM at 12V. I'm not sure what a typical 8000 series motor does at 12V so maybe that's a data point to help understand their usefulness.
Anyway I got to taking it apart.
Here you can see all the innards in the gearbox on the top of the motor. It's unbelievable how tight these mesh, and how smooth they move. I'm guessing this motor wasn't cheap when new.
Finally, here's what's under all those gears. A nice bearing there! ...and a drive gear.
The point of all of this was mostly a learning endeavor. I wondered if we could use these old motors in our locomotives. I left out the picture of the top plate by itself, but it has a few "spindles" on which the intermediate gears spin, and some other channels and whatnot (likely for guiding grease against the gears). If those could be machined off, the output gear could be used almost like a splined output shaft on a transmission or transfer case to drive something else.
I see a few potential options.
- Make flywheels that mate up to this gear. The "back side" of the flywheel would need a "hole" that was the mate to this gear. The flywheels would also need some output shaft on them to then connect to a drive shaft or worm.
- Remove the gear and use the output shaft that is already there. This would probably be difficult as the shaft is much thinner than on other motors I've seen used for model locomotives our size.
- Machine the gear itself down by grinding off the teeth to give a larger shaft diameter to which to connect.
- Make a 1:1 gear that could drive a shaft just below center. Seems like that could be tricky too.
I'll leave it to the experts here to comment on the possibilities. The first step is machining down the "lid" so that the gear can be accessed freely without all the intermediate gear spindles. Another option would be to make a whole new lid for the motor, but I don't know how feasible that is given the tight tolerances in the motor and the fact that there is a bearing there that needs to be supported. Again, I'll let the experts comment. My next step is to take apart the 6.3:1 motor to see if at least the output gears are the same size.
Anyway, hopefully this will save folks from tossing away $25 or so (what I spent on the two motors) thinking they might be able to use them directly in a locomotive as a drop-in replacement. And maybe, we'll get some options to actually modify them and use them so they don't end up in a landfill.
