mmagliaro

I bought a handful of the 10mm x 10mm x 15mm long square-body Minebea motors from eBay and did some testing on them. I have not tried on in a locomotive yet. Starting voltage is 0.55v (free running). By 1v, they make enough torque that it is rather difficult to stop the shaft by gripping it with fingers, so I expect these would make plenty of power for a model loco. By 3v, I could not stop the shaft by gripping in my fingers no matter how tightly I could squeeze. 0.5v 15 mA 6v 26 mA 9v 68 mA 12v 202 mA warm. Given the big jump in current about 9v, I would follow the suggested "6-9" volt rating on these and not push them above 9v, so use some in-line zeners to limit it to 9v. I did some brutal "stall testing" on it. Gripping the shaft with pliers, a full stall for 30 seconds at 6 volts pushes the current to 325mA. The case got very warm, but the motor did not fail and afterwards, it still ran as well as before (would still start at 0.5v, and still drew the same current as before). I repeated this test at 9 volts for 30 seconds. The current was a positively brutal 515 mA. The case got too hot to touch, but the motor still survived and still ran as well as before. I disassembled a few. The armatures are definitely 6 pole. They all run whisper quiet and they start up so slowly that you can count the revolutions of the shaft by eye. These are typical qualities of a high end Faulhaber or Maxon coreless, so I was impressed. I took them apart because I wanted to see if it was possible to convert them into dual-shaft motors by bracing the armature and pressing the shaft partially through it. The short answer is "yes", but it requires attention to detail when reassembling. Mark the case and be sure to put it back together exactly the same way, and to be very careful with the finger brushes. It took me 3 tries of disassembling, pressing the shaft through, and reassembling before I was able to do this and still have the motor run as well as it did before I took it apart. Although in all fairness I have not tested one in a loco, the motors are very smooth, powerful, and quiet. And they withstood full stalls and high heat without self-destructing, so I think they could be used in place of a Mashima. They can run slower than a Mashima 1015 or 1220 without cogging, and they have more power for their size than a Mashima. HOWEVER, they are not built as robustly. The Mashimas have heavy bearings and tough brushes in them. I doubt these Minebea would tolerate abuse, heavy shaft loads, or stalls/overloading as well as a Mashima can. Given the precipitous climb in current between 9v and 12v, I suspect that over 9, we start hitting mechanical limits, possibly the bearings, so I would limit the voltage to 9. A couple of back-to-back 3.3v zener diodes inline with the motor would do the trick there. I would test with an ammeter when running in a loco to see if the current climbs over, say, 150 mA. If it does, I would limit the voltage further with higher voltage zeners. It will limit the top speed of the loco, but most model locos run way too fast as 12v anyway. Sorry I do not have a set of photos of all this. But I've played around with these and wanted to dash off what I had found.

Brilliant trick of sanding the bottom of the dome over a piece of tubing like that. I have just turned two brass domes of my own, and rough-filed the bottoms, but I have been puzzling over how I would get that bottom curvature to at least be close to my boiler radius. Great idea. Incidentally, turning model parts in a hand drill works well for a lot of things, including even turning down wheel flanges. It's not as accurate as a lathe, of course. But in my experience, we really don't need that level of precision in our models. A decent electric drill turns true on its axis, and we aren't putting a lot of cutting pressure on it. The normal issues that one addresses with a lathe such as run-out and spindle deflection really don't matter as much as you might think for situations like this. After all, a dome turned within .005" would look perfect to our eyes, while a machinist would probably think that .005" of error would be "terrible" on a lathe. Thank you for the post. It is a real help.