Is knocking the guts out of a motor a commutative event?

Well… after much frustration, my team managed to use team 1334’s arbor press to get the pinion gears off our FP motors. These luckily happened to be the required 15-tooth sun gears required for our AM planetary transmission, especially because our didn’t ship with the sun gear it was supposed to.

However… after the relief and delay of having to bring our parts to 1334’s shop, I was a little over eager to put thins back together.
My question is… if the back cap of the motor falls off, is it recoverable by pushing it back in? If it appears to operate is it then fine?

Been there, done that. That’s why I buy motors from AM with pinions already installed.

You can no longer trust that motor. Even if you get the end bell back in place the motor most likely will have internal damage that will cause premature failure.

Bah… well it was worth a shot, and it had taken minor damage already. Just losing an advantage I hadn’t expected in the first place.

We have another one though… Should I use an arbor press to install the gear this time?

Arbor press is the only way to do it. Be careful and remember that the shaft goes past the pinion so use a small socket or something or you will bend the shaft.

I hope others who have done this a lot chime in with better advice.

We have pressed our fair share of pinions on and off FP motors.
You have to make a suitable support for the pinion to press
the shaft out, and a very square (to the shaft) hammer blow
is often required to break the shaft loose before you press.
When pressing a pinion back on, you have to remember to
support the shaft at the back of the motor, and not the motor
itself, or you will ruin the motor. The last bit of travel does
require a shim with a hole in it, or a small socket, to get the
shaft to stick out of the pinion properly.


Pressing the pinion off definitely requires an arbor press with a custom milled bar stock to go under the pinion, and around the shaft, to hold it. Or, you can crack it off with a nut buster. Pressing the gear back on can be done either in an arbor press, or a large milling vise. The force of pressing needs to go straight through the armature shaft, not the motor can or endbell. Also, if it’s too tight, it’s not difficult to bend the shaft.

We have taken it off using a gear puller, with no issues. Don’t know about getting another pinion on, as we never had to do it.

We’ve been doing this a lot this week (and several stages are much easier with our new arbor press, I will say). The number one easiest way to get a pinion off a motor shaft is with a purpose-made pinion puller. Basically, there’s an outer structure which goes under the pinion and around the shaft, and then there’s a thick pin attached to a screw mechanism that pushes against the shaft. The pin is slightly smaller than the shaft, so it pushes through the pinion and all the way out (in theory). This week, we found that our pinion puller had been broken at some point, so I used the screw part of the puller in the arbor press and cut a groove in a thin steel bar to hold the bottom of the pinion. Worked pretty well. However, some of our pinions were too close in to the motor, so we had to, uh, “finesse” them off a bit with a flathead screwdriver and a hammer to get enough space to put them in the custom puller. ::safety::

Getting a pinion onto the shaft can be interesting. The only way I’ve done it so far is to use a vice with a plate that is thinner than the distance between the motor terminals so one plate pushes on the back of the shaft, and the other plate pushes on the pinion. That’ll get you all the way until the pinion is level with the shaft. You can then use either pliers that are slightly open or some kind of metal tubing to push the pinion on the rest of the way. Remember to always leave space between the motor and the pinion, otherwise it will rub, and you will never be able to get it off.

The arbor press method would probably be the same, except you lay the pinion on the table (with the size disk removed), and push the motor shaft into it (with the motor upside down) with the bar on the arbor press. I cannot account for the “correct” way as to do this, but this will work if you’re lined up COMPLETELY straight, and it shouldn’t break anything.