Thursday, December 22, 2011

How to Re-wire a Potted Motor, Part 11: Armature & Worm Removal

With the grease wicks removed (and the motor brushes if they were willing to come out), it is now time for us to crack the motor open. In this entry we will learn to remove the motor cover and perform the crucial task of safely removing the armature. We will also learn the difference between a worm and a worm gear.

First we need to remove the motor cover. It is held on by two screws on either side. Here you see one of them. (Although the one on this machine, from the 1930s, is silver, if you have a 15-91 or 201-2 from the 1940s or ‘50s, this screw may be black.)

Begin unscrewing it--but not all the way.

Get it to about this point, so there’s enough space for the cover to come off and then some. But make sure the screw is not so loose that it’s going to fall out.

Do the same with the screw on the other side.

Then, pick up the motor with both hands, oriented as shown in the photo. (My left hand is absent because it’s holding the camera.) Be sure not to grab the bobbin winding assembly with your left hand. Using your right, gently wiggle the motor cover off. It will likely be on there quite tight, so it may take some doing. Continue wiggling it gently and patiently and it will come off.

You should be grasping the left side (but not the bobbin winder) with your left hand as you wiggle with the right.

Once it’s off, we see this. You can set the motor cover aside as we won’t need it for a while.

Next, turn the assembly around so it’s like this.

Look inside the circular part of the housing and you see this:

This is technically called a “worm.” The part that it fits into, the fiber gear that we left on the handwheel, is called the “worm gear.” People will often mistakenly refer to the part we’re looking at here as the “worm gear.” Be aware--if you speak to a technician, for example--that they’re two different things. The worm is shaped like a cylinder, the worm gear is shaped like, well, a gear.

Look inside the left end of the motor. We see the butt end of the armature nestled inside. Don’t touch those copper wire strands; the only place you ought to touch are the metal plates indicated by the red dots in the photo. Those metal plates encircle the entire armature.

You should be able to just get your fingertips onto those metal plates. As you do, spin the armature in place.
As you rotate the armature...

...the worm rotates along with it

As you spin it with your left hand, look inside the housing. Observe that as you rotate the armature, the worm also rotates. That is because it is attached to the armature shaft via what are called set screws or grub screws. Those are screws that don’t have any mushroom heads, they are all shaft.

We need to locate those screws. There are two and they are both located on the right side of the worm. They may be obscured with old grease. I use a Q-tip and a small amount of rubbing alcohol to dab at the right side, rotating the worm, until I can see a screw slot.

Don’t waste your time cleaning the worm, because a) in a second we’re going to have it out of the machine, where it’s much easier to clean and b) if you wipe a Q-tip on it too vigorously, you’ll find the cotton all starts to unravel on the sharp teeth of the worm, leaving you with a lot of strands you now have to clean off.

Once you’ve exposed a screw slot, rotate the armature until you can find and expose the second screw slot, so that you know where both of them are. Now we need to remove both of these screws in a very specific way.

Warning: The Point of No Return

Folks, everything we’ve done up until now is reversible and even if you’ve broken something, replaceable. But now we're about to go past the Point of No Return. If you strip a screw while trying to remove the worm, it’s Game Over and your attempted repair of this motor is finished. So please follow these instructions carefully!

First off it’s absolutely imperative that you use the correctly-sized bit to fit this screw, and that bit is a Brownells #150-4. (If you know of another manufacturer's bit number that also fits perfectly, please let us all know in the comments.)

Secondly, if you’re new to screwdrivers and bits you should read the entry about using proper hollow-ground bits.

Thirdly, you must have practiced enough with screwdrivers and slots that you can consistently keep the driver completely perpendicular to the screw’s face, without letting it wander off on an angle. Trying to unscrew something at a bad angle is how you strip a screw.

Fourthly, you must have a screwdriver handle that lets you provide good downward force, to drive into the screw.

Lastly I should point out that if you’re a lefty, you may find this difficult.

If you’re all set, let’s proceed.

Loosening the Worm

Using your left hand, rotate the armature to a position where you can access one of the screws in the worm.

Place your screwdriver into the slot, and press it into the screw firmly. Keep your left hand on the armature to prevent it from rotating around, which would mess up your connection with the screw. Keep the screwdriver handle dead perpendicular with the screw face.

Once you feel you’ve got a good connection with the screw and that nothing is going to slip, unscrew it, pressing into it firmly. Remember that this screw may not have been loosened--ever.

Here’s a tip: You don’t need to unscrew it all the way, just loosen it three half-turns from its original position. (That’s technically 1.5 full turns, but I find it easier to count out three half-turns while doing it.) That will be enough to remove the armature. Loosen both screws in this manner.

Removing the Armature

Once each of the two screws in the worm have been loosened three half-turns, with your left hand you should now be able to slide the armature out of the housing.

However: On a small percentage of machines, you may find the armature’s metal plates--those same plates you’re grabbing with your fingers--are being "blocked in" by the black enamal wrapping on the field windings. If that’s the case, do NOT try to force it out; leave me a comment and I’ll prepare a separate entry for what to do.

If you’ve gotten the armature out, congratulations. Remember to hold it only by the metal plates as you handle it. (You can safely hold it from the shaft too, though it’s likely quite greasy.)

Once you’ve got the armature out, inspect the base of the shaft closely; you should see a thin fiber washer on it, as seen below.

If you don’t see the washer, check again to make sure it’s not just stuck to the base of the shaft, perhaps under a thin layer of grease. If it’s definitely not there, look into the motor, as in the photo below; the washer may be stuck to the bearing.

I can see the shininess of the brass bearing, so there's no washer stuck here.

If you find the washer stuck to the bearing, pluck it off carefully with some tweezers, and stick it onto the armature shaft, where it belongs, for safekeeping.

The armature can provide some important clues as to the health of the motor, but we’ll get to that later. Although I would normally clean it at this stage, I’m going to shuffle the order a bit as I’m sure you’re eager to see if the wiring on your motor is salvageable, which is an entry or two away. So set the armature aside for now, preferably wrapped in plastic to keep dust off of it and to keep any grease on it from migrating around your work area. Be sure not to lose that fiber washer!

Now that you’ve removed the armature, the worm may have fallen from between the two bearings it spins between (or it may be held in place by all of the old grease).

Either way, you can pull it out with a pair of tweezers or angled forceps, my tool of choice. The tolerance--the space between the worm and the bearings--is often quite tight, so you will only be able to remove it by pulling it straight out, as opposed to at an angle. That’s why I like the precision the angled forceps afford.

Once you’ve gotten the worm out, you can clean it off using rubbing alcohol. Don’t forget to finish removing the set screws and clean those as well. Then set them aside in a safe place.

Lastly, you can now clean all of the old, nasty grease out from under where the worm was mounted.

To clean it, I use a Q-tip dabbed with rubbing alcohol, because with the armature removed, there is no chance any alcohol will run down the shaft and into the motor.

Go on to Part 12: Detaching the Field Core


  1. Thank you for posting all this. A dear friend gave me her 15-91 after she discovered her family thought a porch would be a good storage place. This will help me get it back into great condition.

  2. I just keep learning new things in every step, Thanks,
    Diana in CNY

  3. OK I can't wait for Saturday when i can get into my own motor.......

  4. Once you have in your possession one of these machines, its hard not to become an avowed connoisseur.

  5. These are fabulous. Period.

  6. I have loosened the worm screws, but the armature does not want to come out. Suggestions?

    1. Impossible to diagnose the problem without more details and preferably, photos.

  7. I have loosened the worm screws to the point of almost coming out. I carefully grab the metal part of the armature like the instructions, but it does not want to come out of the housing. I cannot see anything obstructing the shaft. perhaps there is some old grease clogged up inside or something. What are my options to get the armature assembly out so I can continue the restoration. I will hopefully have some pics soon for you to see.

    1. You are still not providing enough detail; it's like telling a doctor your stomach hurts, and then asking him what's wrong with you. Are the metal plates hitting the windings? Are you getting any movement or zero movement? Does the shaft spin freely? Are you able to rotate the worm independent of the shaft?

  8. The metal plates are not hitting the windings. I am getting zero movement. The worm does not rotate independant of the shaft. When the shaft spins, the worm spins as well.

  9. Now we are getting somewhere. If you have loosened the screws the appropriate amount (as indicated in the tutorial) and yet the worm will not rotate independently of the shaft, that means old grease has hardened into a glue-like substance that is keeping the worm stuck fast to the shaft, preventing the armature's removal.

    This is easily remedied. First off, remove both screws all of the way. Then, using a Q-tip, dab a few drops of rubbing alcohol both into the screw holes and on either side of the worm. Allow it to penetrate in, then try rotating the worm independently of the shaft. Eventually you'll find the worm breaks free. Rotate the worm and shaft in opposite directions to break up that old grease, then you should be able to remove the armature no problem.

    1. I have applied isopropyl alcohol as instructed to the screw holes and the area at the ends of the worm. It still will not budge. The worm and the shaft will not move in opposite directions.

    2. Keep at it, it will. If you've done it correctly, there's nothing else but old grease holding it in place.

  10. I have applied isopropyl alcohol as instructed to the screw holes and the area at the ends of the worm. It still will not budge. The worm and the shaft will not move in opposite directions.

    I had a similar situation. I noticed that there was another Bakelite screw at the end of the armature shaft. I removed this and the end of the shaft was exposed. I placed a brass drift on the exposed end of the shaft and ever, ever, EVER so gently tapped on the drift with a very small hammer and the worm was loosened on the shaft. I had the entire assembly on a mat on my bench with the shaft and brass drift horizontal, so that if the shaft were suddenly released nothing would fall to the floor, but I stopped as soon as I sensed a tiny motion of the armature shaft and removed the armature by hand. If anyone follows this method, please proceed with caution, and good luck to you.

  11. Superb tutorials but stuck on the removal of the armature from a potted motor. screws on worm loosened 3 turns. Armature turns independent of the worm now. armature moving couple of mm out but is then blocked by the windings. any advice welcomed!

    Adrian McDonald Scotland

    1. A small percentage of motors have armatures that are trapped by the enamel-wrapped field windings. The only way to get the armature out is to spread those windings further apart--easy does it--with your fingers, and then easing the armature out. This is one of those "three-handed" tasks.

      There is, of course, a chance that you will irreversibly damage something by doing this--but what have you got to lose? If the motor's not working now, the answer is nothing!

      For the record, I've successfully spread stubborn windings on several motors and not had a problem. Hopefully yours will go the same way.

      Do take care not to seize the armature by the fine copper wiring, but instead try to get purchase on the metal plates, as indicated in the tutorial. I find latex gloves give me that little extra bit of purchase.

    2. Instructions worked a treat. Armature out and fibre washer retrieved. Many thanks.

    3. Note to all who come across this problem, Adrian wrote:

      "The armature is out! I left the motor in the kitchen for 12 hours to warm up - my workshop shed is decidedly chilly in winter - and the windings then eased fairly easily and the latex gloves were a big help. So many thanks - and on to the next steps."

      Didn't occur to me that some of you might be working in unheated spaces, and it makes sense that in a warmer environment, the windings will be a bit easier to bend.

  12. Hello Rain,
    I'm on this step on my 201-2 motor. A machine I just got and the motor did run but seemed a little slow. I've just got the cover off the motor and the copper wiring is black not copper colored. Should I be concerned about this?
    I had no oil or grease on my bushings or springs they came out clean. I have not tried to remove the armature yet. Is this black just discoloration over time or is it a sign of something bad?
    Any thoughts?
    DeAnna .... I would not have been able to even get this far without your wonderful blog. Thanks :)

  13. I believe a Chapman bit #68 works for that worm!