The secret life of sewing machine motors

Including a look inside a motor. 😮

I have this Sew-Tric motor from 1960s that had seen very little use, but was making a horrible screeching noise.

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I thought it would be a waste to throw away a perfectly good expensive motor, so I decided to open it up and see what was making the noise.

Careful: Never touch the copper wires!

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My cat is a junkie, he gets high on glue and lubricants! He gets to it so quickly, it’s scary. I can’t leave anything out, or he’ll be licking the oil off the machines or the glue off the sticky tape! Urghhh…

The source of trouble

But he wasn’t the source of the screeching. 😉 It was a small amount of rust on the rotor and the strator (the ribbed steel bits).

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The fit between the two is quite tight, so that little bit of rust made enough of a protrusion to make the rotor scrape the strator, hence the noise.

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So I polished it off with a bit of high grade sand paper (grade 5000).

Re-assembly

That was easy. Now for the hard bit: putting it all back together!

First of all remove carbon brushes if you haven’t done it yet. At this point they should just fall out if you shake the casing with the taped up coils.

Next, the mummified copper coils have to fit into the strator which has to fit into the casing. Fix it in place with long screws and use cable holders to keep the wires under control.​

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Prepare the rotor: it needs two washers on its short end, a normal one followed by a thin warped one.

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Insert the rotor into the strator, the rotor’s end should fit into the hole or socket in the butt of the casing. This may require a bit of “pursuasion” to get past the taped coils.

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Check the wires. They should not be getting into the fan like in the photo above! Use cable ties to keep them securely away from the blades. This is a bit of a fiddle.

Rotate the rotor by the axis – it should move freely.

Place two washers onto the rotor axis like you did it for the other end.

Now put on the other half of the casing. This is tricky because you have to align four things at the same time: the two free wires have to pass through a hole going to the plug, the two long screws should fit through their respective holes, and the rotor axis should go through the hole in the butt end.

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Fix the screws with nuts lightly and check that the rotor moves freely by rotating the axis. If it is stuck, you have either over-tightened the nuts, or something is interfering with the blades, in which case you need to open the case and try again.

Now insert the carbon brushes. They are basically sticks of graphite with a spring. They should be longer than 7mm, if they are too short, it’s time to get some new ones.

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Stuff them in and screw on the caps.

If you look through the grid on the back of the motor, you can see the carbon brushes touch the rotor ring.

Next, attach the lamp if you’ve got one.

Wiring the socket

Now for the wiring! There is a schematic for the socket embossed on the lid. Work out which is the right way up. 🙂

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All you have to do now is connect the right wires to the right pins. One of the pins takes two wires, the others take one each. The earth pin is screwed onto the casing as it is metal. Note that the motor itself must not be earthed.

There are no colour markings in the schematic, it’s not important.

Mounting onto the sewing machine

An external motor is mounted behind the main column using a special bracket. Most motors have two mounting screws in a row on the side where the axis comes out, such motors take a straight bracket, of which there are many interchangeable variants. Some motors have mounting screws in different places, so they need their own brackets to match those screws.

All of those brackets however mount onto the same place on the sewing machine: where you also can mount a hand-crank. It’s a ridge with a hole with a bolt under the balance wheel. The ridge fits into a groove on the bracket keeping the motor vertical. Some early machines were only sold with a treadle, so they don’t have such a mount point. It can be made, but you’ll need to be creative since you haven’t got the ridge.​

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I forgot to take a picture without the belt. 🙂 But you get the idea!

Old machines work very well with a rubber motor belt. It stretches and gives a good grip. It goes over the groove on the balance wheel that normally takes a treadle belt.

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But before you can attach the belt to the motor, you need to mount the pulley. Simply slip it onto the shaft but don’t tighten the screw, as you will first need to determine the right place for it.

Adjust the belt tension by moving the motor left and right on the bracket, and moving the bracket up and down on the ridge. The rubber belt should be slightly stretched.

Turn the balance wheel and allow the pulley to slip on the motor shaft and settle in its perfect position. The pulley can go onto the shaft either with the screw collar towards the motor or away from it, whichever is best for your configuration. Tighten the screw.

Testing

That’s it! There is only one thing left to do: plug it in and hold your breath! 😮

Buy me a coffee 🙂