Classic UK Minitrix Models  -  Motor Spring
Repair
Fixing the brushes spring.

Basically, you can't.  It has to be replaced with another.  The spring takes a full turn and a half turn around the brass stud - it will not fit over the stud, and the stud cannot be removed without breaking the fragile base into which it is fitted - unless you are careful and lucky.

However, depending on the spring you replace it with, it may just be possible to ease it over the top of the stud without having to remove the stud first.  It is definitely worth a try, as taking the stud out is not guaranteed to succeed - and if you break something (very possible) the resulting repair project will take a little time.

Here is the stud after being removed.  The narrowest end at the bottom of the photo is the part that fits inside the hole in the fragile base.  The base will bend and suddenly crack if it isn't provided with support.  But pliers firmly gripping the top, a pull with a twist / untwist action will see it come out.  Some force is required because you will see from the photo that the very bottom of the stud is flared.  It has a hollow section, and when assembled is pushed into the hole and flared to secure it in place.

It follows then that removing this stud will have enlarged the hole.  You can see how tight I was holding it from the marks left by the grip of the pliers

Here is the motor with the fragile base removed.  Note the retaining tags on the left hand side end plate, and the hole in which the tags were located.  A small screwdriver was used to gently lift the tags just enough to clear the hole.  The same on the other side, and the end plate slides to the left in the photo, enough to remove the left hand end of the fragile base.

A better view of the metal tag.  I have lifted this tag too far.  Just enough to clear the hole is all that is needed. 
The spring is one of many that I recovered from the sliding metal cover from some old floppy discs.  (Do you remember those ?)  It is much easier to get a wire that is already sprung, but if not it is possible to make a wire springy by hardening and tempering it.  Make the shape first.  I believe that you heat the wire all over to a dark (cherry) red and then quench it in water.  Clean the wire with emery so that it is shiny and then heat the wire all over until you get the dark blue tinge, and let it cool.  The wire has to be heated uniformly.  I've not done this since I was a school kid, and metal work was not one of my best subjects, so look up the proper technique.  This information is not to be trusted !

The spring above has been wound just over twice to form the 'eye'.  This is too much - there is too much metal to fit around the shoulder of that brass peg.  Insert a shaft of the right size into the eye of the spring (I used a watchmaker screwdriver shaft), and unwind the spring to the shape shown in the photo on the left.  It won't be perfect, but it doesn't matter.  It only needs to keep the carbon brushes against the commutator of the motor.  It doesn't need to exert a massive force.  If it does, it will slow the motor and wear out the brushes.

The underside.  There isn't much clearance between the underside of the fragile base and the spinning armature of the motor.   On this attempt, I didn't make that much of a mess of the hole, and thick superglue was applied before I pushed the brass stud into place - suitably cleaned up.  Don't forget to put the spring on first !  An alternative is to use epoxy resin in the hole.  Apply it before inserting the stud.  Twist the stud so that the epoxy flows around, wipe off the underside and leave it to set.  There is no strain on this component, it is just holding the end of the spring in place.
Relocate the fragile base plate in the motor - locate the tab at the brushes end first, and drop the other end (left end in the photo) into place. 

Reposition the end cap......

... and press the tags back into the locating holes.

Refit the brushes.  This is an extremely awkward process.  I found that I can fit the first one through the slot in the fragile base.  It has to be held in position with the spring - so that ties up one hand and it can be awkward to fit the second brush as the spring, fingers etc get in the way.  Instead I insert the second brush from the other side.  Still fiddly and it takes a while to get it right.  I don't swear much, preferring to leave such language for special occasions.  This is one of them. 

Don't forget the insulating sleeve.  It doesn't matter which brush it insulates, as long as it insulates one of them.

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