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MG Midget and Sprite Technical - Wiper Motor Brushes

I am replacing the brushes in my wiper motor. The new brushes I received have ridges or grooves on one end (see pic). The old brushes were worn down and smooth on both ends.

I am assuming that the grooved end goes against the motor armature. Is that correct? Are the grooves parallel or perpendicular to the segments? My assumption (again) is that they do go against the armature and that they would be perpendicular so the edges of the rotating segments don't catch and chip the brush. Am I on the right track? What is the purpose of the grooves?

Does anyone know why these are so expensive? The ones I bought were $20 and ones I found on Ebay are $32. And that is without shipping, of course. These seem like they are nothing more than a small block of graphite, similar to pencil lead.

Thanks for any information you may have about these.

Paul

Paul Noeth

Hi Paul, How does the spring push the brush against the commutator? Maybe the corrugations are part of this mechanism. As an aside, I have found great improvements in performance can be had by "blueprinting" the stator to armature clearances . e.g. make sure that they are equal both sides, use a feller gauge or similar to check and adjust if possible. OK this was on another type of wiper motor but I assume that the principle should be the same.
Hope this helps
Cheers
Rod
R W Bowers

Rod,

There is a single spring that pulls both brushes against the commutator. You can see it underneath the commutator attached to two red fiber insulators that pull against the brush arms.

On the right side of the photo you can see the brushes against the commutator. They make only a small contact area because they are square and the commutator is round. I thought that perhaps the ends are grooved so that they wear quickly and conform faster to the commutator.

Paul

Paul Noeth

Paul
Nice clean up job BTW, I agreed with your reasoning initially then a little voice said " if the wear is acccelerated due to the grooves this will leave a lot of graphite dust inside the motor accompanied by a lot of sparking mmmm? maybe not a good situation. Perhaps the grooves are for cooling at the holder end to stop excess heat being transmitted to the brush holder?? The electrical contact area should be sufficient because the holder is a square cup i.e. 5 faces in contact. None of this is really conclusive however but it's the best I can come up with. A picture from an old service manual is what we need.
Another idle thought, these brushes may be intended for another application and the suppliers in their keenness to please have supplied them as a substitute on the basis that they are almost the same.
Rod

R W Bowers

I unearthed a pair of new old stock genuine Lucas wiper motor brushes complete in their holders and the brushes have little gooves. I think it may be to aid bedding down.
Bob Beaumont

Rod, I don't think the carbon (or graphite) dust would be any worse because of the grooves, in fact, I think it may be less. The brushes will eventually conform to the commutator. If they grind their way through the solid brush end, versus grinding through the grooved end, more material will have to be removed before the brush is fully bedded.

To me, especially in light of Bob's comment, it seems to make sense that the grooves would be against the commutator.

Bob, do the grooves in the Lucas part run in line with the brush holder arm (perpendicular to the commutator segments) or cross ways (which would make them parallel to the segments)?

Thanks fellas, for helping me unravel this little mystery.

Paul
Paul Noeth

Hi Paul

I have checked and the grooves are cross ways with the brush holder arm and parallel to the segments.

I originally bought the brushes prior to overhauling the wiper motoer in my '61 Sprite. When I stripped it, I found the original brushes were in good condition and the grooves were still evident!

Hope this helps

Bob
Bob Beaumont

Lads you have the answer, albeit unusual but we shouldn't be surprised if there was one accepted way to do something and one different way the BL boys usually picked the latter.
Cheers Rod
R W Bowers

This thread was discussed between 22/09/2012 and 29/09/2012

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