Replacing brushes

I'm not sure if this is the best group for my query but I'lll give it a
try. I have an electric lawn mower that was made by Sears way back in
1975. The nameplate rating is 120VAC - 10A. No other information is
available. The mower has an electric brake feature when shut off and
the mower has worked fine for many years (obviously). Yesterday while
mowing, the motor started to slow down and then died. I suspected worn
out brushes. I took the motor apart and indeed one of the brush wires
had disconnected causing the brush spring to carry the current until it
lost it compression due to extreme heat.
I didn't bother even checking Sear for replacement brushes as they
stopped supporting this tool many years ago. So I figured, how hard can
it be to find some replacement brushes. I went to my local ACE hardware
and found a set of brushes that were the correct size including the
brush wire guage.
I noticed that the motor commutator was very black probably from brush
dust and heat. I pulled the armature and chucked it in my drill press
so I could clean up the commutator. I used 600 grit emery cloth on a
flat stick followed by 1500 grit and then the paper side of the cloth
to clean up and polish the commutator. I put the new brushes in and
fired it up. It seemed to run fine although there was sparking from the
brushes. I proceeded to mow the lawn but after about 5 minutes of
mowing the motor started to bog down. Now it runs very rough with lots
of sparking from the brushes.
1. Was there a basic flaw in my repair process?
2. If so, what should I have done differently?
3. Is it possible the brushes are not of the right type even though
they are the correct size (e.g. not the correct composition of carbon
for that motor type)?
4, What do you suggest I try next?
Thanks in advance for your assistance.
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Did you clean out (undercut) the commutator gaps? If not, you could have a build up of cu/c dust which is bridging between windings.
Did you shape the brush faces to the commutator diameter?
Yes there are different brush compositions.
You need to strip it down, see how much the brushes have worn, whether they have worn equally and evenly, how much dust is present, whether the gaps have been bridged.
What could have happened is that the new brushes only had a small point of contact when first fitted, that part of the brushes have worn down quickly and the brush is now spanning adjacent windings. So a bit of brush tip profiling may be called for.
Or it could be that there was a build up of copper and carbon in the gaps. Because the composition is too soft..
Or, if the brushes don't conform to the comm profile, you may have bridged the gap with extruded copper - extruded by the very high brush pressure on the small area that the brushes act on..Especially, if the brush composition is too hard.
So, all sorts of possibilities...
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"I used 600 grit emery cloth "
Real emery cloth residue is conductive, if used it has to be cleaned out thoroughly.
Reply to
George Jetson
Sue, Thanks for your quick reply. I did not undercut the commutator contacts but I did visually check for copper smears/dust as a result of my cleaning/polishing. I didn't see anything but I ran the edge of my fingernail through each slot. Perhaps I should have concentrated more on that area (i.e. used a magnifiying glass and a toothbrush).
I did not check the match of the new brush surface to the commutator but the new brushes did have a concave contact surface and were the same thickness (1/4 ") as the old brushes.
It was of course hard to tell if the brush composition is to hard or soft. Maybe a teardown will help answer that question (boy, where is that CSI lab when you need it).
One thing I didn't mention in my original post was that I'm not entirely sure, but it may be that just one of the brushes was sparking when I first fired it up. If that was the case (I just don't remember), is it possible that there is a problem with the windings, either armature or field?
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George, Thanks for your reply. I wasn't really emery cloth, it was silicon carbide wet/dry sandpaper. I just used "emery cloth" as a generic term. Nevertheless you are right, I probably should have done a more thorough job of cleaning.
I'm going to break it down again and see how much damage is present. If it still looks repairable, I will be sure to thoroughly clean it to eliminate any residue.
After I wrote my post, I started wondering if perhaps the brushes weren't the only problem. Is it feasible that the windings are damaged? The mower was not bogging down when it died and I did not detect any burned wire smell when I first took it apart, but maybe the "damage" is subtle.
Reply to
George, An update. See my latest response to deastrom's post. Thanks again for your input. John john wrote:
Reply to
Sue, An update. See my latest responst to deastrom. Thanks again for your input. John Pal> > People,
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Ah, loose commutator bars. Something I didn't mention before. The individual bars are held down using various techniques on machines. Sometimes just a tight winding of cord. Larger machines often use steel wire (insulated from the bars), or a steel ring (with an interference fit and insulated from bars). Small machines sometimes use just epoxy or other glue. Obviously if they work loose, they can shift and start to bounce brushes, vibrate the wires connected to them, and literally fly-apart (as yours has apparently).
Repairing one with missing bars is not worth it except in large machines. Oh well, you did get 30 years out of her.....
Reply to
Not really work repairing, but thanks to 'daestrom' for an interesting read.
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