(starter) motor failure

[madderscience]

OK, this is NOT my MR2 (happily) but in fact the starting motor in my (very non-green) 1962 land rover.
However there are way more motor experts here than in the rover crowd so I will post my question here.

The starter motor in my truck (which is basically a scaled down version of any of the DC motors you see in conversions, 4-brush cylindrical commutator, series wound, etc.)   failed recently and upon taking it apart it turned out the commutator and brushes had failed badly.  This occured fairly suddenly (within the course of a few starts).   The commutator seems to have started flaking off bits of copper which shorted out the bars and fouled the brushes (which are still intact though), damaging it to the point where the whole thing shorted out and quit working.  There wasn't any sign of carbon scoring or excessive heat but perhaps this was the result of arcing or what have you?   The brushes weren't even fully seated yet though this starter was rebuilt 5 years ago.

Ideas?  Don't want it to happen again, or to have anything similar happen to my EV.

-Brian

[Stew-2]

Well, back in my RC days a guy would take his brand new 540 motor and run it off of an AA or two submerged in water. This would seat the brushes to the commutator and the water would carry away wear particles and keep it cool. The voltage was low enough that running it submerged in water was no problem. You would normally run it like this until the brushes were fully seated, which could take several days.

I don't know if this kind of break-in procedure would be kind of silly to do with a starter motor, but yeah, 5 years is a pretty short life.

[leitmotif]

IF the commutator is not too badly arced
1.  turn it on a lathe and take out the bad spots.
2.  Undercut the mica between the commutator segments
3.  Throw away the brushes you have
4.  Seat the new brushes using sandpaper (I think from ancient days and 300 Kw DC side of a motor generator - thank GOD that is way in the past it should be emery NOT carborendum).  Lift brush and slide sandpaper under it abrasive side to brush and slide sandpaper back and forth holding it tight against commutator.
5.  Clean with low pressure air 10 psi or so
6.  While you are at it get new bearings and maybe the throwout assy - and assemble motor

Dan Bentler

[StorminN]

The commutator seems to have started flaking off bits of copper which shorted out the bars and fouled the brushes (which are still intact though), damaging it to the point where the whole thing shorted out and quit working.  There wasn't any sign of carbon scoring or excessive heat but perhaps this was the result of arcing or what have you?

Do you have any pictures, Brian?

-N.

[madderscience]

I've already made an attempt to clean the commutator so I don't have any pictures of how it came out of the car.  I ended up patching together a different starter motor though from the parts of four junk ones I've collected and it works WAY better than the failed one ever did. 

I think what may have happened is that the starter that was rebuilt probably had its commutator turned down so much that the brushes did not fit it properly and never made good contact, perhaps causing excessive heating and or arcing, and eventually the debris from this built up enough to cause a sudden failure when things started shorting out.  There were no environmental reasons for it to fail (water, dirt, etc) that were evident so this is my only guess.

Anyway the "new" starter is a slightly newer design and it has a disc commutator and its brushes are properly seated, if a bit worn but it cranks like crazy compared to the failed one.

No more hand cranking.  Yay.   Of course no sooner did I get the starter fixed did my brakes fail, but that is totally off of topic.   Ah, the pleasure of a fine british automobile.