dang PM lawn mower motor

I tried running a permanent magnet lawn mower motor in an application with a
shop-made fan. I guess I didn't figure out the air flow right because it got
real hot and the rear sleeve bearing siezed. I was able to get the motor apart
and other than a little plastic melting around the edges back by the rear
bearing, it seems mechanically sound still. Yet when I try to slip the armature
back into the sleeve bearing it no longer slips smoothly in. Obviously there was

some local galling - my question is should I hone the bearing, polish the shaft,

or both? I don't want too much clearance. It would be extremely tough to mount
the rear end cap on a lathe, so any honing would have to be with very small
brake cylinder type stones in a hand drill (don't have a Sunnen with a tiny
mandrel handy).
Grant
Reply to
Grant Erwin
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Probably the right way to do it would be to polish the shaft and turn a new bearing.
If you just want to get it running again, I'd make an expanding mandrel out of AL, charge it with fine valve lapping compound and work it in and out of the bearing a few times. Even just an AL rod, a couple thou smaller than the shaft with lapping compound might be enough to clean it up.
I suspect you'd never get all the lapping compound out and that long-term reliability might not be that great. OTOH, it doesn't sound like it's going on the space shuttle either...
Reply to
Jim Stewart
Lapping the bearing sounds like the way to go. I would suggest no coarser than 400 grit lapping compound As Jim said it may be difficult to get all the lapping compound out of the bearing. I have had reasonable success with paint thinner ( mineral spitits? ). Run a saturated patch in the bore many times until it comes out "clean". Even then, there may be microscopic particles of compound imbedded in the bore. It is this quality of "imbeddability" that makes sleeve bearings work as well as they do. With a hard shaft (journal) and soft bore, imbeddability says abrasive particles in the bore will be imbedded beneath the surface and do no harm.
Bob Swinney
Reply to
Robert Swinney
How about cleaning the bearing out with a slotted shaft (Like a 3" long piece of 1/4 " diameter rod with a 1" long slot hacksawed down from one end.)chucked in a drill with an appropriate length strip of crocus cloth (or stronger abrasive if needed.) stuck in the slot and wrapped around the shaft in the direction of drill rotation.
It's worked for me...
My nosey mind just has to ask, wuzzat a line voltage lawn mower or a battery powered one?
Jeff
Robert Sw> Lapping the bearing sounds like the way to go. I would suggest no coarser
Reply to
Jeff Wisnia
It was a Black & Decker plug-in lawn mower that I got free used in 1996 and scrapped it for the motor, which I rebuilt with a new front end bearing. I also glass-bead blasted the case and painted it, plus I designed a machine around it. It's dang inconvenient having the motor crap out on me. The hole is pretty small for the split rod technique, too small to get a finger into. I'll go look at everything carefully with a 10X lens and think some more.
GWE
Jeff Wisnia wrote:
Reply to
Grant Erwin
Last time I looked into a similar project they were using 110v weed wacker motors > It was a Black & Decker plug-in lawn mower that I got free used in 1996 and
Reply to
bamboo
A lot of the cheaper motors use 8mm shafts, the bearing housing is often 22mm - just the same size as a 608 skate bearing. Just press out /in/on on and off you go and they work very well. . you might be lucky.
in case anyone's concerned about the speed it woks out to skating at around 12mph..
Grant Erwin wrote in news: snipped-for-privacy@corp.supernews.com:
Reply to
mike
It might be a little slow going but in the old days of model airplaning we always lapped our engines cylinders to sleeves and crankshafts to bronze bushings by using some Pepsodent tooth powder and light oil. Work in a crosshatch for about a minute. Very smooh. It all rinses out also.
Reply to
daniel peterman
Sometimes you can get a needle bearing the will fit in place of the bronze. I say go for the meadow look!
Reply to
Tom Gardner
OK, I dug out my 10X visor and really looked at the motor tonight. It wasn't siezed in the bearing at all. Rather, the plastic around the sleeve bearing had melted and flowed until it touched the end of the commutator rings that the brushes slide on. Eventually the plastic solidified and stuck onto the commutator rings, and that's how the motor seized up. I put the armature in the lathe and turned off the melted-on plastic, and then I used some 1000 grit wet/dry in a slit pencil spun on a drill motor to lightly polish the ID of the sleeve, which had some melted plastic in there too, and now the motor goes back together and the shaft turns, but it appears that something that had located the armature shaft axially has melted away or moved, since the armature can now drop back a little and then it fouls on the housing. I think I can make a little brass ring bushing and that will step it out away from the sleeve bearing enough. If that fails, it's back to the drawing board. But now I'm hopeful I can salvage this motor.
By the way, this sleeve bearing looks neither bronze nor oilite. It looks like cast iron to me. It's definitely silver colored and very smooth. And with great certainty I can say that there's no way in hell a 608 bearing would fit back there, not without remanufacturing the entire back end of the motor.
GWE
Reply to
Grant Erwin
Grant Erwin wrote in news: snipped-for-privacy@corp.supernews.com:
Grant, Use a wave washer back there. They are specifically made for axial location of electric motor rotors. Smiley, I think is the manufacturer.
Reply to
Anthony
But if you don't cut the lawn., it'll produce more oxygen, absorb more CO2, and you could reverse global warming single handed. Big Al would be so proud!
Reply to
Tom Gardner
Grant sez:
" . . .> By the way, this sleeve bearing looks neither bronze nor oilite. It looks like
Hey! If it looks like a duck, wallks like a duck. . . . . My guess it that it is sintered iron.
Bob Swinney
And with great
Reply to
Robert Swinney
So, what cooled the motor to keep that melting thing from happening when it was in it's original grass cutting application?
BTW, I've never encountered a PM motor in what would appear to be an "AC only" application like your lawn mower. Have PM magnets gotten so cheep that it's less costly now to use them rather than than field windings? Or maybe it's a size/energy density thing.
My inquiring mind wants to know!
Jeff
Reply to
Jeff Wisnia
Also commonly known as a "bellview" (sp?) washer
Gunner
"If I'm going to reach out to the the Democrats then I need a third hand.There's no way I'm letting go of my wallet or my gun while they're around."
"Democrat. In the dictionary it's right after demobilize and right before demode` (out of fashion). -Buddy Jordan 2001
Reply to
Gunner
No. Belville springs are dished washers, and usually (not always) associated with very high compression forces. Wavy washers are specifically for shaft axial take-up applications, and are available in compression ratings down to a few ounces.
We use Belville springs as the "final cushion" for high pressure die-pressing operations where the die pins must equalize on the multiple loads.
LLoyd
Reply to
Lloyd E. Sponenburgh
I pulled apart a black-and-decker 110V electric lawnmower awhile back, and I'm pretty sure it had permanent magnets.
For cooling, there was a centrifugal fan on the shaft below the deck, some holes in the deck near the center, and the plastic motor cover was left about 1/2" above the deck.
Maybe a perm. magnet motor has better torque characteristics for mowing? Maybe a lower startup inrush (good for when you are on 100' of extension cord)? Or maybe the perm. mag motor was better at meeting a weight or cooling target.
Wait, I think I have it. I recall that if you turned the switch off (maybe even if you unplugged it) the motoir would stop spinning fairly quickly. The perm. mag motor was probably so they could apply a braking force.
My makita chop saw has a brake, I wonder if its via the motor or an external brake?
Dave
Dave
Reply to
spamTHISbrp
The fan. It's plastic, and it was cracked. In desperation, I figured out how to weld up the cracks using an old soldering iron.
They are used on millions of electric lawn mowers. I don't know anything about the economics other than you can get them nearly free sometimes.
GWE
Reply to
Grant Erwin
The switch on an electric lawn mower is a squeeze-type dead man switch. It has 3 positions - completely let go, no AC power to the motor. Fully squeezed, full AC power to the motor. In between, no AC power to the motor but the brushes are shorted together, which brakes the motor.
GWE
Reply to
Grant Erwin
One I had was old enough that it had no dead-man switch. Just an on-off rocker switch.
Dave
Reply to
spamTHISbrp

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