Electric motor machining repair (Sir John?)

[alsinaj]

Greetings! Just had the motor in my 30-year-old table saw seize. Motor is 3HP, single phase. One bearing (25/52/15mm) was shot, the other seemed good. Replaced shot bearing, cleaned and re-greased the other, and reassembled. Motor now runs smoothly when first started, then an audible vibration starts that lasts 20 or 30 seconds, then goes away, then comes back ... I have only one clue to the problem: if I tighten the 1/4" bolts that secure the end bells to what I would consider a normal torque, the shaft seizes. If I back one or more of the bolts off little by little, eventually the shaft frees up. By then, at least one bolt will be only a little more than finger tight.

My not-so-local motor repair man says that the problem is probably warped end bell(s) and that sleeving and re-boring the end bells would cost about the same as a new motor, not to mention 4 hours off driving to drop-off/pick-up.

So, my questions are:
1) Does this sound like an end bell warpage problem, or something else?
2) If end bell warping,
a) how can I tell which end bell is the problem (or both)
b) how to set the work up accurately on the lathe for boring, sleeving, and boring to size? (The end bells are not-very-meaty aluminum castings.)
c) instead of repairing the bearing seat, would it be easier to turn a plug mandrel to the bearing OD (52mm), press the end bell onto the mandrel, then turn the end bell mounting flange to be concentric with the mandrel/bearing seat, and make up paper shims to space the end bell out to its original location?

Thanks for your suggestions.

JA

[J Tiers]

Are teh bearings self-aligning? it is not that uncommon in some motors, so it is possible, just not that likely.

If not, line reaming would seem to be in order.

[bob_s]

On re-assembly did you make sure that you maintained the original orientation of the bells?

[DS_park]

Some thoughts based on past misadventures I have had with electric motors.

Is the new bearing pressed all the way on the armature? If not set all the way in, the end bells could be putting too much pre-load on the bearings. Bearings (both ) need to be fully pressed in and square in the end bells. Are the end bell surfaces that go onto the motor clean? Is are the motor end bell mating surfaces clean? Do they mate fully? Did you mark the end bells to the housing so that they went back exactly as original? Two bolt end caps can be reoriented 180 degrees. Can you try re clocking the end bells to see if there is a happy orientation. You may have to add shims. The new bearing might be a little different in dimensions from the old causing binding. Shouldn't happen but it is the only different part.

You may have tried most of these. As mentioned above, most of these were experienced the hard way by myself.

[BCRider]

Also were the bearings a press fit into the end bells? If so how did you remove the bearings?

THis is a bit of a last ditch question since I've yet to find a motor where the bearings were not a reasonable slip fit in the end bells but press fitted to the shaft.

If you think it's really close something else you could try would be to tighten the screws down correctly then give the end bells a couple of whacks with a wood mallet to shock them a little. The idea being that the bearings might just crack loose and finish seating themselves. I say wood for this as it'll deliver a proper shock load where rubber or dead blow mallets will deliver a slower build and release. You want the shock for this trick to work if it might work.

[John Stevenson]

Are teh bearings self-aligning? it is not that uncommon in some motors, so it is possible, just not that likely.

If not, line reaming would seem to be in order.

How do you line ream two blind bores 52mm diameter with out the bearing rocking about like a prick in a shirt sleeve.

[rodelu]

I've found in older american-made machines a ball bearing identified as 88505, that looks like a 6205. The outer race is 15mm wide BUT the inner race is 16.7mm wide. I cannot tell this is the source of your problem, might be worth checking.
Couldn't find a reason for the wider race; a regular 6205 was used, with spacers that took care of the difference.

[John Stevenson]

Good answers to the problem above except one,
Electric motors are very crude and they never worry about end float much, just using a wrinkly washer behind one bearing.
The fact that you have to everything so loose suggests that the bearings are not fully home of you have a new one of the number Rodelu mentions although we never see bearing such as this, this side of the pond.
So cheapest option [ free ] is check the build up and re-assemble and report back.
If the end bells have distorted, [ very common ] these can be reclaimed at no cost but lets cross that bridge when we come to it.
DO NOT touch the bearing bore whatever you do, you will only make it worse. Difference between a good fit and scrap is 2 thou at most.

[J Tiers]

How do you line ream two blind bores 52mm diameter with out the bearing rocking about like a prick in a shirt sleeve.

Not, if they are ball bearings, but ball bearings don't generally do that, so I am questioning it. I've never seen a ball bearing lock up with the sort of minor alignment issue he seems to be talking about. I've seen lots of journal bearings that do that in motors, no ball bearings. You can ream plain bearings, because the shaft stickout end isn't blind. (not all ball bearings are in pockets, either, lots are in thru holes in motors, or even in separate bearing holders, because it's easier to machine. With those there are screw-on covers. I'd still not line ream those though)

Maybe it IS ball bearings, since he seems to be quoting a size. If he means it IS a ball bearing, and not a journal, obviously that approach is not the answer and he is going to have to align the bells. The bearings will already be fitted and anythng you do will eff it up.

Seems like something is really strange though, you should NOT be able to lock up ball bearings.

If it DOES lock up, AND it really has ball bearings, THEN it may be that the alignment issue is not in the ball bearings, but there is a rub between stator and rotor. THAT fit is more like a prick in a ****, and it has to be right, or it will rub and not work.

Then he might need to have a look at the seats for the bells, might be junk in one that shoves it off to the side, or something stuck to the rotor or stator. Maybe the motor has to go back together with the bells in just one orientation vs the stator housing, or it will rub.

[Peter.]

You can stop a ball bearing turning easily enough with too much axial load.

[John Stevenson]

Not, if they are ball bearings, but ball bearings don't generally do that, so I am questioning it. I've never seen a ball bearing lock up with the sort of minor alignment issue he seems to be talking about. I've seen lots of journal bearings that do that in motors, no ball bearings. You can ream plain bearings, because the shaft stickout end isn't blind. (not all ball bearings are in pockets, either, lots are in thru holes in motors, or even in separate bearing holders, because it's easier to machine. With those there are screw-on covers. I'd still not line ream those though)

Maybe it IS ball bearings, since he seems to be quoting a size. If he means it IS a ball bearing, and not a journal, obviously that approach is not the answer and he is going to have to align the bells. The bearings will already be fitted and anythng you do will eff it up.

Seems like something is really strange though, you should NOT be able to lock up ball bearings.

If it DOES lock up, AND it really has ball bearings, THEN it may be that the alignment issue is not in the ball bearings, but there is a rub between stator and rotor. THAT fit is more like a prick in a ****, and it has to be right, or it will rub and not work.

Then he might need to have a look at the seats for the bells, might be junk in one that shoves it off to the side, or something stuck to the rotor or stator. Maybe the motor has to go back together with the bells in just one orientation vs the stator housing, or it will rub.

So basically Jerry, once distilled between plan bearings, ball bearings, blind bore pocketed bores it's obvious you cannot see the build up and don't have a clue.
I thought the OP explained it well enough when he mentioned BALL BEARINGS and even gave the size.

[J Tiers]

Don't be a twit.

I've fixed a fair number of motors, but I have never seen a ball bearing motor that could be stopped by a little tightening of a single bolt out of the four typically present. I've seen several sleeve bearing motors that can be.

Made me not even believe it was ball bearing.

warped is possible, I suppose. Odd that it would suddenly warp to where it does not work.

Did the bearing that was "shot" spin in the housing? If so the seat could be messed up. Was it?

If seat messed up, how do you know it's in straight? Yes they usually are slip fit, did they go in easily?

Not only are they slip fit, but one at least usually has room to shift axially so that expansion with heat can be accommodated. So there should be no issue with too much axial force. But there might be a burr in the bearing seat that is jamming the bearing short of its proper location. Maybe due to besring spinning.

Whacking the shaft with wood hammer seats sleeve bearings, maybe it will work on ball bearings. Do it axially as well as radially.

Are they pocketed or are there end covers and thru bored bearing bores? If thru bored you can see the ends of the shaft and bearings, and check for seating etc.

Might be good to be certain it is not a rub between stator and rotor, if it really "siezes" and is not just progressively harder to turn when tightened.

Ok it frees when loosened. Does it suddenly sieze when TIGHTENED?

[RWO]

If the bearing bores are warped or misaligned or otherwise screwed up, they can be easily fixed by boring them true but oversized and installing tolerance rings: http://www.usatolerancerings.com/

RWO

[John Stevenson]

Don't be a twit.

I've fixed a fair number of motors, but I have never seen a ball bearing motor that could be stopped by a little tightening of a single bolt out of the four typically present. I've seen several sleeve bearing motors that can be.

Made me not even believe it was ball bearing.

Some village somewhere is missing its idiot

[Mark Rand]

We had a 1000hp experimental turbine seize because a spacer ring on the shaft put 3 thou too much preload on a 7" bore ball bearing.