Forum Thread

Replacing bearing in electric motor

dealgate 6,368 1,085 December 3, 2015 at 03:49 AM
I was using my table saw last night and started smelling burning. A few minutes later the blade would not turn. I took it apart and found that one of the bearings on the motor was fried. A new motor assembly is $175, which is more than I paid for the entire table saw (10 years ago). Since it is just the bearing I figure I could replace it myself. Problem is I think it is pressed on. I could not budge it at all. So I am not sure how to remove it, how to find the correct replacement bearing, and how to put it back on. Is anyone familiar with this procedure?

In the below pic (not mine) it looks very similar to my broken one. The bearing on the far right in the pic is what needs to be replaced.

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#2
Usually bearings pressed on shafts are essentially unremovable without a very high risk for sever damage to the bearing and shaft. It may even be heated and installed in which case I'd say it's just about impossible.

If the only alternative is tossing it out, I'd try to cut off the bearing with a dremel tool, being very careful to not score the shaft in the process.

You should be able to find bearings that fit in the same "envelope" (physical dimension) as the old one but the question will be if it's up to OEM specs - e.g. speed rating, lube type, blah blah blah. But again if the alternative is $175 for a new motor (or a few hundred for a new saw) then I'd risk the $$ on my "best guess" for a suitable bearing and buy it. The worst that could happen would be the motor fries itself again and you have to buy a new motor.

There is a slight chance the motor could fail catastrophically but I can't predict that.
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#3
I've had great success with just a bearing puller and some patience. Look closely on the bearing and you'll find a part number. Most use a commonly available bearing size to save the maker some money. I always replace both bearings so I don't have to replace the other old one in the near future. A press is the best way to install the new bearings but it can be done with some pipe, or a deep well socket and LIGHT hammering. Just be sure that you apply pressure ONLY to the inner race of the bearing as you push it into place.

Try to get the old bearings off before you buy new ones. If you end up breaking something, then you need a new motor (or saw) anyway and won't have to return the new bearings. Smilie It's already broken, so the worst you can do is spend some time trying to fix it and it will still be broken. Wink

If you're not mechanically inclined, you might want to ask one of your friends that is to help you. This kind of stuff is easy for me but I've been doing it since I was a kid. Everything is easy when you know how to do it. Wink
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#4
I am mechanically inclined so will definitely try the repair. The bearings look like they are around $10. I have a gear puller that will probably work and enough sockets that I should be able to find one that fits against the inner race. Good idea replacing them both. I'll see how the bad one goes and if easy, do the front one too. Should I torch/heat up the bearing or will that ruin the grease pack?
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#5
I would try it first with just a press. It usually works fine. Definitely don't use a torch on the new bearing, but you should be able to heat it in an oven to 200 degrees F for just a little extra ease of install.
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#6
Perhaps I'm too late, but NEVER heat up the new bearing. That will likely "liquify" the grease and allow it to run out (even on so-called 'sealed bearings'), then you would have a dry or inadequately lubricated bearing.

Great pic you provided, and that helps with solving the replacement issue.
If you use a puller, it's far better if the jaws catch on the inner race. If you can cut off the outer race with a dremel, that will make working with the inner race much easier. Cutting the inner race with a dremel would be hard, due to the larger diameter of what appears to be the inner spacer on the shaft.
You may be able to do a diagonal cut on the inner race, taking care not to hit the shaft or the spacer. If so, only go 3/4 way thru the inner race, then use a cold chisel (on an angle, not directly down) to make the final break, to free it.

The other alternative would be to 'jury rig' some simple stand that would support 2 pieces of angle iron.
Get the flat edges of angle iron to sit under the opposing edges of the inner race, with the 'keyed end' of the motor/shaft facing downward, and the entire assembly only supported by the angle iron under the bearing.

Then place something like a block of aluminum on top of the end of the shaft (to prevent damage to the shaft) and strike the block on end of the shaft sharply with a hammer, jarring the bearing loose. Make sure you have something 'cushioning' under the motor assembly, so that when it 'falls free' it will not be damaged.

That's just my 2 cents...
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#7
Good news! I used the cheap gear puller below and was able to remove the bad bearing! I had to clamp the puller on with a vice grip (channel lock) and I thought it was going to break as it is just a cheap chinese POS. I used a small breaker bar and slowly turned and eventually I got a "pop" which I thought initally was the tool breaking. But it is off and I ordered a new one. It seems like a 10x30x9 bearing which is pretty standard. It actually measured 9mm in the center but I think it is considered 10mm since I could not find any 9mm centers for sale.

Getting the new bearing back on is going to be the real trick. Hopefully the socket trick works as I do not have a press.

Oh I am not going to replace the other bearing it seems good.



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#8
If your shaft is actually 9mm diameter, and your new bearing is actually 10mm inside diameter, it will result in MAJOR problems. Just letting you know, as a 1mm difference is a HUGE amount.

If it were me, I'd use a micrometer or a dial/digital caliper to measure the shaft and I wouldn't use any bearing that differs from this diameter by any more than 0.005 of an inch max. Any more clearance than that would result in significant armature/shaft vibration and 'roar'.
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#9
I think it is 10mm shaft and bearing. I'll know this weekend. Do you think a drill press could press in the bearing?
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#10
Quote from dealgate View Post :
I think it is 10mm shaft and bearing. I'll know this weekend. Do you think a drill press could press in the bearing?
You shouldn't need any press at all. The old bearing typically is hard to remove due to age, excess overheating, and tight factory fit. The new bearing should be virtually identical to the shaft diameter, and in most cases it will tap on fairly easily by just using a socket on the inner race and a very light tap with a hammer. Most times they can be pushed on by hand, even tho the fit is without any clearance or 'play' (as it should be).

Check the fit as you slide it on. If its a hair tight, you can use fine sandpaper or emery cloth to 'polish' the shaft a little (only smoothing surface, yet not removing over .002 - .003 of metal). That, plus a little lubricant on the shaft and you should have no problem at all.
You just want a snug fit, without any play at all between the shaft and inner bearing race.
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#11
I would recommend applying ice to the shaft and applying heat [Careful how much heat and the method] to the bearing.
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#12
It is a VERY normal and common practice, even for those in the profession, to polish the shaft with 320-600 grit sandpaper, just to get the surface irregularities removed, which results in a smoother shaft with tighter tolerance deviations. I can see from the OP's pic alone that the shaft has noticeable surface irregularities straight from the factory machining process.

Heating a ball or roller bearing is guaranteed trouble! If you're talking heating it at low temp with a hair dryer, then that won't do didly squat to expand the small ID of the inner race.
While it's ok to freeze a bearing to help shrink it (for its OD clearance), its NEVER good to heat a bearing, as this causes the grease within the bearing to liquify into 'low viscosity oil' and run out of it, leaving the bearing under-lubricated.

Believe me, I've changed over a hundred electric motor and equipment ball/roller bearings, and if the replacement bearing is the proper size, then installation on the shaft is a breeze. No heat/ice/press is even required.
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Last edited by Accster December 11, 2015 at 10:12 AM
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#13
Quote from dale_101798 View Post :
I would recommend applying ice to the shaft and applying heat [Careful how much heat and the method] to the bearing.
I was thinking of using my Ramset.
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#14
I'd have no issue smoothing the shaft. Depending on the diameter, I've used a drill press to hold chafts center and then use emery or polish the OD of the shaft as it spins in the press.
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#15
Quote from Dr. J View Post :
I'd have no issue smoothing the shaft. Depending on the diameter, I've used a drill press to hold chafts center and then use emery or polish the OD of the shaft as it spins in the press.
If the measurements are correct, then the motor shaft is 9mm and the bearing I ordered is 10mm. There would be no way to make that fit correctly. I hope the measurements are off.
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