Motor bearings and more

[Sonic EscapeParticipant @sonicescape38234]

I want to have a silent low rpm motor, something around 70rpm,  and after a bad experience with a new motor + reducer from a dubious brand I decided to buy something used. Since I lived in Spain now, I picked this motor eléctrico trifásico. I like how it looks without the usual cooling fins. I guess the low power to weight ratio of this motor makes them unnecessary. Since its speed is already so low I think I could reduce it to 70rpm in a single step with a pulley.

That pink twisted cable was rolled inside the junction box. There is no marking about it. But when I brought the motor outside the resistance between the two wires was 97Ω and it started to increase. I think there is a temperature sensor inside.

I don’t have yet my inverter to test the motor but I noticed that it was not spinning very easy. I found this to be strange because I think the motor was not unused since the rubber seals on the junction box are intact. So I had a look inside.

There is something wrong with the top bearing. I’ve usually noticed that worn bearings feel like they have sand inside. But this is different. It feels like it’s out of grease. And it hangs a little, but it’s always in the same position. Like it has some dent inside. So I suppose I have to replace it? Or I’ll wait for the inverter to come and give it a test?

Also another thing. I want to have a look on the other bearing. But how I could pull it out? The bottom of this motor is flat. So it sits in a blind hole. Also I don’t know how to pull the rotor with enough force without damaging the windings.

[noel shelleyParticipant @noelshelley55608]

If old stock the grease may have dried out. A very small screwdriver may allow you to remove the shield to inspect wash out and replace the grease. then carefully press in the shield. To remove the other end gently tap the end casing off but be prepared for the wiring needing to be disconnected MARK all leads and their positions. IF the NDE bearing comes with the rotor then just do the same as the DE, if it stays in the casing pry out the shield as before. Good Luck. Noel.

[MacolmParticipant @macolm]

Also, this is what can happen if a ballrace cage fails. Despite an apparently fatal failure here of this bearing in a vacuum cleaner, it still ran, scraping every second rev. Before it gets this bad there may be a variety of symptoms.

[DiogenesParticipant @diogenes]

In the video it looks like there are ends of some material protruding out past the lip of the shield – either an internal  seal is damaged or it’s foreign matter.

 

[gerry maddenParticipant @gerrymadden53711]

Sonic, the action of the bearing in the video is indicative of a bearing that is radially tight by a handful of microns.

Is there any detectable axial play in the outer ring? If not, remove the bearing and check the shaft seating diameter. This is probably oversized or not very round.

Gerry

 

[Diogenes]

This is the bearing without the shield. It shouldn’t be more grease inside? I flooded it with WD40 but nothing changed.

I was able to remove the rotor from the back cover. Lacking many tools I was creative with the baby lathe wood shipping box.

Fortunately the bottom bearing remain attached to the rotor instead of the blind hole. This bearing is fine. It feels like a new one. Both have the same amount of play. Barely noticeable. I’m going to order two new ones. Bearings are cheap.

If I’ll use C2 bearings, like this one for example, it will be more quiet than a regular bearing? Or it will require too precise alignment of front and back motor covers?

On Diogenes Said:

In the video it looks like there are ends of some material protruding out past the lip of the shield – either an internal  seal is damaged or it’s foreign matter.

 

I didn’t noticed anything inside. Just the lack of enough grease.

 

On gerry madden Said:

Sonic, the action of the bearing in the video is indicative of a bearing that is radially tight by a handful of microns.

Is there any detectable axial play in the outer ring? If not, remove the bearing and check the shaft seating diameter. This is probably oversized or not very round.

Gerry

 

That could indeed explain the behavior. Next week my bearing extractor will arrive and I’ll check this.

[gerry maddenParticipant @gerrymadden53711]

In view of the internal condition, this is almost certainly a tight bearing giving that cogging effect.

Unfortunately when you remove the bearing, if you are unable to pull it off from behind the inner ring, you could well create some new notchiness from balls being forced over the sharp raceways shoulders. However this will be quite ‘long wavelength’ compared to the notchiness due to the tightness due to a bad seating. I’m sure you will notice a difference though.

Yes, you will need to fit new bearings. But you will need to check and correct the seatings first or new bearings will not be quiet.

Finally, don’t use C2. These are bearings will LESS than normal radial clearance for special instruments etc., the last thing you want for a conventional electric motor. You should use normal clearance (CN) OR C3. Are there wavey washers inside the housings ? If yes, both clearances are ok. If not, NORMAL is probably the right spec.

Gerry

[Sonic Escape]

On Sonic Escape Said:

 

… I didn’t noticed anything inside. Just the lack of enough grease…

That looks like enough grease. Don’t be tempted to fill the bearing with grease because the churning effect as the motor turns can lead to overheating.

George

[gerry maddenParticipant @gerrymadden53711]

… in fact if there isn’t a wavey washer at one end of the rotor, and there is room for one, you should fit one before you re-assemble.  Someone may have missed it out during some previous rebuild.  (A light axial preload is the only way to make these radial bearings run quietly/smoothly in a motor application.)

Gerry

[Sonic EscapeParticipant @sonicescape38234]

There was a wavey washer at the bottom of the blind hole. But I don’t understand it’s purpose. It makes contact with the bearing outer race. But this is useless since the bearing can’t move relative to the blind hole. Since it is press fitted there. Is not like the bearing can move axially and the washer is holding it in one place.

[DiogenesParticipant @diogenes]

It might have felt like a ‘tight’ fit when cold, but probably designed to have the potential to creep as the motor warms up, the washer is there to keep the bearing against it’s shoulder on the shaft – don’t forget that the housings will expand radially as well as the casing expanding axially if hot.

[noel shelleyParticipant @noelshelley55608]

As Gerry has said use C3 bearing fit for electric motors and fit the wavy washer in the blind hole. When assembling the motor use a soft head hammer to tap things all round to ensure true alignment of the ends and stator housing, even the slightest misalignment will cause things to run tight. Good Luck, Noel.

[gerry maddenParticipant @gerrymadden53711]

As Diogenes says, the outer rings will move axially as the alloy housings expand, aided by vibration…. provided of course that they are the correct size!

When fitting the housings over your new bearings, heat them a little to make them easier to slide on.

However the purpose of the wavey washer is to provide a constant axial preload to the bearing. This will put all the balls into load simultaneously and continuously. Without it, the bearing will have a small load zone (in line with the radial forces, which every ball will have to enter and leave as the bearing rotates. This makes noise and vibration.

Another common source of noise is the rings being out of square, particularly the inner rings. So ensure the shaft seating shoulders are clean and the rings properly pressed up against them.

Gerry

[Author]

Posts