Electric motor bearing fit damaged by overgreasing?

[lostmeche]

Hello!

We have a 4160V, 500HP, 1200RPM 5000 frame electric motor used in vacuum pump application. It is belt driven. The motor took a crap after the DE bearing temperature shot over 300 degrees and the shaft seized up. We pulled the cap and saw the a substantial amount of grease on the windings. We saw that the shaft was also damaged. As if the inner race spun on the shaft. We got a report back and the shop said the reason the motor failed is due to over greasing. The bearing housing fit on the DE is also damaged. So I believe it spun on the shaft and it spun in the housing.

The kicker is that the motor was just rebuilt 4 months ago and at that time both bearing end were sleeved. So we believe the sleeves didn't hold up and the shop is blaming us because they don't want to warranty a $50k motor.

So my question is if over greasing can cause a bearing to lose its fit. I know a over greasing is the major failure of bearings. I don't have much experience so I don't know in what ways over greasing can damage to a bearing. The DE bearing is a NU324 and the ODE is a 6318 insulated bearing.

 

[SwinnyGG]

What do your maintenance plans show for re-greasing of these bearings?

If the bearings are over greased and the grease gets cooked, the bearing overheats which causes the failure. Overheating the bearing overheats everything it touches- so yes, over-greasing a bearing can eventually lead to damage to the bores the bearings are installed in, especially if the failing grease stays in place long enough for the bearing to seize.

 

[JJPellin]

If the motor was overgreased to the point that there was grease in the windings, you have no warranty claim.

Johnny Pellin

 

[lostmeche]

jgKRI: It was 89g/81g DE/ODE every 3 weeks. That makes sense but they just gave us a vague report. I believe they are just taking the easy way out.

@JJPellin: The motor didn't fail due to the grease on the windings. If it failed because of that then I wouldn't have made this post.

 

[JJPellin]

That was not my point. So much grease was applied that it had filled the grease cavity and squeezed out into the windings. This is evidence of severe over-greasing. I would probably add grease to these bearings every six months. Every three weeks seems excessive.

Johnny Pellin

 

[romke]

How did you decide how much grease was needed for regreasing, how much grease was in the bearing after the overhaul and were the new bearings more or less run in under controlled conditions or immediately put into full service?

 

[electricpete]

I'd have to agree with others that the grease quantity added seems excessive (what is the bearing type?) and it is very plausible that overgreasing could cause the symptoms. That's certainly not proof that overgreasing caused the failure... BUT to cast doubt on that scenario we'd need evidence of an alternate cause. Got any pictures?

The kicker is that the motor was just rebuilt 4 months ago and at that time both bearing end were sleeved. So we believe the sleeves didn't hold up...

I've got to say we've had many motor bearing housings sleeved and I haven't seen any failure due to the sleeving. It's also hard for me to imagine exactly what stresses would lead to failure of an installed sleeve (after installation the sleeve is in compression within the housing, with additional compressive stresses from bearnig load over portions of the sleeve bore... sort of a passive role). If you want to support your theory, maybe there is some evidence you can find by inspecting the failure.

...Otherwise the best you can do is ask for all available records about the sleeving - what is the sleeve material and sleeve thickness and diameter prior to installation, housing diameter and roundness prior to installation, final machined diameter. Were any screws used at the sleeve/housing interface. If you're very lucky you might use their responses to prove the sleeve was too thin, screws too big, or not enough interference. There are certainly other ways that the sleeving might be problematic, but absent anything in the paperwork or inspection it doesn't seem like you have any grounds to blame the sleeving.

=====================================
(2B)+(2B)' ?

 

[Michael-Lu]

The best you can do is to ask about all the available records for the sleeve - the thickness of the sleeve material and the thickness and diameter of the sleeve before installation, the diameter and roundness of the casing before installation, and the diameter of the final machining. Are there any screws on the sleeve/shell interface? If you are lucky, you can use their answer to prove that the sleeve is too thin, the screw is too large or there is not enough interference. Of course, there are other ways that can cause problems with the casing, but there is nothing missing in the paperwork or inspection, and there seems to be no reason to blame the casing.

Supply Forever Global Industry Co., Ltd.

https://www.supplyforever.com/

 

[dArsonval]

Apologize for bumping this older thread. (I need to escape the Electric Motor forum more often

Excessive grease is problematic and can be a cause for a motor to fail, but it's likely not the root cause of the motor's failure in this instance.
The OP noted that in previous history of the motor, both end bracket fits were "sleeved" during a repair.

The OP also states the Drive End bearing is an "NU" type bearing.

Noting that the motor's mechanical repair was focused on the end bracket housing fits and that they were sleeved in a repair...
the D.E. shaft journal is the focus point and likely the contributing factor to this failure.

Here's what [can] happen in a repair facility related to NU bearings.

First of all, an NU bearing has a different shaft fit dimension than a radial ball bearing fit.
If a machinist is not paying attention, the shaft fit can be mistakenly machined for a radial ball bearing fit.
An NU fit is tighter than a radial ball bearing motor fit.

(How do I know this? Because I, myself have committed this mistake once-upon-a-time.)

Secondly, romkee's suggestion about test running the motor in "controlled conditions" is valid.
A motor with a drive end "NU" bearing must be tested with the shaft out-fitted with
a side thrust load in order for the bearing to function properly during a test.

With no shaft side thrust load on the bearing during an initial start-up test, the bearing can
be ruined at that instance. It usually is evident during the test by its squealing and noise making.
What happens at that point, (if no one knows any better) is... more grease is pumped
into the motor to temporarily silence it during its test.

So, in my opinion.... there is ample reason to suspect the repair. Perhaps more so, the test procedure.

The D.E. fit was possibly incorrect due to the wrong tolerance machined.
Or, (and additionally) the fit was incorrect... period.

My guess is... an incorrect or (not within tolerance) drive end bearing journal shaft fit is what caused the beginning of the cascading failure events...
not the excessive grease.

Unfortunately, as noted by JJPellin the excessive grease if pumped in place by the end user would void a warranty.

Yes, the grease contributed to the excessive heat, etc. It made a mess all over the winding...
and at first evaluation it was [stamped] as the cause of the motor failing.

I don't believe it is.

Here are published tolerances used by Electric Motor Repair facilities in the U.S. for a number 24 bearing journal fit.

Radial Ball Bearing Shaft Fit: m5 tolerance class, Max. 4.7257 inches Min. 4.7250 inches

Cylindrical Roller Bearing Shaft Fit: n6 tolerance class, Max. 4.7262" Min. 4.7253"

An additional thought. The motor is in a belt drive application.
This means the failure was likely accelerated by possible over tightening of the belts.
Typically, the opposite drive end goes out first when belts are too tight, but because we know the
drive end "fit" is suspect, I believe it began its failure at that location first.

John