bearing damage of 3500kw motor 1

[fiaz1404]

Hello,

We have 3500kw motor, NDE bearing#NU1048. Today motor NDE bearing was lubricated with approximately 40 grams grease. Type of grease was as per motor manufacturer recommendation. After twenty minutes of greasing, smoke was observed from bearing and motor was stopped immediately. We checked the bearing and found bearing seized and damaged heavily. Please see attached picture for damaged bearing.

Is it possible that such a severe damage has some relation with greasing which was done 20 minutes before this failure or it is due to some other reason? Please note, this was a new bearing which was replaced one month ago.


Thank you,

Ahmed

 

[waross]

The old rule of thumb was to lubricate once a year, whether it needed it or not.
Small motors got one pump of the grease gun, larger motors got two pumps and the largest motors may get 3 or 4 pumps. When there was some doubt as to if the motor needed grease, it was not greased until the following year.
How was the bearing lubricated?
Was the drain plug removed and the bearing allowed to drain?
40 Grams seems like a lot.
In 20 minutes the bearing may still have been draining.
My guess:
The bearing was improperly installed or was damaged while being installed.
Someone noticed that the bearing was in distress and tried to save it with a lot of grease.
At this point the bearing was minutes away from total failure and was probably beyond repair already.
The human factor:
When a worker has a job that doesn't go well for some reason or other he will often keep an eye on the problem area for awhile. Someone may have known the bearing was in distress for some time before the failure.
How was the bearing greased when it was replaced. Common practice is to remove the drain plug, introduce grease into the top fitting and then let the bearing drain the excess grease.
Over greasing when the bearing was installed is a possibility.
More bearings fail from too much grease than from too little grease.
If the bearing failed due to over greasing a month may be a more reasonable time frame than 20 minutes.
The grease may have had no effect or it may have extended the time to smoke from 10 or 15 minutes to 20 minutes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I like that explanation best, makes perfect sense per my experiences.

The majority of problems from over greasing of electric motor bearings are related to what's called "churning" of the excessive grease in the bearing. It cooks the grease and can harden it, increases load, shortens life etc. etc. etc. But one other possibility I was taught was that seals are good for up to about 500 PSI, but many grease guns can put out 1500 PSI of pressure and blow out the seals if you over do it. When that happens, the grease liquifies, runs out, often into the motor housing, and your bearing is left with NO grease. Failure in 20 minutes is then very possible, if even that long.

"Will work for (the memory of) salami"

 

[electricpete]

NU1048

We don't have any big horizontal machines with greased bearings in that motor hp range.
That is one big cylindrical bearing. Info attached.
Bearing bore - 9.45"
Bearing OD - 14.17"
Bearing dynamic load rating - 118,000 lbf

What speed is the machine?
What would you estimate the rotor weight? If that's not available, what would you estimate the entire motor weight to be (rotor is typically 30% of total).
Answers to above may shed light on whether it may be prone to skidding based on low load combined with temporary poor lubrication conditions from injecting fresh grease. Ordinarily skidding wouldn't result in rapid bearing failure, but still it would be interesting to see how the bearing fits within application guidelines (I'm guessing it's oversized, and unless it's relatively slow speed it may not even be suitable for grease lub based on D*N criteria).

Another scenario that comes to mind is that some very large contamination was pushed into the bearing and got caught between the rollers and cage, causing one or more rollers to stop rolling. Could be there was some seriously dried or (Jeff's comment) previously cooked grease in the cavity that got pushed into the bearing working area during relubrication and acted like contamination (jamming between roller and cage).

For smaller ball bearing I would wonder about shields/seals being pushed into the bearing by grease gun, but I doubt this huge roller bearing had seals or shields. Jeff's scenario of grease running out and causing failure in 20 minutes...I'd have a hard time buying that for any size motor fwiw (would take a lot longer than that before the tiny quantity required for lubrication remained).

40 grams is around 1.3 ounces. That does not seem unreasonably large amount to me for this big bearing, although more info would help...

Can you describe the grease type, grease addition frequency, and (as Bill mentioned) any discusion of grease adding technique? Is the machine running or secured while adding? Add through a grease zert (hopefully)? Plug left removed to provide path for expulsion during run after greasing? Any grease observed coming out anywhere during greasing? Anything unusual reported during greasing before the smoke? When was the initial grease charge in the housing established? (most likely the last bearing replacement... when was that). Was the initial grease the same type as used during relubrication? Was the machine unusually cold when started?

Based on absence of mention in op, I'm guessing no other maintenance (other than relubrication) was done immediatley prior to the failure?

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[electricpete]

When was the initial grease charge in the housing established? (most likely the last bearing replacement... when was that)

OK, just saw bearing was replaced one month ago. Apparently ran good for a month and then had this problem during first relubrication?
By the way, why was the bearing replaced?


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[electricpete]

Also, was any vibration recorded since bearing replacement? Any indication of bearing distress prior to relubrication? (non-sync frequencies present?)

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[electricpete]

Does the machine have bearing outer ring temperature monitoring? Any records of how hot it ran after installation and prior to relubrication?

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[waross]

Another scenario that comes to mind is that some very large contamination was pushed into the bearing and got caught between the rollers and cage, causing one or more rollers to stop rolling.

Well said Pete. That thought was hovering on the edge of my mind but the words to express it didn't come together.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fiaz1404]

Sorry for delay in reply,

Waross,

Bearing was completely filled with grease when it was replaced last month, yesterday it was first time greased after bearing replacement.

Electricpete,

Motor rpm: 895
Motor weight 24000 kg
Grease type shell alvania rl3
Freq 100gram/2000h
Yes, machine was running when grease was added
Greasing tool is hand operated grease gun
Grease drain was kept open
No grease coming out observed during greasing
Yes it was the same type of grease.
When greased motor, atmospheric temp was zero deg centigrade and bearing temp was 30 deg cent grades.
After replacement the bearing this republication was first activity on the motor.
One month ago old bearing was damaged
Over all vibration was less than 1 mm/s throughout the month except two weeks before it went up to 1.2mm/s for two three days and then again less than 1mm/s up to bearing failure. During failure vibration raised up to 10 mm/s. This vibration is taken by sensor installed horizontally on bearing.
Before lubrication temp was 30 deg cent grades while after damage it was 89 deg recorded by this sensor

 

[waross]

Pete's suggestion of contamination moved into the bearing working area by the new grease is looking good.

two weeks before it went up to 1.2mm/s for two three days

This may be an indication of contamination. Maybe not.
Thanks for the kind comment Jeff.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[squeeky]

There are a couple of worrying statments

The bearing was filled with grease. This is too much. See above comments on over filling.

The grease used was recommended by the manufacturer. Of the bearing or the motor? Normally the motor will have a standard rating which will include ambient temperature. This is normally 30 degrees Celcius. At zero degrees it's another story. There was a previous post someone in Canada having a problem with motor bearings stored in sub zero temperatures. The liquids will solidify at different temperatures and thus separate and become non-homogeneous. This may happen at a less extent whether lubricant is meant to operate between 80 and become100 degrees. The Shell grease meets all those requirements. I did note that the vicinity is given as the100 cst at a40 degrees and only 10.2 at a100. So at a30?? It is recomendedfor low temps. So rule out lubrication being incorrect.


One month is 672 hours. Why did you lubricate? 2000 hrs is about 3 months.

Some motor OEMs have the grease ports closed for sealed bearings but this is unlikely but not impossible on this size motor. Have you checked that it is clear? Back to over lubrication.

Was the input great nipple cleaned before grease pumped in? Contamination.

Don't rule out installation methods with overheating the bearing to alignment.

Your motor is rotating relatively slowly but if your shaft or steel work near the bearing is magnetized you can get spark erosion. If your NDE bearing is not insulated you could get circulating currents but then you should see something on the NDE bearing too. If the lubrication has insulated the shaft, which it should do if the lubrication is working, the static builds up on the shaft and when high enough to overcome the insulation, it flashes to earth via the bearing. You may pick this up with an ociliscope with one lead on the shaft and the other to earth. You would need a Gauss meter to pick up low magnetism ora compass if it is strong enough. Shaft grounding rings would be required. Try Impro /Seal.

Please keep us updated. Thanks.

 

[electricpete]

Forgive me stepping in…

One month is 672 hours. Why did you lubricate? 2000 hrs is about 3 months.

You missed the quantities posted:
100gram/2000 hrs = 0.05 g / hr
40 g / 720 hrs = 0.056 g / hr.

So they are exceeding recommended rate by only 10%. And for a given rate, smaller portions at shorter intervals provides better lubrication than larger portions at longer intervals.

Having the bearing itself being "full" of grease does not sound like a problem to me (there's not a lot of empty space in cylindrical roller bearing). If we're worried about to much grease added initially we should ask if room was left in the bearing housing cavity (somewhere around 50%).

Good points about cleaning the inlet fitting.

I haven't plugged in the numbers for minimum load calc or max speed calc yet. Will post that later.


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[macmckim]

Based on the bearing damage, the bearing was skidding in the raceway and then became jammed, causing the extensive
damage /wear patterns on the rollers. I beleive there was some contamination / materials in the end-bracket's grease tube.
Possible the repair shop did not clean the grease tube out prior to re-assemblying the motor. 40 grams of grease is nothing for a bearing of this size. When the motor was regreased, the "crud/ foreign material" was pushed into the bearing
by the the new grease. It would be great to see more pictures of the bearing and end-bracket (where the grease comes into
the bearing journal area)

 

[electricpete]

My intuition was wrong - the simple application calculations show no concern:

Minimum load to avoid skidding is met:
SKF website indicates minimum load for NU1048 at 895rpm is 7,100N.
Mmotor = 24000 kg
Mrotor ~ Mmotor / 3 = 8000 kg
Wrotor = Mrotor*g = 78400 N
Wrotor / 2 = 39200N >> 7,100 N

Dm*N is below max for grease lub:
Dm*N = 300mm * 895rpm = 268500 (units mm*rpm)
For medium load CRB used as non-locating bearing, SKF suggests max Dm*N = 300,000




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