Motor Journal Bearing Oil Foaming

[vibegeek]

I have a large horizontally mounted ac induction motor with journal bearings, mechanically oiled with oil rings. The free end or opposite drive end bearing is experiencing oil foaming. I am looking for possible causes and ramifications of this conditions. Any help appreciated.

 

[electricpete]

Possible Causes:

1 – Erratic oil ring operation. – Inspect in viewing window.
2 – Improper level – check.
3 – Oil contaminants – sample.
4 – Foaming tendency. (related to #3). Take a sample of oil. Shake it up vigorously for 10 minutes. Let it sit. Bubbles should be gone in an hour. If not, suspect, contaminants and investigate further with oil analysis or with ASTM foaming tendency test.
5 – Malfunction in oil supply. (is this a forced oil system or a simple reservoir?).

If bearing temperature and vibration remains normal and oil level and viscosity are acceptable, I would not be too concerned about the bearing in the short term.

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[vibegeek]

Thanks for the quick reply.

-The oil ring appears stable, non forced system.
-The oil sample results have not come back yet, but I don't anticipate problems as the motor is freshly rebuilt and new oil installed.
-There are vibration issues with the motor. The vendors own data shows multiple harmonics of 1/2RPM in the shop. We are seeing multiple harmonics of runspeed though amplitudes are not excessively high at this point.

Basically we are trying to determine what we've got on our hands and how severe the problem is.

 

[electricpete]

Rebuild is the most likely time for contamination, which is a possible source of foaming.

1/2 times and multiples are definitely not a good sign.

Neither are multiples of 1x but not quite as concerning.

Either way it most likely adds up to looseness.

I am guessing you are monitoring on the bearing housing? You can miss a lot there on a sleeve bearing machine (prox probes are better but of course not always available).

Tough call. No-one can know for sure. My best guess would be the foaming is contamination and unrelated to any vib problem (other than they both may have their origins in your recent overhaul). Evaluate the vib problem on it's own. If you get an opportunity to do prox probe monitoring that can provide better diagnosis.

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[aolalde]

If the oil grade is adequate for sleeve bearings, size and speed, I agree with electricpete; your main source for foam formation should be oil contamination. Was the oil reservoir washed with soap?

 

[rmw]

I vote for contamination. Ring lubrication indicates to me that you probably don't have forced lubrication from a common sump. If you did, your contamination would be uniform and would be occuring in both bearings. The fact that it is occuring in just one (assuming that you don't have a mechanical problem with this bearing) suggests that something got into this bearing that doesn't belong there.

rmw

 

[electricpete]

"Ring lubrication indicates to me that you probably don't have forced lubrication from a common sump"

I disagree with that logic. Our feedwater booster pump motor and pump has forced lubrication from a common sump feeding bearing reservoir with oil rings on bearing. Oil pump feeds from sump to reservoir thru orifice, then overflows from reservoir standpipe and drans back to the sump. Keeps the reservoir cooler than a static reservoir. It's a common configuraiton.

I agree with your other comments.

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[54494060]

Assuming the Motor has thermocouples at the bearings and you didn't mention any evidence of bearing temperature problems, the slinger ring and oil are performing their desired function. I agree with the contamination issue mentioned above. The rebuild shop probably cleaned the bearing journals with solvent and possibly didn't get the outboard journal cleaned properly. If this is the case, suggest you drain the existing oil, thoughly clean the journal with alcohol and add new oil.

12fish
As Iron sharpens Iron so does a man sharpen the countenance of his friends.

 

[waross]

Hi electric pete
I think the important words are "forced lubrication". Forced lubrication is not the same as a continous supply and overflow to maintain the optimum level in the bearing reservoir.

 

[electricpete]

waross – I appreciate you pointing out if something I said sounds wrong. There are many times when I am mistaken, and I'm glad to learn when I am wrong. In this case I don’t see it.

My objection was to the following sentence:
“Ring lubrication indicates to me that you probably don't have forced lubrication from a common sump. If you did, your contamination would be uniform and would be occuring in both bearings.”

I agree there can be different interpretations of the term “forced”.
1 – “Forced” as you are suggesting meaning hydrostatic lubrication (as opposed to hydrodynamic) injected under pressure at the bottom of the bearing such as oil lift system (not common in motors) or turbine hydrostatic lubrication.
2 – “Forced” meaning a pumped supply of oil through an orifice to a vented reservoir.

I assume rmw meant definition #2 and I further clarified my assumption by describing in my reply what I understood him to mean by “forced”. This seems like a safe assumption since #1 is very rare on motors. If rmw thinks I am using the wrong meaning, he is free to correct me.

Either way, the main point of his comment had nothing to do with the lubrication method but whether or not there was a common lubrication sump. Once again, I do not agree that the presence of oil rings means there is no common sump. Do you agree?

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[waross]

Yes electricpete.
You have explained your meaning very well and I accept your explanation. What is also important is that you have removed any possible confussion of those who are not directly involved but are following the threads to increase their knowledge.

I have seen descriptions of pressure prelubrication systems but I never have worked with one.
I (and four others) have had to "roll" large machines with oil ring lubrication to establish the oil film before starting. One machine was a 3000hp motor driving 5 ring lubricated single bearing DC generators. When dry, the breakaway torque was about. 2800 ft/lbs.
That was 2x200lb. men hanging from a 7 foot coupling spanner. After about 3 revolutions the oil film would be established and the rolling torque would be much less.
I wish that pressure prelube systems were more common with ring lubrication systems.
respectfully

 

[hardmettle]

I was reading above oil ring lubrication foaming problem. I have seen foaming whenever there is 1/2 rpm vibration. And foaming goes away as soon as this subsynchronous vibration goes away. Vibration geek can yu tell me amplitude of overall vibration , 1x vibration , 1/2x vibration. I am very much interested in this problem as I myself work in development of large 500-20000hp motors.
Also are you using flood lubrication system. What kind of motor it is Fincooled or Open type enclosure. Which side bearing drive end and non drive end bearing is showing foaming.