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I HAVE EDITED THE TITLE TO RELFECT A NEW FOCUS OF THE THREAD. 1/8/08.

Can anyone give me the fault frequencies for the following bearing:
Torrington 240BA51 or 240BH51, MRC 148R

I have a the following for MRC 148R
FTF 0.35
BSF 1.45
2xBSF 2.9
BPOR 2.8
BPIR 5.2


What we are seeing on this machine appears to be harmonics of 12.38 orders with running speed sidebands. Although the sidebands seem often higher than the main harmonic peak.

I can't really pick ou the periodicity in the time waveform (even though I suspect it should be 12.38 orders).

The machine is 324rpm. True pk/pk accleration (from TWF) is 25 g's. The ringdown frequency of the impacts on the TWF is around 100,000cpm which is around where the harmonics of 12.38 orders reach their highest level.

It sure sounds like a bearing fault, but I would be more comfortable if I could match it up with bearing frequencies.
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Thanks Danny. Your timely information helped fill in the picture very well before the big meeting we had today on this motor.

The fault frequencies above are for MRC 148R from our Entek database. Our Entek database doesn't list those Torrington bearings. There are interchangeable bearings. While I don't expect the fault frequencies to be exactly the same, I'm surprised they are that much different.

David - This is a vertical 3500 hp motor rigid-coupled to an axial flow deep draft pump (circ water pump). Kingsbury thrust bearing on top of the motor. The bearing of concern is a rolling bearing at bottom of the motor which from the drawing looks like an angle contact bearing according to the drawing cross-section (can anyone tell by those bearing part numbers?).

We will be be following this condition very closely over the next two months. So I will be posting some more details soon and then asking some questions about severity.
Pete,

The closest Torrington numbers I can match are for a 240BH02, this may be in the same series as the 240BH51, I don't know. The information I have on the 240BH02 is as follows:
Single Row Maximum Capacity Ball Bearing - Open Type,
Dimensions are: ID = 9.4488 in or 240.00 mm,
OD = 17.3228 in or 440.00 mm,
Width = 2.8346 in or 72.00 mm

I hope this helps, it may get you in the ball park. Have you tried contacting a Torrington Rep?

Travis
Thanks Travis. I have not been able to locate any of those numbers on-line yet. Will keep trying.

================================

Now I'd like to change the subject to developing a monitoring strategy for the bearing until it can be repaired.

I realize that forecasting failure is not a precise business. But we are asked to watch this motor very carefully over the next two months until a maintenance availability at which time the motor will be removed and the lower bearing replaced/inspected (we will also check as-found alignment).

If the trend before then indicates that the motor is in imminent danger of failing and won't make the full two months, we have a strategy to take the motor off-line if it is really necessary (it is preferred to wait to the scheduled availability if possible).

I will be looking to develop a monitoring strategy based on temperature and vibration. For example, if temperature increases to 150F on bearing housing or true peak acceleration doubles, or we see an increase in the number of fault frequency families, we would ask to secure the motor.

Any suggestions or comments?

Attached is a comparison of TWF and spectrum for the last two measurements (12/11/07 and 1/7/08). Additional vib data was posted previously.

Attachments

Pete,

It looks from the twf that amplitudes have dropped slightly since December. Was the bearing recently lubricated? I would increase the frequency of the readings to at least once a week and consider lubrication when assessing the peak to peak value in the twf.

I would also collect Spike Energy readings and auto-correlate the twf if you have that capability.

I'm also with David on trying a lower fmax.

I'm betting it makes it to shutdown, but I don't have to hear the $25,000/hour lecture from the Plant Manager. Eeker

ps I don't have much thermography experience but the fact that the shaft is hotter than the bearing housing would seem to support an inner race defect.
IMO there is a possibility that (based particularly on 12/11/07 TWF: single sharp impact every revolution ) there is a crack in the inner race. Significant temperature increase ( +25*F) observed in the bearing area could be attributed to inner race creeping on the shaft rather then to a spall/pit in the race. If this is the case then fault progression is non-linear.

David
Pete,
This is a very interesting case. I'm with danny here, and suggest you shorten the measurement frequency. Once a week would be good for starters but i'll consider taking more radings when you approach the 2 months limit you speak of.

You should read also the Energy Spike levels, if they change rapidly over time, you'll be in trouble.

In the meantime my advice is to lubricate the bearing to soften the spectrum, and if its lubricated by oil, take a sample and do a particle count, i bet it's really high
"I'm not sure whether it was added or original... is there a way to tell?"

I don't know if there is a way to tell if a bearing has been added to the DB or not. I add them all the time when I find info I am certain about. The fact that what you found in yours is so far out from what Danny found would make me suspect someone added one with the wrong info (maybe calculated from physical dimensions or something). However, I don't know for sure. Just curious.

As far as monitoring, I'm with Steve. I would set up statistical alarms, and put a sister machine in the list of those measurements I was setting the statistics with. 2 sigma + 10% and 3 sigma + 10% or something similiar. Manually set the storage spec to "no stat" on those measurements you know are bad if the auto "Exclude data outliers" doesn't catch them. I find it doesn't always work real well, no matter who ole Grubbs was Smiler, or what the "Median of Absolute Deviation" is supposed to catch. Big Grin Collect data like Danny says, weekly.

My 2 cents.

Dave
Grubbs is a tricky fella. A lot of times, I will just export the data to Excel and calculate the average and standard deviation. Then I'll sort the data and see how many measurements exceed the avg + 3 sigma threshold, toss those measurements, and recalculate the average and standard deviation. If you don't have a lot of data, this gets rid of the outliers pretty quickly. If you do have a lot of data, the outliers don't affect the values very much.

Dave - Do you find that the extra +10% helps you much? I can see where you are coming from (having some extra buffer), but justifying it (technically or to management) may be difficult depending on your circumstances.
No Steve, I don't think it helps that much.

There are some trends that the plain old 2 and 3 sigma just doesn't seem to give much wiggle room. They look tight to me. A good running 1800 rpm pump in the 0.0? ips vibe range looks like it needs a little more room to roam, so I give it some percentage. Wink

It's just a mind thing with the "extra buffer". I've always done it.
And you're right, if I had to explain a rationale for setting them that way to someone who was beyond "bs'n", I'd have trouble. Razzer

Dave
The kinematic equations show this simply. Otherwise think of it as this, in one shaft revolution a point on the shaft passes some of the rolling elements, and what this point doesn't pass in 1 revolution (outer race fixed) these rolling elements have passed the starting point on the stationary race -- otherwise the inner race would have passed these balls.

--> sum of BPFI + BPF0 = number of rolling elements.
Thanks forr the comments and suggestions. Still thinking about some of it. A few responses.

quote:
David G
Do you set such a high Fmax=100,000cpm for such a slow machine?


Fmax was selected high for investigating the abnormal noise and suspected bearing defects. We also have routine route data with Fmax I think 10,000cpm where we can get better resolution on the low frequency stuff.

quote:
Danny Harvey
Was the bearing recently lubricated?

It was greased around last week of November. Shortly before the unusual noise was first heard.

quote:
IMO there is a possibility that (based particularly on 12/11/07 TWF: single sharp impact every revolution ) there is a crack in the inner race.

You can't tell from the powerpoint, but if you zoom in on the TWF, what you see is not impacts spaced at 1x, but an envelope peaking at 1x. Within that 1x envelope are BPFI spaced impacts. It corresponds to the spectral pattern of harmonics of BPFI with 1X sidebans.

quote:
Do you have enough historical data to calculate some statistical information? Instead of relying on a 100% increase (doubling), you could use the average plus 3 standard deviations. You could consider using data from the sister motors as well

We don't have a lot of history on the high Fmax acceleration measurement. I tend to think we are beyond 3 standard deviations compared to sister motors and there is no question it is much different.


quote:
In the meantime my advice is to lubricate the bearing to soften the spectrum,

We have thought about that. But we are also mindful that lubricating can sometimes make vibration worsen drramatically from one previous experience.


NOW – NEW INFORMATION:

You recall that on Monday and Tuesday morning, we had seen bearing temperature at approx 115F on housing and 130F on shaft (hotter than all the others). This afternoon (Wed) , we recorded bearing temperautre at 150F of the housing and 170F on the shaft Eeker ! Vibration level was stable at previous levels. Later in the evening, back down closer to 115/130 again.

What do you think would cause that?

Up until now I have been linking the temperature symptom and vib symptom under the assumption that something like misalignment increased the loading which caused both 1- incr in temperature 2 - bearing degradation. Now I'm wondering whether the temperature might be a result of the bearing degradation (which only occurs in final stages of bearing failure?)

We're not sure whether this might be a daily cycle or random variation associated with redistribution of the grease, or how it bodes for the future. We will be monitoring very very closely for the next several days.
Pete,

I have had two cases similar to what you are describing. One of my experiences was a motor bearing with irratic temperature spikes after I had diagnosed a bearing outer race defect. Vibration levels, as best as I can remember, remained pretty much constant.

The other case sounds very similar to your sitiuation. The motor is a 2000 hp, 1800 nominal rpm, ac motor driving a large ID fan. I had diagnosed early stages of a bearing defect. This motor has the internal RTD's so we store constant bearing temps and we can trend them. I had noticed bearing temps fluctuating during the hottest part of the day, this occured during July and August so ambient temps were at times into triple digits. But on two or three occasions the bearing temps would increase dramatically higher than usual for no apparent reason and later in the evening would return to near previous levels. During these temperature increases I could see little or no increase in vibration. Several weeks later vibration levels started to increase and more difinitive bearing defects started to show up in the vibration. The bearing temperatures never did increase the way they had earlier in the degradation of the bearing.
Like you, I wondered if this was caused by ambient increases, or lubrication redistribution.
In my case the bearing was replaced 6 to 8 weeks after the temperature spikes. Failure analysis showed a spalled outer race which originated from electrical eddy currents.

I hope this will help with your bearing. I would monitor the bearing very, very closely but from my experience you "may" be OK for the scheduled replacement.

Travis
Thanks Thud. That is some good info. Now that I think about it, I am coming to the general conclusion that grease lubrication can be a very transient phenomenon and changes in temperature when lub is poor are to be expected. This fits in nicely with another observation I have on some other motors with large bearings (6313+) running at 3600rpm. I have often found the grease discolored and races stained appearing to be overheated, but the bearing housings don't appear hot. I think the grease gets hot, then redistributes (grease location and oil partioning) and "cures" itself for a little bit. I have read somewhere others that concluded grease lubrication can be very dynamic, but I can't find where I read that.

UPDATE:

We took some shaft riders measurements:
On the motor shaft just below the motor:
11 mils N/S (all are pk/pk)
13 mils E/W

On the pump shaft just above the stuffing box
8 mils N/S
16 mils E/W

We monitored this for a few days. Saw the temperatures bounce up and down somewhat randomly without any correlation to ambient temperature.

We have pulled the motor. During removal, checked as-found alignment and found:
5 mils TIR N/S (2.5 mils offset)
15 mils TIR E/W (7.5 mils offset)

Checked hold-down bolts - tight. Checked pump lift - normal.

I'll be watching the motor disassembly at repair shop. Planned checks are:
Inspect for proper as-found grease quantity, grease location, grease consistency, evidence of mixing or contamination
Gather Grease samples from grease input area, from just above bearing, and just below bearing
Check as-found endplay
Check as-found shaft and housing fits
Check as-found rotor-stator airgap
Check upper bearing insulation resistance
Inspect bearing for evidence of upthrust, pre-existing defects, pitting from current, and other degradation mechanism
Check shaft runout during as-left uncoupled run

Does that sound like a reasonable set of checks? We had considered as-found uncoupled run, but believe it would not contribute much. We had considered checking upper radial tilting pad bearing clearances, but that requires extra disassembly and does not appear related.

Any comments: Could the relatively small misalignment be related to the bearing fault? What do you think of the shaft rider data? Any other inspections we should do?
I just realized I never came back to update this thread with our findings after we pulled the motor. Here is a brief summary of the case, including inspection findings:

MOTOR DESCRIPTION:324 rpm 3500hp vertical motor. Kingsbury thrust bearing on top, greased MRC 148R angle contact bearing (arranged for momentary upthrust) on bottom. Rigid coupled to single-stage axial flow power plant circ water pump.

INDICATIONS: The bottom bearing went south. We picked it up at around 30g's true peak/peak in TWF. (!) Spectrum shows harmonics of BPFI with multiple large 1x sidebands around each of those BPFI harmonics In the final stages before motor was removed from service, the bearing housing started heating up.

[NEW]: INSPECTION RESULTS are attached slides 8-14: Inspection confirmed the bearing was in bad shape. (slides attached). The inner race had severe spalling. The flakes from the spalls had migrated thru the bearing grease and embedded in the phenolic cage. The cage was broken in one location but still doing it's job.

LESSON LEARNED: It was certainly an easy diagnosis, and should have been caught/removed from service earlier. One thing it does show is that bearing housing temperature can increase in the later/severe stages of bearing failure. In this case I think it was likely the metal particles (from inner race spalling) embedded in the cage causing the increased friction heat (remember ball/cage interface causes the majority of friction even in a normal bearing, so you can imagine what it does to have metal particles embedded in the cage). There are of course other possible causes of bearings running hot (overgreased, overloaded), but if you see hot bearing along with other indications of severe defect, that is a datapoint that should push your concern level higher than the same vib indications without housing heating IMO.

Attachments

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