Bearing noise due to overgreasing ?

[edison123]

Two 3000 KW, 6 Pole, 994 RPM sq. cage motors driving reciprocating gas compressors have NU 244M + 6244C3 bearings on DE and NU 244M bearing on ODE. Motor OEM (Hyundai) specified to charge grease (Alvania 2) every six months of continuous running with 2100 gms in DE and 1000 gms in ODE, which is the original total grease quantity for a new motor.

After regreasing as per above OEM schedule and quantity, we hear a high frequency squeal from both the motors. This noise is cyclic with a 3-4 second duration between peaks like a beat. No such noise before regreasing. We do not have any SPM or other noise measuring devices at present. No abnormal vibrations and the bearing temps not more than 60 deg C.

1. Did regreasing introduce this noise and if so, how ?
2. Normally, for such slow speed machines, regreasing amount is small and interval is large. Why in this case, the OEM has recommended a full grease replenishment within six months of continuous running?
3. After regreasing the above entire quantity, not a bit of old grease came out of grease outlet. What happened to all the old grease (3 kilos) ?
4. Do we need to open up this motor immediately and check the bearings, grease etc?
5. Finally, is it not abnormal to use rolling element bearings for such a large capacity of 3 MW ?

 

[electricpete]

1 - I am pretty sure the regreasing did cause the squeal although I can't say how (did you follow proper greasing procedures.. clean drain port, run with drain open or relief after greasing, etc).

I have seen bearing do very similar thing after accidental overgreasing. (we got a new motor from the factory, someone wasn't sure if it had grease in the bearings, instead of disassembling to check they attempted to add grease until it came out... stopped attempting after injecting about 16 ounces on 6313 bearing cavity... it did come out when they ran the motor).

I am not sure of the reason. It has continued to intermittently make that noise ever since. But not hot. We'll get a look at the bearing soon. Suspect possibly extra grease causes skidding??? (I'm not sure if that makes sense... I do know skidding is somewhat lub dependent, but we do have normal weight of the motor load on the bearing)

2 - good question.
3 - It is very normal NOT see grease appear at any time during regreasing or run after regreasing. The bearing cavity is usually intially 1/2 full. Subsequent charges start to fill the remaining half. I feel better if I don't see any grease... if I do then I know there is too much in there.

4 - Our motor has run for 3 or 4 months. After initial regreasing first 14 minutes the noise was almost constant on 5 seconds off 5 seconds back on etc. Then it got less frequent. Still it remains and you usually hear it if you wait a few minutes. No guarantees on your situation, but we have gotten some life out of ours. My gut feel is that you are safe to run for a few months if necessary perhaps with extra monitoring but you will eventually need to take a look and may have to replace those bearings.

5 - Ball bearings with grease lubrication are supposed to be limited in the neighborhood of D*N ~ 300,000 where D is shaft diameter in mm and N is speed in RPM... above that they supposedly may overheat. For oil lubrication the same bearing can run a little faster. For horizontal motors, I never see ball bearings on machines above a few hundred horsepower (few hundred kw). For vertical motors I often see greased ball bearings on the lower guide bearing even for large... have a 4000hp 1200rpm vertical with ball bearings on the bottom (232R). I never understood why the 300,000 DN limit doesn't apply here for vertical motor lower guide bearings... perhaps because those bearings don't see much load?

 

[edison123]

epete,

As usual, your comments are precise and helpful. I am also not worried unduly about taking out the motors out of service right now since a planned shut down is around the corner.

I still can't digest the fact about regreasing quanity and interval given by the OEM.

 

[edison123]

The guide bearings in vertical motors do not see much of radial loads.

But usage of rolling element bearings in such large capacity (though at a medium speed) is the first time I am seeing in my 18 years experience. May be the motor designers who participate in this forum (teco comes to mind) can shed more light on this.

 

[stardelta]

Sounds like it could be a skidding roller in one of the bearings. Its quite a common occurance on overhauled machines and we see/hear it all the time. It will probably disapear when the grease flows fully round the bearing and when the motor is mechanicaly loaded.

 

[jbartos]

Suggestion: The grease should have been approved equivalent by the Hyundai since there may be differences in grease composition, viscosity, heat conductivity, etc.

 

[edison123]

stardelta,
The bearings were regreased about a month back but beat like noise has not gone. Both the motors are running on load, one at 100% and the other at 50%.

jbartos,
The grease used for regrease was from OEM.

Another interesting thing I noted was that the stator ammeter fitted near the motor shows a swing of about 10 to 20 amps in tune with the bearing noise. When the noise is hitting the maximum, the current also hits the maximum. Any ideas on this phenomenon ?

 

[electricpete]

Going back to the original problem… I can throw out one more possible scenario. Excessive grease increases friction torque. This can lead to intermittent/excessive spinning of the outer race within the housing. Less likely would be spinning of the inner race on the shaft. Spinning of races is one thing that is well known to come and go intermittently and make a chattering noise.

Another scenario... contamination introduced during greasing? Hard to imagine that would give these symtpoms.

Oscillation of motor amps in similar time to bearing noise… that is very strange. I have never seen or heard anything like it. I have no idea but I can throw a few wild ideas:

Intermittent high friction in the bearing is actually adding load to the motor. It's hard to believe there could be any sustained difference in real power loading and still not produce a very hot bearing housing. But perhaps it is very momentary binding which causes the current to swing high. (is the curent high for only a short time?). If this scenario were true obviously a big cause for concern.

One other possible explanation – maybe there is an actual mechanical load (pump?) increase going on. More torque on coupling causes it to stiffen up and can add load to bearings. So load affects both current and bearings. Kind of a stretch.

Hard to imagine any way a bearing could affect the air gap (magnetizing current).

Just wild guesses. I'm sure you have a few gueses of your own.

Good luck. We will be going into our motor within a week or two and I'll let you know what we find.

 

[edison123]

pete,

Spinning of outer race simultaneously at two motors. Unlikely.

The connected load is a gas boosting reciprocating compressor, whose performance has not changed a bit.

I am also first time seeing current oscillation with bearing noise (it entirely coincides with the noise level, max 10 A / 20 A swing with maximum noise amplitude.

Somehow this bearing noise and the stator current swing are related. but how ?

 

[GusD]

Hello edison123.

I have a tough time to believe that your problem is related to overgreasing.
When we overgrease a bearing, its operating temperature
will increase and if there is no room for that grease to expend to,bearing may fail in short order.
The problem related to overgreasing would be noticed right after you run your motor under load and short after your regreasing.
Your noise is most likely coming from the Roller Bearing NU244.Roller bearings have a peculiar behavior when running at no load.They have a tendency to "squeel" due to skiding of the rollers when motor is operated at no load.
Note;The Rotor wieght does not load up the bearing sufficiently to eliminate this condition.For this reason one should not let motors with roller bearings run for extended periods without load.Ball bearings are not as prone to the "skiding" condition.If bearings skid for a long time they will score the races and in fact destroy the bearing.
As for the grease quantities recommended by Hyundai for regreasing of their motors,I have reason to believe that they are excessive.I had the same concern with Hyundai motors and contacted the Manufacturer about their recommended amounts for regreasing of most of their motors.We have 600/800/1500/2500 and 5000 Hp motors.The 5000 hps are sleeve bearings all others are Anti-friction
Balls and Rollers.After I contacted the Factory we reduced the recommended amounts by large amounts.I remember one particular motor that called for something like 800 Gr and we brougt this down to 200 Gr for a 6 month regreasing schedule.
As for the fact that you don't see any old grease being expelled from the bearing,it may be because the grease in the drain pipe may have hardened suficiently to the point of not letting grease out.I would remove the drain pipe and you should see grease coming out of the cavity when you grease the bearing.If it does not come out of the drain pipe it is possible that it is going into to the windings.
Edison, contact your Vendor and make sure that they reevaluate their recommended amounts for bearing relubrication.As I mention before,you have to be carefull when running Roller Bearings without load.They will be damaged if "skiding" happens and motor continuous to run.The Overgreasing may destroy the bearings but not to cause the noises you reported.The overgreasing fauilure is fairly rapid ,either in bearings or seals.

Good Luck.


GusD

 

[edison123]

Thanks GusD for your suggestions. I too think the regreasing amount (too high) and interval (too low) are abnormal.

Both the motors are running, one at 50% (150 Amps) and other at 100% (300 amps) loads.

You said you contacted hyundai about their grease schedule and amount. Did they give any explanation for their original recommendation for such grease schedule and amount ? How did you finally arrive at the regrease quantity on your own or on hyundai's advice?

 

[automatic2]

Was the drain port cleaned prior to greasing the bearing(s). What you may be experiencing is an overgreasing of the bearing (lack of old greases ability to exhaust). The result is a displaced seal/shield or worse, the overheating of lubricant which can lead to immediate bearing compromise. The frequency is very likely early signs of bearing failure. Don't leave it too long as your bearing seats will erode.

 

[electricpete]

Out of curiosity - Is there a belt load or flywheel on this machine? Those would be determining factors for load on the bearing (along with rotor weight). The only

Is there an rtd monitoring the bearing outer ring temperature? Given this size of machine I would think there would be. If not then judging bearing temperature from housing have to consider where the bearing is in relation to the accessible surface measurement.

It is right to question the notion of overgreasing based upon temperature. The example I provided otherwise does suggest overgreasing resulting in noise and no high temperature, although I have no real explanation. To add a little to that example: ite was clearly overgreased (16oz added in 6313, then motor ejected most of the grease when started ), started making the noise, and never overheated as indicated by bearing housing surface temperature. It was a new motor never run at our facility so there is small possibility that it was simply a problem unrelated to our overgreasing... BUT we installed 5 other new motors, didn't overgrease any of the others, didn't see noise on any of the others. Draw what conclusions you will. I'll know more shortly.

 

[electricpete]

I see now the roller is in the ode. So belt or flywheel even if present on shaft end would not explain the roller. Why is there roller in ode? Perhaps that is just an easy way to allow motor rotor expansion... control position on de and use roller on ode end. I have never seen it done that way on smaller antifriction bearing motors.

Is the following sequence correct?
- New motors.
- Operated ok for awhile.
- First regreasing was performed
- immediately started making noise

I know you have followed the manufacturer's recommendation for grease type... I would ask them if they could provide evidence of the type of grease which was in the motor when new so you can check compatibility for yourself.

 

[DanDel]

My first impression about 'where did all the old grease go' would be what GusD touched on...that the grease was under pressure when being pumped in and it ended up going past the seal and into the windings. This may account for the cyclic noise and current draw.

 

[electricpete]

I didn't pick up on the fact that it is an enormous quantity of grease.

Using SKF guidelines
Gp = 0,005 D B
where
Gp = grease quantity to be added when replenishing, g
D = bearing outside diameter, mm
B = total bearing width (for thrust bearings use total height H), mm
For 6244 we have D=400, B=65 => Gp=130.
1000 grams is way too much.

Now, I look at some other parts of the SKF guideance:
"For large roller bearings having a bore diameter of 300 mm and above, the high specific loads in the bearing mean that adequate lubrication will be obtained only if the bearing is more frequently relubricated than indicated by the diagram, and the lines are therefore broken. It is recommended in such cases that continuous lubrication is practised for technical and economic reasons. The grease quantity to be supplied can be obtained from the equation below for applications where conditions are otherwise normal, i.e. where external heat is not applied
Gk = (0,3 ... 0,5) D B ´ 10–4
where
Gk = grease quantity to be continuously supplied, g/h
D = bearing outside diameter, mm
B = total bearing width (for thrust bearings use total height H), mm"

** In summary it looks like they are recommending continuous grease-feed for grease lub of large roller bearings. Is it possible that 1000g/6mo and 2100g/6mo represent quantities to be added by continuous feed (in conjunction with self-releiving path), rather than as a single re-lub?

 

[electricpete]

Apply the above formula above for continuous relubrication of NU224. It has same D and B dimensions as 6244.
Gk = (0,3 ... 0,5) D B ´ 10–4
Gk = 0.4 * 400* 65 * 1.00E-04
x24 hrs/day
x30days/mo
x6 mo
=4,500 grams per 6 months.
Now this is only a factor of 2 off from 2,100g, where the calculated batch lub quantity was a factor almost 10 off. This seems to suggest again that the quantity may have been intended for continuous relub. Manufactuer can surely clarify. Maybe I am completely wrong and they have something special about their motor.

To Dan - how does grease getting into the winding cause current fluctuation? I am just curious to understand.

 

[electricpete]

Going back to the question of whether ball bearings and grease lub are appropriate on this large motor:

I may have botched up the DN rule before (I think D is mean diameter, not bore). For 6244 and NU244, SKF lists the max speed for grease lub as 1500rpm. Derating for higher load than assumed or external heat sources.

SKF's speed rating is based upon thermal considerations at an assumed load. The assumed load is that which gives an L10h life of 150,000 hours. Now I understand as you said that lack of load explains how vertical motors use these big grease lubed rolling element bearings at higher speeds.

 

[edison123]

As per SKF General Catalogue, relubrication interval for our bearings (220 mm ID) is about 3000 hours for roller bearings and 6000 hours for ball bearings for a speed of 1000 RPM. This works out to 4 month (roller) and 9 months (ball) of continuous running. (These intervals can be extended by two times if the bearing temperature did not exceed 70 deg C.)

If the relubrication interval is shorter than 6 months, then replenishment grease as per DB rule (0.0005 D B grams) is 130 gms at intervals of half of the relubrication interval (i.e. every 1500 hours for roller bearing and 3000 hours for ball bearings fro our case). SKF recommends a maximum of three grease replenishments after which the entire grease has to be replaced. If the relubrication interval is more than 6 months, then all grease had to be replaced after six months. So, apparently six months is the upper limit for any grease in use and I guess that Hyundai was right after all.

For vertical shafts, the above relubrication intervals have to be halved. Could anyone give valid reason for this restriction for vertical shafts?

 

[electricpete]

I can guess that on vertical shafts there is more of a concern for grease running out but that is just a guess. Anyone else?

Regarding SKF recommendation to REPLACE (vs replenish) the grease after 6 months, for smaller motors I know that very few people will actually replace the grease because it generally requires motor disassembly.

I don't understand how you say the Hyundai is following SKF recommendations. SKF recommends 130g at a time (for either bearing by my calc since they both have same D and B dimensions) whereas Hyundai is recommending 1000 or 2100g at a time. What am I misunderstanding?

Also as a side note it appears SKF recommends continuous relub for this roller bearing. That recommendation is not followed.

A quick comment on minimum load to prevent skidding. SKF recommends minimum load to prevent skidding is > 3% of dynamic load rating for roller bearings and 2% for ball bearings. Since roller bearing has higher load rating to begin with the minimum load is much higher for roller.

Can you identify which end the noise is coming? If the outboard end (with the roller) that is consistent with skidding.

Dynamic load rating per SKF of the NU224 is 172,000 pounds.
To prevent skidding half rotor weight W/2 > 0.03*172,000
W > 10,000 pounds. Is your rotor weight at least this much? Note that skidding is lub-sensitive.

Sorry to do a lot of talking on this thread. I take it as an opportunity to review this stuff. Hope others will continue to contribute as well.