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 ?

 

[edison123]

Hyundai says "charge 2100 gms in DE and 1000 gms in NDE every six months of continuous running". These are total amounts of grease used in the first fill by Hyundai. So, that agrees with SKF recommendation to replace the entire grease after six months.

As far 2100 gms in DE, remember there are two bearings NU244 + 6244. The noise is definitely coming from DE.

The total motor weight is about 14 tons. So, I guess rotor weight will at least be 6 tons. Remember, this noise is coming even when the motor is fully loaded or 50% loaded. So, obviously it is not related to load.

SKF recommends continous relube only with roller bearings of ID over 300 mm.

I still cant understand the stator current hunt in both motors after relubrication.

 

[electricpete]

You are right and I was wrong about continuous relub. Bore is 200mm<300mm as you say.

Using SKF guideline based on 0.005*D*B (same DB both bearings) I come up with same quantity for either bearing, although the replenishment interval is different.

The SKF recommendation to replace the entire grease every 6 months involves removing all of the old grease from bearing and cavity and adding new grease. You probably have to disassemble the motor to do it, unless you have some way to clean out all the existing grease in-place. SKF does not recommend adding that grease all at one time to a bearing without removing existing grease.

 

[electricpete]

(correction 220mm).

On the subject of loading to prevent skidding, it is the radial load on the bearing that is of interest, not the torque loading applied by the pump/compressor or whatever. In your case with no belt or flywheel it is very much dependent upon the weight of the rotor.

 

[electricpete]

&quot;...this agrees with SKF recommendation to replace the entire grease after six months.&quot;

I think you are interpretting the SKF guidance wrong. &quot;Replenish&quot; means to add more to the existing. &quot;Replace&quot; means to remove all existing and repack again. Not many people choose to replace (and I don't think that's what you did) even though SKF recommends it.

 

[electricpete]

I wasn't paying close attention to your configuration. Two bearings on DE... roller and ball. One bearing on ODE... roller.

That would change the way to compute minimum loading. Presumably each end carries half of the rotor load, but how to divide among the two bearings on the DE??? IF we assumed that DE load divided equally among those two (a big assumption), then we would double the minimum rotor weight required to 20,000 pounds. But it seems like it is very tough to assure minimum radial load on two adjacent bearings. Small change in tolerances, internal clearances, temperature etc might cause one to hold more than the other. Might certainly be a good question to ask the manufacturer how they maintain that minimum load.

The fact that the noise coming from the roller with less load (DE) is consistent with skidding.

 

[electricpete]

Out of curiosity, how is it connected to the compressor? Belt or coupling? What type coupling?

 

[edison123]

I quote SKF verbatim

&quot;When the end of the lubrication interval has been reached, the used grease in the bearing arrangement should be completely removed and replaced by fresh grease&quot;

&quot;In order to be able to renew the grease fill it is essential that the bearing housing is easily accessible and easily opened..... After removing the used grease, fresh grease should first be packed between the rolling elements... When the housings are less accessible but are provided with grease nipples and exit holes or grease valves it is possible to completely renew the grease fill by relubricating several times in close succession until it can be assumed that all old grease has been pressed out of the housing. This procedure requires much more grease than is needed for manual renewal of the grease fill&quot;

&quot;The six month limit represents a very rough guideline recommendation and may be adapted to fall in line with lubrication and maintenance recommendations applying to the particular machine or plant&quot;.

 

[edison123]

The motor is directly coupled to the compressor line shaft.

 

[electricpete]

I'm sorry. You are right about what SKF says. I see it on their CD-Rom. Still it sounds like horrible practice to me which invites overlubrication and I have not heard of anyone using it.

There is some other hardcopy SKF guidance that leads me to believe this method is less effective for large bearings, and also requires the machine running and look for grease expelled during the process.

 

[edison123]

pete,

In the two bearing arrangement in DE, the ball bearing does not carry any radial load. It is only used as an axial locating bearing to prevent axial movement of the rotor. As a matter of fact, the ball bearing, while tight in the shaft, has a clearance fit with the housing (the outer race is loose in the housing). So, in this case, the rotor weight is carried by the two roller bearings only and not by the ball bearing.

 

[GusD]

Hello edison123

The Bearing arrangement is pretty common on motors with Roller bearings.The Ball bearing Outer race is not captive and is kept snug by the springs compressing the outer race.
On motors with mid size bearings, the housing on the ball bearing side is usually machined .5 mm oversize.
As you say ,the ball bearing does not carry any radial load and is simply in place to locate the rotor in its Mag center.
As per my previous post regarding greasing concerns with Hyundai motors,if you like you could contact me at this E-mail--gdalmeida@shaw.ca.
I'll send you the info on what we had to do to deal with the problem.

Take care

GusD

 

[edison123]

GusD,

I do not know whether you recd my last e-mail to you. If not, could you pls mail your Hyundai details to eewemf@myrealbox.com ?

 

[edison123]

To all,

Today we had an opportunity to shut down one of the motors. When we removed the DE bearing front cover, lot of grease was seen to have collected in the cover (The cavity seems too big, may be the OEM designed it such a manner to collect the excess grease there ?). Also, grease had turned from original light yellow color to dark brown indicating high temp (though our probe fixed on bearing casing showed only 60 deg C).

Removed some excess grease from both bearing front covers. Motor restarted but the cyclic noise persists. So, asked for machine shut down to pull the rotor out and check the bearings. Probably this may happen by month end. Hope nothing goes awry with the bearings before then.

 

[electricpete]

Discoloration of the grease within a short time but no sign of high temperature is as you say strange. Some obvious guesses:
1 - Temperature detector is not positioned properly or some other instrument problem. Although depending on the setup maybe you have also been able to confirm using your hand that there was not excessive temperature?
2 - There was actually a different type (color) of grease installed by OEM. When mixed it caused a change in characteristics.
3 - Some type of fine wear products
4 - Contamination.
Although we haven't used it, I understand that rotating equipment grease analysis is offered by many labs like Herguth. I would suspect they can tell you if the grease was intermingled or exposed to high temp or contamination.

 

[jbartos]

Suggestion/question: Please, how is the motor started and controlled? Sometimes, various electrical currents flowing through the bearings damage them. The time may somewhat coincide with regreasing.

 

[edison123]

pete,

The temperature detector is fitted to the front face of the front bearing cover. As I said, the front bearing cover has copious space to accumulate the excess grease, which I suspect, acts as a heat shield. This would account for not so high temp readings both by probe or by hand. So, we plan to shift the temp probe to read the bearing outer race during tear down in two weeks.

The greases (original and replenishment) are supplied by OEM. Contamination - Nah, since this is a gas turbine station with clean environment. Wear products - what would that be ?

I still go with overgreasing (because of grease discoloration) but I can't account for the cylic noise with attendant current swing.


jb,

The motor is DOL start since no ghosts of bearing currents there due to soft starters or VFD's.

 

[GusD]

Hello edison123

I just revisited your original post to see what I had missed regarding some of the questions.
As you hinted about too large a motor for the bearing arrangement.I totally agree that this drive is probably a toss up in between Anti-friction bearings and sleeve bearings.All our 5000 Hp Hyunday motors (11 metric tons) are of the sleeve bearing type.True they also have a higher speed (1800).All of the 2500 Hp,1800 rpm are fitted with anti-friction bearings.So I would say your motors probaby fell on the grey area of bearing application.
Regardless the bearing type,it should not experience abnormal noise.
I find it interesting about your correlation of noise with surging amps and the fact it is driving a compressor.
The compressor cycling load is most likely the cause of hunting.As for the &quot;Hi-frequency&quot; noise,it still there even after removing some excess grease.You say that vibration analyses have not indicated any problem.The noise may appear to be coming from bearing housing but it may not necessarily be from the bearing.You have grease slingers,spacers and possibly a snap ring to keep the assembly in place.Any one of this can cause some noise.
I don't know if the motors were overhauled or new,but there is a possibility that the Outer race of the Ball bearing is not being made captive by the tension springs and it slides against them under shaft torsional forces?
As for the installation of the RTD ,it should be mounted as close to the outer race as possible.Normally a hole is drilled on the bearing housing just short of going through it.If the bearing temps don't go higher than 70C under full load conditions,I don't think your motor is at risk.
You have to make sure that temperature measurement is reasonably accurate.If noise remains as is and temp. rise stays put,I would keep monitoring motors until one of them changes.

Good luck Edison
on inside the cause of huntingis probably edge syour specific questions.

GusD

 

[jbartos]

Suggestion: The bearing might be inspected since large motors may experience another type of currents, (no the VFD ghost current) that can potentially be detrimental to the bearings. There are currents flowing through the bearings due to motor magnetic circuit asymmetries. Visit

http://www.reliance.com/pdf/white_papers/Inverter_Driven_Indct_Mtrs_Shft_Brngs.pdf

for a homopolar flux condition creating bearing currents.

 

[king144]

I've seen motor bearings that suffer from &quot;caved-in&quot; metal shields due to the hydrualic pressure of over greasing a bearing cavity. I supose there is a chance that the shields could begin rubbing on the bearing's ball carriage.

However, most large bearings don't have shields and the over-greasing problem in their case would have to be explained by some other phenomena. Perhaps, over-greasing interferes with the seperation of oil from the wax or carrier in some heavy greases, thus increasing noise and reducing bearing life...

Richard Anderson
GP Mill, Camas, WA.

 

[electricpete]

On 7/12/03 I described a bearing which I thought we had overgreased and was making noise. Today bearings were replaced and inspected. There is some mixture of old and new grease but much less than expected. Electricians think the cause of the noise was in fact lack of grease. I'm not sure what I learned but now I am sure that there was not excessive grease present in this motor that was making the noise. Also none of the races were spinning. I am having the shop cut the bearing open for inspeciton.
Sorry for the detour.