New phenomenon in Vibrations of Electric Generator 2

[INDAR]

Hi guys!
A new phenomenon has been seen in a Run Down of an electric generator. It is shown in the attached graphic.

The machine has ball bearings with grease in two sides. One side is fixed and another one is non fixed in axial direction. The horizontal line is appeared only in 15% of manufactured machines. Why?

The colour-map generated is obtained in a Run Down test. So there is no electric supply. The machine goes to 2000rpm and then the electric supply is switched off. Now, the rotor speed goes down until 0rpm. The horizontal line appears near 1600rpm.

Do you need more data? Wich the reason is that generates this horizontal line?

Thanks in advance.

 

[electricpete]

Thanks, I didn't see that attachment

The next fb-waterfall...

What does fb stand for? How does the 2nd attachment relate to the first? Different machine? Different bearing on same machine? Different run on same machine?



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[INDAR]

Hi guys!
The 2nd attachment is not related to the first one. This is of another machine of the same type. The testing was the same: Run Down.

I am sure that the problem comes from the bearing zone. I don't know if there is some looseness, the size of ball bearings, maybe the Non Drive End that has mechanical play in axial direction... By other side, the design of this area and the ball bearings was recommended by SKF, so I think this should be OK.

Besides, as this is a production line, I can't do a lot of testing.

I would like to find some information about this phenomenon.

Best Regards,

 

[INDAR]

Hi Guys,

I have get the full information in pdf with very good graphics of this testing. There is no more information.

This one is from another machine. The horizontal line appears in vertical and axial direction.

For its application, the vibration level is high.

Best Regards,

 

[SomptingGuy]

Well, there's no .5, .4, .43 or similar order in any of those plots. All I see is the expected 1st order dominance cutting through resonances, exciting harmonics.

- Steve

 

[GregLocock]

Ah, no at 850 rpm there is a lot of activity at all the integer order lines, that looks either like a signal acquisition problem (distortion), or else something is going off in a non linear fashion.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

Agreed, it does go a bit AWOL in some directions at that speed. Hadn't thought of distortion (or simple overloading), always a possibility.

- Steve

 

[INDAR]

Hi guys!

What is AWOL? I don't understand this word.

In the beginning I think so. But after some measurements (15% of measured machines)and after check it with a hand analizer, I think the measurements are fine. The channels (accelerometers) weren't overload.

The measurements are made with the same equipment and the same projects always. So I think the problem comes from machine, exactly from bearings.

Besides, although bearings are well lubricated, there is a screeching noise from the bearings. (This screeching noise dissapears with the running of machine as well as the temperature of machine raises - maybe the grase of ball bearing is more liquid and the lubrication is better-). But in some machines that appears this noise its behaviour is fine. Not horizontal line appears. >:-<

Somebody has see this horizontal line before in other machines?

Best Regards,

 

[MotorVib]

For the bearings take a look at NTN or SKF Web page, they have an on line informasion about bearing freq's.

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[GregLocock]

All I can really say is that it looks like a 'clunk' to me if the signal is OK.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[electricpete]

It looks as if everything is confirming some kind of resonance around 870 or so. The excitation is sometimes 1x and sometimes this 0.43x which we have postulated by be bearing fault frequency FTF.

Try googling: "bearing fault frequency" FTF

Meanwhile, if you have a bearing handy, you can easily convince yourself of the physical significance of this fraction around 0.43 (exact fraction can be calculated from bearing geometry or from bearing part number).

I have done it in the attachment, although I think I need to explain what is shown in the attachment since the visual cues are not well displayed.

The bearing starts out with blue dots at the 12:00 position on all three components: inner ring, cage, and outer ring.

Then I rotate the inner ring one full rotation so that it comes back to the 12:00 position. Where does the blue dot on the cage end up?.... around the 5:00 position.

During one full revolution of the shaft, we have seen 5/12 ~ 0.4 revolutions of the cage. And if we had one large rolling element, we have seen 0.3 revolutions of that element.

If shaft is traveling at X rpm, then that rolling element will pass by a given location on the outer race at a rate of 0.43 * X rpm.







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[SomptingGuy]

Ok, just being thick here...

Is that 0.43 based on the contact radii ratios of the solid parts of the bearing? If so, is 0.43 some kind of optimal ratio?

- Steve

 

[electricpete]

It is a slightly different number for each bearing depending on geomtry as you mentioned. Could be maybe 0.38 to 0.45. I don't know the number for this bearing.

0.43 = 13hz/30hz is the ratio that I calculated between the resonant frequency and the running speed frequency from the first attachment. I have ASSUMED that this is the value of the FTF for Indar's bearing.

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[INDAR]

Hi guys!
Some doubts:

1. What's AWOL?
2. What's FTF?

I do not understand what it gives you the information ratio between the resonant frequency and the running speed frequency.

Best Regards,

 

[SomptingGuy]

AWOL means Absent Without Leave, a military term. It's often used to describe something that's out of control, off the scale, etc. I shouldn't have used it really, sorry.

- Steve

 

[electricpete]

FTF stands for Fundamental Train Frequency. It is the frequency that the cage travels around the bearing. It is also the frequency that a given ball travels around the bearing. It is one of the 4 common rolling bearing "fault frequencies" (FTF, BSF, BPFO, BPFI). If we compare a ball going around bearing to earth going around sun, then FTF is analogous to one orbit per year, BSF is analogous to one rotation per day. Please google bearing fault frequency and you should find a lot more info.

I do not understand what it gives you the information ratio between the resonant frequency and the running speed frequency.

My discussion was in the context of the first attachment. We see evidence of a resonance at 13hz which is excited when machine frequency is 30hz.

If the source of excitation is one whose frequency is proportional to machine speed, then the proportionality constant in this case is Fexcitation / Fmachinespeed = 13/30 ~ 0.43

Bearing fault frequencies are proportional to machine speed. For the case of an 8-element bearing, we might have something similar to:
BPFO = 3.1 * Machine Speed
BPFI = 4.9 * Machine Speed
FTF = 0.4 * Machine speed
(where the exact multiple can be determined from the bearing part number or geometry).

Looking at these, we see the FTF proportionality constant (~0.4) is very close to the proportionality constant suggested by the data (0.43). So we make a guess that the fault may be one which varies at frequency FTF.

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[INDAR]

Hi guys!

Through an exchange of mails, roller bearings manufacturers have confirmed that the "nonlinearity" could come from the bearings. This rigidity depends on the type of bearing, size and clearance / preload.
I have to talk with them by telephone. I'll let you know.

Best Regards