Hello everybody.

I'm a newbie in the field of FFT spectral analysis. I have a problem with exceed axial vibration of overhang centrifugal fan. I get 3 spectrum of 3 direction at the bearing next to blade. I have some questions:
1. I have some low frequencies (1hz, 2.5Hz).
2. I see the amplitude in axial direction at 187.5Hz is too hight. It's approximate to the result of SKF's tool: "SKF Bearing Calculator".
What wrong with my bearing and how to solve this problem?
Thank you for your time.

Axial directionHorizontal directionVertical directionskf 



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Original Post

Seems to me you answered your own question.   187.5 Hz peak evident in all directions. It is ~ 7.7X running speed and, according to your own bearing frequency calculator, it is an outer race fault. I get the same result at :


You may see this frequency as a series of peaks in the higher frequency ranges which you don't show.

You would likely see this as a series of impacts if you look at the time waveform.

"How to solve the problem?"     --> Change out the bearing. Confirm the outer race defect is indeed there (I would suspect a single large spall) and determine what might have caused this problem to occur (dirt, poor installation, lubrication, etc.)

Review this video from Mobius:    https://www.youtube.com/watch?v=dEn2Qvh_qjc

The 2.6 mm/sec (running speed frequency) doesn't seem too high to me.

Wouldn't worry about the very low frequencies.....Likely the result of integration of your accelerometers to velocity units. 

Post the results of what you find....

Jim P

Hi EveryBody!

I'm TrịnhNgọcLâm'S colleague . We are  think mini Frequency relate to the rotational frequency of the outer ring and 187.5hz to the over-rolling frequency of one point on the outer ring. But we don't see some mark on outer ring and the vibration is only axial direction. Please help our.


Quote:  But we don't see some mark on outer ring and the vibration is only axial direction. 

I am confused. I clearly see the 187.5 Hz peak in ALL 3 Directions.

I don't understand what you mean by "we don't see some mark on outer ring".  Have you already looked at the bearing?

Please post time waveform and higher frequency FFT plot if you have it.

Jim P

A 187 hz impact will be spaced (1/187) = 0.005 seconds. It is hard to see that in your 2 second long time waveform. Can you show a FFT and TWF for a 2,000 hz or 4000 hz range?

Based simply on amplitude levels, it would appear that the outer race defect (the initial evidence suggests its there) would be in the NDE bearing location.

Hi Trinh & Tony.

As per my experience in cement industry for the overhung centrifugal fan, high 1X axial vibration compare to hor/ver is likely relate to shaft bend influence or bearing cocked/skewed. Is there any Enveloping/PeakVue value did you measured? 

9mm/s at axial direction for me is high since axial not always contribute a vibration problem. Try measure shaft roundness w dial gauge. If ok, the answer is change the bearing to get a peace of mind. Because the peak frequency is related to bearing as what mention by our leader ivibr8.


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