High axial vibration in a centrifugal fan | AMP Maintenance Forums

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Hello all,

Do we observe high axial vibration (and more than radial vibrations) in all centrifugal fans ?

I have a case where overhung centrifugal fan driven by motor and coupled with belt drive. Following overall velocities (10-1000 Hz RMS) recorded,

Axial: 15 mm/s, Horizontal: 12 mm/s, Vertical: 8 mm/s

I have also attached spectra (freq vs RMS velocity). What are the possible faults; to me looks like looseness.

Similar cases (high axial vibration for centrifugal fans) reported earlier in this forum. I was wondering whether its machine configuration, fan operation, or common design issues (in once case it was a weak bearing block).

Thanks

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How can we read the two spectra? There is no vertical amplitude scale, and the horizontal axis does not have good numerical and consistent values (such as 100, 200, 300 Hz) or useful small divisions. Looks like there may be 1/4 x shaft speed present that is not normal.

Axial vibration on a centrifugal fan is normally not dominant. Possible causes of high 1xSS vibration in axial direction are shaft/belt sheave misalignment, 2-plane rotor unbalance, thrust bearing damage, rotor or seal rub, loose bearing or fan base supports, or resonant structure/foundation.

Walt

4/27/2211:32 AM

Registered Member

Thanks Walt for your comments.

Amplitude is in mm/s RMS velocity; 1X is 30 Hz.

4/27/2211:20 PM

Registered Member

@Registered Member posted:
Thanks Walt for your comments.
Amplitude is in mm/s RMS velocity; 1X is 30 Hz.

Why is vertical scale labeled "9" at top left and have 10 divisions?

Registered Member

Hi John, that's 9 mm/s RMS, values at discrete frequencies can be roughly estimated from this value.

Registered Member

@Registered Member posted:
Hi John, that's 9 mm/s RMS, values at discrete frequencies can be roughly estimated from this value.

Strange; 9 mm/sec is 10 divisions? Why not 9 divisions?

Registered Member

Is this fan fed from both ends? If yes, then this is usually the result of uneven damper settings, and/or the fan is not centered in the fan housing.

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