# Structural resonance Vs Rotor resonance

Dear Members ,

i would like to know the following information on Structural Vs Rotating resonance .

Can we minimize Rotating resonance of a rotor by balancing .please share your experience .

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Original Post

yes. The natural frequency of the rotor can be changed by adding or removing mass. Resonance occurs whenever a forced frequency nearly coincides with the rotor natural frequency.So, if you are having resonance of a rotor at particular frequency you can go for balancing to avoid resonance at that frequency.

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Resonance involve the system and not just one component.  A natural frequency primarily involving one component may be named for that component.

Balancing can improve either resonance if it contains the forcing function (or is synchronized to it).

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Dear sir ,

what if a rotor (For example a I.D.Fan Impeller)has resonating at the particular speed and the structure does not have the resonance at the particular speed .But the Vibration increase due to rotor resonance .Does that impeller resonance minimized by balancing ?

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Unless you measure on the impellor, it is unusual for vibration to increase at typically measured points.

Impellor vibration can be affected by rotor vibration.  As far as impellor resonance (natural frequency), it is not likely to be at 1X.  This vibration could well be decreased by balancing, but without measurements we never know -- assuming this is 1X vibration.

Note:  Campbell's paper (Campbell diagrams and all that) in steam turbine wheels included details on natural frequencies.  Campbell used Southwell's theorem to find wheel natural frequencies running compared to when stationary.  Den Hartog has an example of a spinning chain, where he uses Southwell's theorem, too.  One can see that the (depending upon mode shape) natural frequencies increase while spinning.  In Den Hartog, we see the spinning chain with no static stiffness has a natural frequency at running speed (if I recall correctly); so an impellor with inherent stiffness would have higher natural frequencies - some mode shapes could be an exception, but for the 1X, I doubt it.

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Kishore kumar posted:

Dear sir ,

what if a rotor (For example a I.D.Fan Impeller)has resonating at the particular speed and the structure does not have the resonance at the particular speed .But the Vibration increase due to rotor resonance .Does that impeller resonance minimized by balancing ?

Shaft vibration levels in this case could be reduced by balancing the rotor but more details are needed to confirm that. Some machines can show high shaft vibration while the external structure does not  vibrate.

If the vibration frequency is not 1X, balancing wouldn't help. If the phase is not stable, balancing wouldn't help. If the problem is not unbalance, balancing wouldn't help.

Regards- Ali M. Al-Shurafa

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I wouldn't say this data had steady phase, but the rotor was successfully balanced with one shot.  Of course at higher vibration levels the phase as more steady that this after balance plot.

Red data is from at speed, 3000 rpm.  Green is coastdown, and blue is the start.  The machine ran happily in the middle between the extremes with steady load after reaching temperature.

Any point to this?  Reflect on assumptions when making broad statements.  Rules of thumb have assumptions, too.

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