Whirl,unbalance and critical bearing stiffness

Joined: 04/07/2023

Post 07 November 2023 20:16:53

Whirl,unbalance and critical bearing stiffness

Good Day !

I have questions concerning whirl,unbalance and critical stiffness this time!

I am trying to:

Compute the asynchronous and synchronous whirl on my system.
Do an unbalance force analysis
Estimate the critical bearing stiffness of my system (5% of the value of the stiffness which would make my 35k rpm critical speed impossible to reach)


I am not seeing any whirl output in AxRD, instead i notice in the Harmonic analysis an Amplitude-Frequency response, is it the tab which indicates the whirl of my system?
If yes i would have a question about the excitation frequency, my target speed is going to be 35k rpm is it right to only input for omega_0

If not , what is the way to evaluate omega 1 and 2 ? ( a specific formula to apply).
What would be the procedure to compute the asynchronous whirl on AxRD ?


Based on my critical speed simulation, I have the behavior of the shaft depending on the mode. Does the unbalanced analysis apply only for the 1st mode? if not how can i compute it for n modes ?
Does this formula sound correct to compute the unbalanced force of my mass elements on my shaft ?
IF(N)= mass*Radius*2*PI*f

Critical stiffness

To estimate my critical bearing stiffness (5% critical value which would make my target critical speed impossible to reach) would it be best to use the critical speed map and do a graphical estimation ? If not what would be the procedure to estimate such parameters ?

Thank you very much for your time !

Have a nice day !


Shayman-Reza Labadlia

Joined: 06/11/2023

Post 08 November 2023 10:00:09

Re: Whirl,unbalance and critical bearing stiffness

Hello Shayman-Reza! Thank you for joining our forum!

Please, find answers to your questions below.

1. Whirl

The spin whirl ratio input is possible for the Critical Speed and Critical Speed Map analyses settings in AxSTREAM RotorDynamics. Using this option, you can calculate undamped forward synchronous critical speeds with an inputted value of spin whirl ratio equal to 1, and undamped asynchronous critical speeds with an inputted value of spin whirl ratio different from 1(e.g. -1 for backward whirl). More details about this option are presented on our Wiki page by the link http://wiki.softinway.com/id/2695/rd-critical-speed

To study the synchronous response due to applied unbalance forces, you can perform the Unbalance Response analysis and obtain a rotor vibration amplitudes and phases, orbit plot and other results. You can find more details about this analysis on our Wiki page by the link http://wiki.softinway.com/id/2697/rd-unbalance-response

The Lateral Harmonic analysis allows to estimate the rotor response due to harmonic excitation, modeled by harmonics adjustment coefficients Omega0, Omega1 and Omega2. You can evaluate the rotor whirl by inputting the corresponding value for Omega1 coefficient (1 for synchronous whirl and different from 1 for asynchronous one) while the Omega0 and Omega2 should be equal to 0.
Also, it is necessary to specify the unbalance force X2 or Y2 in the “Lateral Forces” tab with its location of the rotor and magnitude similar to Ux (or Uy) inputted in the “Unbalances” tab in the Unbalance Response analysis.
More details about this analysis are presented on our Wiki page by the link http://wiki.softinway.com/id/2699/rd-lateral-harmonic

2. Unbalance

The Unbalance Response analysis shows the peak amplitudes at a damped critical speed in the Amplitude-Frequency Characteristic plot depending on the specified speed range in the analysis settings.
If you want to obtain the rotor response for different damped critical speeds, just increase the considered speed range in the analysis settings.

To calculate the unbalance (note, not unbalance force) required for the Unbalance Response analysis in AxSTREAM RotorDynamics, please refer to the API 684-1-2019 standard (SP6.8.2.7).

3. Critical stiffness

Yes, the best approach to choose the necessary bearing stiffness to reach the target undamped critical speed is to perform the Critical Speed Map analysis.

Kind regards,
Anna Perepelytsia

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