Part 6 - Rotor Response Analysis and Mode Shapes and Imbalance Spec

[@RotorDynamics]

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[@thomasmathew4469]

Hi Team, I was of an impression that the 1st mode shape would always be cylindrical. Can you please elaborate on the factors that affect mode shape and do a video on mode share vs critical speeds.

[@RotorDynamics]

Sure, Will do. And, thank you for your inquiry. In response to your question about mode shapes, it's worth noting that while the assumption of the first mode shape being cylindrical holds true in certain scenarios, mode shapes can indeed exhibit variability due to factors like geometric considerations. For instance, in machines involving significant overhung mass, the first mode shape may manifest as a rocking (conical) mode, deviating from the typical cylindrical expectation. These nuances highlight the influence of specific structural features on mode shape characteristics.

[@cck1496]

Good video..I have a stupid question... What is the difference between "stiffness" and "damping"? In my understanding, they are proportional to each other...The stiffer the rotor, the higher the damping... Can you correct me please? Thanks for your excellent work.

[@RotorDynamics]

Stiffness and damping are both properties that describe the behavior of mechanical systems, especially in the context of vibrations and oscillations. Here's a brief explanation of each and the key differences between them: -Stiffness refers to the resistance of a material or structure to deformation when subjected to an external force. -Higher stiffness means that the material or structure will deform less for a given force, whereas lower stiffness means it will deform more. -Damping refers to the dissipation of energy within a mechanical system, which reduces the amplitude of oscillations or vibrations over time. -Higher damping leads to quicker dissipation of energy and faster decay of oscillations, while lower damping results in longer-lasting vibrations.

[@斗士007]

thank you very much from China

[@RotorDynamics]

Thank you. I hope you enjoyed it. If so, please like & subscribe. :)

[@dannysong1659]

I have a question - from 1:49, bearing support stiffness is not a fixed number? Can stiffness be changed as speed increases?

[@RotorDynamics]

Excellent question! Indeed, in certain cases, the stiffness of bearing support can be frequency-dependent rather than a fixed value. In practical scenarios, we often conduct impact tests to acquire the Frequency Response Function (FRF), which helps us understand the behavior of the system at different frequencies. By incorporating this FRF into the rotor dynamic model, we can account for the varying stiffness as speed increases. I appreciate your curiosity, and I'm looking forward to addressing this topic in a future video. Stay tuned!

[@dannysong1659]

​@@RotorDynamics Thanks for kind and fast response :)

[@Reach1e]

Hi, I really like your videos. Could you help me with these two questions? 1. How do you know that the 1st mode is elastic bending? Was this stimulated by the XLrotor program ? 2. If the rotor is operating above second critical speed (damped) with a rocking mode, is it sufficient to just do low speed balancing?

[@RotorDynamics]

Great question. To address this, performing a rotor dynamics analysis is essential, and you can compare the results with physical tests. Additionally, conducting a balance sensitivity study using rotor dynamics software will provide deeper insights. If the analysis shows that the rocking mode is dominant, low-speed balancing should suffice.

[@bkoo3472]

It has been a month. Where is Part 7?

[@zeddg8633]

I have a question, when studying rotor dynamic, we plot campbell diagram. I always see the curves going linear and symmetric, but what happen when the curves aren't linear and symmetric? I'm dealing with a project and I can't explain why one of the curves isn't linear.

[@RotorDynamics]

Campbell diagrams are typically used to analyze the stability of rotating machinery by plotting the natural frequencies of the system as a function of its rotational speed. The resulting curves are typically linear and symmetric around the critical speed, which is the speed at which the natural frequency of the system crosses the excitation frequency (usually the rotation speed). However, in some cases, the curves on the Campbell diagram may not be perfectly linear or symmetric. This can be caused by a number of factors, such as non-linearities in the system, such as large clearances or nonlinear bearings, unbalanced forces, or other sources of vibration. In such cases, the Campbell diagram may become more complex, with multiple critical speeds, bifurcations, and other nonlinear phenomena. When the curves are not linear or symmetric, it can be more challenging to analyze the stability of the system. In some cases, it may be necessary to use more advanced analytical or numerical methods to accurately predict the behavior of the system. These methods may include finite element analysis (FEA), computational fluid dynamics (CFD), or other advanced techniques. If you are dealing with a project and one of the curves on your Campbell diagram is not linear or symmetric, it is important to carefully consider the possible causes of this behavior and to use appropriate analytical or numerical methods to accurately predict the behavior of the system. It may also be helpful to consult with experts in the field of rotor dynamics to ensure that your analysis is accurate and reliable.

[@cck1496]

​@@RotorDynamicsThanks for such detailed explanation.... Would you briefly explain, " what are non linear bearings?"... Thanks.

[@nastarantamjidi3917]

Thank you

[@RotorDynamics]

You're very welcome! If there's anything specific you enjoyed or if you have any suggestions for future content, feel free to share. Thanks for watching and your appreciation!

[@teb7002]

Thank you so much for your sharing. I will appreciate it if you can share a paper including rotor dimensions, rd analysis results and test results. BR

[@RotorDynamics]

Hi Talha. I'm using the rotor geometry from the paper written in 1996 "Squeeze Film Damper Bearing Experimental VS Analytical Results For Various Damper Configurations." But, the inputs for the bearing coefficients that I used in my videos are different than what is presented in the paper.

[@teb7002]

@@RotorDynamics Thank you so much.

[@that_dude_shubham]

Dear Sir, I am an engineering student from India. I was struggling to find the time series response of a single rotor system subject to unbalance in ansys transient structural module. Will you please help me in this regard? I have tried to do the analysis by applying a constant force in a particular direction at all times and evaluated the response but it is not as I was expecting. The response that I got is having some transients initially and after that the vibrations are becoming 0 after 3-4 sec for a 10 secs time frame.

[@RotorDynamics]

Hello! I can definitely try to help you with your question. To simulate the time series response of a single rotor system subject to unbalance in ANSYS Transient Structural module, you will need to follow these general steps: 1. Create a geometric model of your rotor system in ANSYS. This should include the rotor, bearings, and any other relevant components. 2. Assign material properties to your model, such as density and elastic modulus. 3.Define the unbalance force by applying a force vector in a specific direction at a specific location on the rotor. This can be done by using the "Remote Force" or "Remote Displacement" boundary condition in ANSYS. 4. Define the boundary conditions for the analysis, such as fixed or free bearings. 5. Define the time domain for the analysis, including the start time, end time, and time step. 6. Run the transient analysis and obtain the time series response of the rotor system. It is possible that the response you obtained with a constant force in a particular direction may not accurately represent the true behavior of your system. In a real-world scenario, unbalance forces would typically vary with time, and the response of the rotor system would also vary accordingly. To simulate a more realistic unbalance scenario, you can apply a sinusoidal or random excitation to your rotor system. This can be done by using a time-dependent force or displacement function in ANSYS.