Shaft - find natural frequency of horizontal shaft with bearings

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Discussion Overview

The discussion revolves around determining the natural frequency of a horizontal shaft supported by bearings on either side, specifically in the context of using Ansys-classic software. The inquiry includes considerations of various types of frequencies and the effects of rotation.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asks how to find the natural frequency of the shaft using Ansys-classic.
  • Another participant questions whether the shaft's rotation speed could affect the frequency, suggesting that appropriate restraints should be applied at the bearings.
  • There is a discussion about whether to calculate the critical angular frequency for whirl or the torsional or lateral vibration frequency, highlighting the ambiguity in the original question.
  • Some participants note the potential need to introduce an imbalance or noise in simulations to avoid overly idealized results.
  • Concerns are raised about the appropriateness of modeling the system as "rigid disks on a flexible shaft" without additional details about the rotor's design.
  • One participant emphasizes that lateral vibration frequencies can be whirl frequencies, noting the importance of considering both synchronous and non-synchronous whirl in the analysis.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the specific frequency the original poster intends to calculate, indicating that multiple competing views remain about the appropriate modeling approach and the effects of rotation.

Contextual Notes

The discussion highlights limitations such as the lack of clarity in the original question, missing details about the rotor design, and the potential impact of rotational speed on frequency calculations.

chkarthiksai
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shaft -- find natural frequency of horizontal shaft with bearings

how to find natural frequency of horizontal shaft with bearings at either sides, using (Ansys-classic).
Please reply, Thank you.
 
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I'm sorry you are not generating any responses at the moment. Is there any additional information you can share with us? Any new findings?
 
Is the shaft rotating fast enough that the rotation might alter the frequency?

If not, then just put appropriate restraints where the bearings are and solve it. You'll need to decide if the bearings give it freedom to rotate like simple supports or only allow axial rotation like a sleeve bearing.
 
It is not clear if you want the critical angular frequency for whirl, or the torsional or lateral vibration frequency.

Simulators are often too perfect to self destruct. You may need to add an imbalance or some white noise while sweeping over an RPM range.
 
I agree the question doesn't specify what the OP really wants to calculate.

And if you do want to include the effect of whirling, without a drawing of the rotor it's not obvious whether a simple "rigid disks on a flexible shaft" model is appropriate, or whether you need to include the flexibility of all parts of the system.

Baluncore said:
It is not clear if you want the critical angular frequency for whirl, or the torsional or lateral vibration frequency.

The lateral vibration frequencies ARE whirl frequencies, but not synchronous ones. In some situations non-synchronous whirl is just as much a problem as synchronous. Even if it doesn't cause the device to fail catastrophically, it can eat up the structure's fatigue life like there was no tomorrow!
 

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