What is static and dynamic stiffness

  Good to know: Ansys Discovery Live: 3 steps to flow simulation in secondsJohnsoldo
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created on: 26 Sep. 2019 04:21   <-- editieren="" zitieren="" --="">   Submit unities:

Hi Guys,

My task is: "A high static and dynamic rigidity in all spatial directions avoids critical vibrations"

Now I'm just thinking about the best way to build this into my simulation.

Static-mechanical analysis: Force as boundary condition -> Results: Deformation in 3 directions -> Force through displacement = rigidity
Harmonic analysis: periodic force as boundary condition -> results: deformation in 3 directions -> force through displacement = stiffness

Does that make sense?

LG John

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ibgross
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created on: 27. Sep. 2019 02:00   <-- editieren="" zitieren="" --="">   Submit unities:For Johnsoldo only

Hello John;

rather not.

Static stiffness = resistance to load
Dynamic rigidity = avoidance of the effects of vibrations

An exemplary study on the topic is available here:

http://publica.fraunhofer.de/eprints/urn_nbn_de_0011-n-1372790.pdf

Optimizing the static rigidity usually means:
Adjustment of cross-sectional area, area moment of inertia and section modulus against external loads with the optimization goal of weight reduction.

Optimizing the dynamic stiffness usually means:
Avoidance of external vibration excitations in the relevant frequency range of the natural modes of the component.

A very complex subject.
And optimization of static and dynamic strength can sometimes be incoherent.

To deal with this exhaustively, nothing works without empirical values ​​(e.g. from standards and guidelines) and / or analytical, numerical methods (e.g. FEM).

When you inquire, I am assuming a task in the university area.
If the task was set as you described it, I find it inappropriate.
Unless you are doing a PhD in this topic.

Maybe my information will help you a little further.
When you have familiarized yourself with the topic, I am happy to provide suggestions.

Best regards
Michael

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arnd13
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created on: 02. Oct. 2019 13:13   <-- editieren="" zitieren="" --="">   Submit unities:For Johnsoldo only

Hi john,

Basically you have correctly presented the methods / formulas for calculating the stiffness.

Michael's remarks are aimed at the fact that you also have to evaluate the calculated stiffnesses: From what value is a stiffness "high"? And if the stiffness is not high, how can it be increased? Sometimes it is also better to reduce the stiffness, e.g. to avoid resonances. Optimizing stiffness is a very large field and probably not part of your study assignment.

------------------
Regards, A.

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Johnsoldo
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created on: 04. Oct. 2019 00:29   <-- editieren="" zitieren="" --="">   Submit unities:

just a question of understanding:
I have the following configuration
The tooth mesh frequency of a milling tool with four teeth that operates at one spindle speed
of 8080 rpm is in mesh at 538.66 Hz. The n-harmonic tooth mesh frequencies are 1.07 kHz, 1.62 kHz, etc.

I have now carried out a modal analysis and have the natural frequency and these values ​​are not identical to the tooth meshing frequencies. So I can roughly say that there is no resonance, right?

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ibgross
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created on: 04. Oct. 2019 01:56   <-- editieren="" zitieren="" --="">   Submit unities:For Johnsoldo only

Hello John;

now we know (a little) more what this is all about.

I can still understand the tooth mesh frequency calculated by you with fz = z fn = 539 Hz.

The eigenmodes you calculated:
- were determined how?
- refer to what?
- have what values?
- do they even play a role here?

I suspect that here (in this frequency range) it is more the storage that plays a role.
Keywords:
- cage rotation frequency
- roll over frequency
- rolling element rotation frequency

Your task and your original question are about static and dynamic stiffness and the avoidance of vibrations.

To do this, it is important to evaluate them first.
And in the frequency range available here, that is usually the storage.

You can find a basic approach here:

https://maschinendiagnose.de/mosaic/_M_userfiles/PDF/Downloads_DE/Fachbeitraege/Maschinendiagnose_Teil_2.pdf

Best regards
Michael

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tmg11
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created on: 04. Oct. 2019 11:47   <-- editieren="" zitieren="" --="">   Submit unities:For Johnsoldo only

hello, these are the simulation results from the system. Finally, I wanted to do an experiment to confirm the simulation results. So the natural frequencies not the excitation frequencies (also the n-facche) tirt resonance or not?

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