# 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 ββ(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.

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 ββ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

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:

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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