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harmonic analysis

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Message 1 of 24
colastanarobo
1678 Views, 23 Replies

harmonic analysis

Hi,

 

I introduced  the vibration of a machine (frequency=10Hz) : HARMONIC ANALYSIS.

The problem is that the results are not clear: there isn't load combinations  with harmonic

Can you explain me this type of analysis

Best regards 

23 REPLIES 23
Message 21 of 24
Pawel.Pulak
in reply to: colastanarobo

Both harmonic and FRF analysis consider loads defined in this load case as the amplitude of harmonic (sinusoidal) excitation and they analyze steady-state harmonic response.

 

In case of harmonic analysis it done only for single frequency. In such case all results (reactions, displacements, internal forces and moments,...) are also amplitudes of harmonic response.

 

In case of FRF analysis it is done for the series of different frequencies from some range (possible to use natural frequencies too). FRF load case is a compound load case containing results (amplitudes as above) for all these frequencies - like many harmonic analysis load cases.  It is also possible to see results not only for single frequencies but some summary results (diagrams) in frequency domain - see mesage 3 and 9 in this post.

 

Additional remarks to defining combinations with harmonic or FRF load cases discussed in previous posts - because results of these load cases are amplitudes the combinations of these load cases with static loads should be used with positive and negative factors to consider opposite signs of amplitudes. So for instance if the combination ise defined with +1.6 factor for the harmonic load case it should be defined also another with -1.6 factor.

 

As concerns considering masses in harmonic of FRF analysis the load to mass conversion concerns constant loads (and related masses).

As concerns some un-balanced mass related to vibration of machine (engine, ventilator, etc) it is usually significantly smaller that the total mass of this machine and it is considered as the amplitude of excitation in approprate harmonic or FRF load case. Converting it to mass is not necessary. Total mass of the machine is considered as static load in some other load case and this load should be converted to mass.

 

I hope these explanations will be helpful.

 

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Regards,


Pawel Pulak
Technical Account Specialist
Message 22 of 24
colastanarobo
in reply to: Pawel.Pulak

thank you
Message 23 of 24

As usual precious and perfect post.

 

Thank you for your commitment


PasProStudio

www.pasquiniprogetti.eu

Structural + Detailing engineers
Message 24 of 24

Hi Stefano,

good to know that this old post is still useful:)

 

But when looking at it I realized that it should be updated.

 

Starting from RSA 2019 the significant change was made for harmonic and FRF analysis in case of models with non-zero damping.

For such models maximum response can be obtained for various parts of the model (precisely - for each degree of freedom DOF) at different time. It means different phase angle of harmonic vibration for each DOF.

In previous releases Robot tried to show only some envelope of these results.

Starting from 2019 release:

  1. harmonic load case for the model with non-zero damping is a compound load case with 360 components - corresponding to 360 degrees of the phase angle for a full period of harmonic excitation. It may be so that for instance the maximum UX displacement in one node is for instance for 60th component (60 degrees phase angle) while the maximum UX displacement in another node is for instance for 196th component (196 degrees phase angle) 
  2. FRF load case for the model with non-zero damping gives extreme results for specific frequency - so they may correspond to different phase angles for corresponding harmonic case

More information in What's New in Autodesk Robot Structural Analysis Professional 2019 and in help referred from it.

 

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If this post answers your question please click the "ACCEPT AS SOLUTION" button (the button visible when logged to the forum). It will help everyone to find answer more quickly!

 

Regards,


Pawel Pulak
Technical Account Specialist

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