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
Solved! Go to Solution.
Solved by Pawel.Pulak. Go to Solution.
Solved by Artur.Kosakowski. Go to Solution.
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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As usual precious and perfect post.
Thank you for your commitment
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:
More information in What's New in Autodesk Robot Structural Analysis Professional 2019 and in help referred from it.
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