Learning Forum

[AR]

Hello,

I have a structure that holds a motor with a cantilever arm, as you can see in the image.

I have to ensure that the structure can withstand the vibrations generated by the motor when it is running. To do this I have to perform a Random Vibration analysis. How can I insert the PSD into the motor and not into the support?

In addition to the PSD, I also have to check against acceleration peaks between different frequencies. This would be done with a harmonic analysis. How can I insert the acceleration only into the motor instead of all the bodies? Well, it is the motor that generates these excitations.

[dlooman]

Yes, that's a common scenario which isn't supported by the Mechanical Gui.  With APDL commands you could use a force psd applied to the motor.  The harmonic analysis could be done in Mechanical.  If Mechanical doesn't allow acceleration to be scoped to a body you can apply it with a commands object:  CMACEL,named_selection,Xacel,Yacel,Zacel where a named selection has been created for the motor body in the gui.

[AR]

Hi dlooman,

Thank you very much for your answer.

So I can insert apdl commands in the Mechanical to apply the PSD to the motor? Can that PSD be for acceleration too? Could you tell me the command or a document where it appears?

I will try to apply the acceleration only to the motor also in the harmonic. 

Regards

[dlooman]

The acceleration of the motor is obtained by applying a force equal to the desired acceleration divided by the motor mass.  Even though the acceleration is actually at the motor.  Applying a base acceleration should produce the same relative results.  The only error is that the base acceleration is applied to the entire model.  Here are commands that apply a pressure psd based on a unit pressure applied in the modal:

sfdele,all,all
psdunit,2,press
lvscale,1! scale pressure from modal
psdfrq,2,,10,50,100,500,1000
psdval,2,0.01,0.02,0.022,0.042,0.03
pfact,2,node! nodal excitation
!dmprat,.01 ! damping ratio
psdcom
solve

[AR]

 

Hi,

I have managed to apply the acceleration to the motor in the Harmonic Response analysis. I leave the process below for other users who have this question:

You have to declare the CMACEL in the modal. You have to apply it to a body, a volume, previously defined as a Named Selection (in my case BODY1). The Named Selection has to be created before solving any of the analyses so that it is passed to the Solver!!

After applying the CMACEL in the Modal, in the Harmonic Response I declare the LVSCALE to apply the scale to the CMACEL that I have applied before. In my case it was 1, but you can declare the CMACEL as 1 and in the LVSCALE with the corresponding value. The SFDELE is to eliminate any residual force that may have been applied.

Regarding the application of the acceleration to the motor for the Random Vibration, I have not achieved anything. Could you help me?

Regards

 

[AR]

Hi dlooman,

 

I just make a little model to try the code but it was impossible for me to do it work. 

 

In case of Harmonic Response, I use the CMACEL with a Named Selection that I made cald "NodosMotor". I also try it with a named selection of the body of the engine. However, in Solution Output I have a warning messages: There is no component named NODOSMOTOR in the model.  Please specify a valid component name to apply a translational acceleration (CMACEL command). The Harmonic Response solve but there are no results, as the acceleration were 0. 

In case of the Response Spectrum, I use "cmsel, s,NodosMotor,node" to select the nodes in that named selection before the "pfact,2,all", but I obtain an error: SELECT COMPONENT NODOSMOTOR

 *** ERROR ***                           CP =       0.234   TIME= 17:41:15
 Component NODOSMOTOR is not defined.

 

Could you help me with that?

 

Thank you very much!!

 

Regards,

 

 

[AR]

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