×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Modal Analysis of an Assembly

Modal Analysis of an Assembly

Modal Analysis of an Assembly

(OP)
Hello,

I've been trying to conduct a Modal Analysis of a simple assembly (the example assembly provided in the help system) in FEMAP. I tried this on a lamp assembly of my own, and since it didn't work the way I'd anticipated, I thought I'd try it on a simpler assembly first.

I just deleted the load on the front surface of the "plunger" and ran a modal analysis, but I'm obtaining similar results as I did with the lamp, in that the mode shapes comprise of the individual components becoming dis-engaged and vibrating irrespective of the assembly "contact" constraints.

How do I fix this? How do I have the mode shapes account for the assembly constraints, i.e., have the plunger not disengage from the base link which are connected using the pin which slides through the holes in the former components?

Thanks.

RE: Modal Analysis of an Assembly

(OP)
Also, I read the "Linear Contact in Normal Modes analysis," and, according to namklof's suggestion, changed the "contact" condition to "glued," but that did not work either. The solver just failed.

It makes perfect sense that a "contact" problem would call for a non-linear solution, and that the "normal modes" analysis requires a linear model.

Does anyone have any ideas as to how to obtain the normal modes of an assembly while maintaining the contact constraints between the components of an assembly?

RE: Modal Analysis of an Assembly

Linear contact in a Normal Modes Analysis requires a pre-stiffened analysis as a subcas: the solver needs to add the differential stiffness coming from the contact conditions, in order to account for the contact constraints.

I copy here the paragraph "Special Note about Pre-Stiffened Modal Analysis", page 8-69 on the FEMAP User's Guide:

"You will notice that during a Normal Modes analysis, that you cannot choose a Load Set in the Boundary Conditions
dialog box of the Master Analysis Case. You can run a pre-stiffened modal analysis in FEMAP by adding a
loading condition to your model, but it does require a subcase to be created in order for it to work properly. The
exact process is to go to the bottom of the "tree structure" in the Analysis Set Manager and highlight the "No Case
Defined" branch of the "tree". Now click the New button and FEMAP will prompt you to create a subcase. Click
Next and you can now add a loading condition to the model using the Loads pull-down menu (you will need to have
a load set for this to work properly). You will notice an additional SUBCASE has been added to the Case Control
section and a STATSUB entry can be found there which will prompt Nastran to run a Pre-stiffened Modal Analysis.
Now run the analysis and your results will reflect pre-stiffened modal analysis. A pre-stiffened modal analysis is
needed when using Linear Contact with NX Nastran SOL 103."

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources