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vibration modes ?

Discussion in 'Simulate for Geomagic Design (FEA)' started by DavidJ, Jul 22, 2013.

  1. DavidJ

    DavidJ Alibre Super User

    Has anyone had any success using Simulate to check natural frequencies of a part or assembly?

    I get as far as specifying all part to be rigidly joined (it is a welded mixing shaft assembly), and choosing vibrational analysis - after that I can't work out how to proceed. Generally the help/examples are a bit thin on explanation.

    Do I have to set this up as a dynamic simulation? - I'd have expected not, just for vibration.

    Any tips most welcome, I'm getting quite frustrated.
  2. Alexander

    Alexander Senior Member

    Hi David,
    Did you manage to produce a vibrational analysis for the above, I know its a few years ago but I have a similar need for a project.
    I have DST Motion but not the FEA which I am considering purchasing if it's up to the job?

  3. Lew_Merrick

    Lew_Merrick Alibre Super User

    To be clear, Dynamic Simulation Technologies should stay away from things they do not understand! Material Properties and Analysis rank quite high on that list. A compatriot of mine (Dr. Alex Deak) wrote a rather scathing review not long after they took over Visual NASTRAN (I no longer remember where he posted it). One of Alex's more "publicizable" comments was, "These people cannot accurately relate room temperature to absolute zero accurately!"

    Now, to be precise, Alex hates virtually all "preprogrammed analysis systems." I had him use FEMdesigner on a project we worked together. Alex's comment was that it (FEMdesigner) was "almost acceptable." [This, as an "evaluation" from Alex is quite high praise.] FYI, Alex was a very senior "systems dynamic analyst" for NASA from shortly after the Moon landing until the early 2000-teens. Alex is one of four people to whom I pat close attention in this realm of things.
  4. Alexander

    Alexander Senior Member

    Thanks for the advice Lew.
    I'm desperately trying to find a way to simulate vibrations for a personal project I have been working on for a few years now.
    The frequencies will be of the audible range 20 to 20,000hz and will be structural borne. I am designing a range of platforms on legs which will absorb vibrations from components placed on the platforms.
    I contacted DST and they said it is possible but it will not take into account the natural frequencies.
    I've looked at FEMdesigner (waiting for a demo download) it sounds okay but does it simulate assemblies and if so how does it calculate the joints of each component.

    I've looked at ANSYS mechanical which seems very good but a bit out of my price range.

    Any help would be appreciated. ;)
  5. Lew_Merrick

    Lew_Merrick Alibre Super User

    Alexander -- You end up having to "re-constrain" the Parts making up an Assembly in FEMdesigner manually to get things right. It is somewhat of a PITA, but apparently the Constraints were not well documented back when it was created. The truth is that nobody (except for a couple of USAF and NASA programs I have run into over the years) does a good job of calculation stresses or strains in joints held together with screw threads. [I have a UN Thread Data sheet that provides the shear area per inch of engagement for internal and external threads. It would require 25 such Tables to cover Metric threads properly.]

    The main issues for Welded Joints is to get the Material Property definitions correct for the welding filler.
  6. Alexander

    Alexander Senior Member

    Thanks Lew, it does make me wonder how any of the FEA software can be reliable/accurate when most of the joints can't be simulated.
    Probably one reason why your friend Alex hates all analysis systems.
  7. Lew_Merrick

    Lew_Merrick Alibre Super User

    Alexander -- Take a look at the difference between a screw's Major Diameter Area and the Effective Tension Area sometime...

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