Natural Frequency

[sapto]

Hi everyone...I have problem to calculate natural frequency..can Pro/Mechanica calculate or analysis natural frequency....I heard Pro/Mech have dynamic analysis.


Thanks


sapto

 

[lbotez]

It sure can handel modal analysis. Go to Applications -> Mechanica -> Structure, setup your model for the analysis (loads don't count) then create and solve a modal analysis. If your model is unconstrained, the first six natural frequencies should be 0 (corresponding to the 6 degrees of freedom for the rigid body), so when asked how many modes should be calculated, be sure to ask for at least 7.

 

[Luis Aguirre]

Mechanica should be able to calculate natural frequencies. THis is done via the analysis type call modal. THe results you will get out this model will be natural frequencies and Mode shapes. Once you run the modal analysis then you can use this results to run a dynamica anlysis (ie. Dynamica time, frequency, shock, random)








Luis

 

[GeorgeTsonev]

do you have some tutorials of natural frequencies and dynamic analysisproblems
Edited by: GeorgeTsonev

 

[dhbigelow]

It is pretty straight forward, the analysis will return a set of "Frequencies" for each computed Structural "Mode" - these are the apex/high-points fo structural sensitivity for your specific design.


What you need to look for is a particular "Mode/Frequency"combination that is within the frequency range you may be concerned about. Specifically, in accoustic applications, a Mode that had a frequency within the performance/usage range may cast an audible buzz.


Another example, would be if your modal response (typically within the first 3-4 calculated modes) was within the operating/vibration range (especially lower ranges)are a good indicator of a tendancy for structual failure due to vibration.


You must take care with this type of analysis to make sure that the design is properly constrained, and is representative of the loads on the structure. Specifically, if you have another component bolted to it, you must make sure that the mounting relationship is constrained to effectively move as "one" and the mass and location of the mouted items are properly represented within the constraints. You can use some special FEA rigid-elements/links to represent connections between the Center Of Gravity of mounted components and the actual design.


A Quick Example, if you are designing a structure that must stay below 20-Hz (either due to acoustical concerns, or a vibration application), and it's first three Mode responses were calculated to be 25-Hz, 33-Hz,39-Hz.


You would have to either change the materials of the design (probably something stiffer) or change the structure of it (rib it, to stiffen areas with more predicted movement, etc). The trick is to balance all cases, and do so in such a way that you are not making it worse. After you run the analysis on the new settings/design, lets say it drops to 9-Hz, 12-Hz, 17-Hz. This would be acceptable and you could call it done.


A tip for not making things worse, is to use non-symmetrical design changes for modal stiffening. This sounds odd, but it it can make things WORSE if you just try to stiffen a design with a "pretty" rib pattern. This esspecially is a problem with flat plates and bracket types of designs that may have a component in the middle of it.


Hope this helps...


Dave

 

[Luis Aguirre]

George,


Do you have access to PTC costumer support web site? You canretrive the following document or call PTC and ask them to fax it to you.


http://www.ptc.com/cs/cs_23/howto/mst2477/mst2477.htm


If you need a little more down to earth theory and use of FEA get the following book. I read it few years ago found some errors but over all is a good book. The book probably has been revised since then.


Building Better Products with Finite Element Analysishttp://www.ptc.com/cs/cs_23/howto/mst2477/mst2477.htm





Luis
Edited by: Luis Aguirre

 

[GeorgeTsonev]

What do you think about Procision v 4.13.

If anybody work with Procision v 4.13 and PRO-E wild fire 2.0

Are there any differences in the analysis results for the same problems calculated with this two programs.

 

[dhbigelow]

AVOID IT LIKE THE PLAGUE! The Glerkin Method is more of a "toy" than a serious FEA method in my humble opinion.


I would not bet the farm on the results - regardless of what the sales guy says.

 

[GeorgeTsonev]

Do you carry out a combine analysis between dynamic
analysis and feasibility optimization, and if you have some tutorials
or other materials would you send them to me or tell me where I can
read such?

 

[dhbigelow]

Sorry - only going from experience in using FEA to solve my past real world problems. You can learn alot from the manuals and by reading the works of others.





Good Luck!





Dave

 

[Nelia]

[No message]

 

[Luis Aguirre]

Hope this helps!


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