How to use results from natural frequency analysis? 6

[msjoey]

G'day everyone!

I have a basic rookie question regarding extracting natural frequencies and mode shapes. So I have 3 different designs of an automotive part for which I have run frequency analysis. The results are in, but how do I use them in the correct way? I have always been told that you pick the stiffest design by choosing the one with the highest (first) natural frequency. Is it really that simple? Or is there more science and art to judging the "best" design based on mode shapes?

Thanks in advance!
Jo

 

[IRstuff]

Seems to me that you should pick the overall best solution to your REQUIREMENTS. If you had a requirement for >100Hz first mode, wouldn't it probably be overly excessive to pick a solution that had a 1kHz first mode? Wouldn't it be likely that this choice would be overly heavy and therefore substantially more expensive to make?

TTFN
faq731-376

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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[kan123]

You also need to understand what loads the part will experience because it is probably the stress or strain that is one of the design drivers (besides weight as IRstuff mentioned).

 

[GregLocock]

"I have always been told that you pick the stiffest design by choosing the one with the highest (first) natural frequency. Is it really that simple?"

No. That is ridiculous advice. Suppose you had a couple of shaft designs for a machine that runs at 3000 rpm. Do you choose the one with a resonance at 50 Hz? or 30 Hz?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[ThomasH]

Hi

I would say that if you pick the design with the highest frequency you will have picked the stiffest design.

Since in short: F = SQRT (S/M)

S = Stiffness
M = Mass

But that does not mean that the stiffest design is the best for your particular situation. Basically, I agree with Gregs statement.

I usually say the frequency tells you when in vibrates, the mode shape tells you how it vibrates. But what is really interesting is often how much it vibrates.

Regards

Thomas

 

[msjoey]

Thank you very much for your feedback! In this case the comparison is done for a fender panel with different rib designs. The optimal design is a compromise between higher eigen frequency and minimal addition of ribs (to comply with weight budget).

I was simply wondering what is the use of viewing mode shapes because so few books and tutorials touch on this.

Much obliged!
cheers
Jo

 

[BigInch]

You typically want to avoid sustained loading conditions that cause your part to oscillate (generally) at any frequency between 0.7 * Fn to 1.4 * Fn

Within that range, any normal deflections caused by a static load can be amplified by dynamic resonance, bouncing effects, like when you jump on a trampoline. If you jump at intervals corresponding to the trampoline surface's natural frequency, you go higher and higher.

 

[BigInch]

You an also experience similar dynamic amplifications at 2 and 3 and 4 (any integer value) x Fn, but they will be successively lower than 1 x Fn

 

[BigInch]

Or a Pogo Stick would be a better analogy I suppose.

 

[BigInch]

But for both of those examples, the object is to experience higher and higher deflections, so consider those reverse examples

 

[stressebookllc]

On a side note, a modal analysis can also be used to find out mechanisms in your model... I have.. its useful, yeah!

http://www.stressebook.com

Stressing Stresslessly!

 

[jmarkus]

Assuming you understand your target (minimum or range) natural frequency, in order to assess or make improvements to the design I always find it is useful to review the strain energy (or better, strain energy density) distribution in the model. The way I think about it is that the modal animation may show the part moving around but the strain energy helps pinpoint whether that area of the part is going along for the ride, or is actually contributing to the overall stiffness. For example, you might put 5 ribs in your part, but upon review of the strain energy density, only 3 of the 5 show high levels - indicating they are helping.

Hope that makes sense.
Jeff