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# Eigenvalue response to force injection with Ansys

## Eigenvalue response to force injection with Ansys

(OP)
Hi dear users,
I was asked to calculate the natural frequency of a small component mounted on the airplane.
I must evaluate also the first five natural modal shapes.

There are no natural frequency in the near and the force injected has the following requirement:

I will create the mesh and impose a fixed constraint at the lower surface near the bolts but I am not sure with kind of analysis to use in Ansys.
I would go for Modal but I do not know how to apply a nominal force wich such a frequency. Do you have any idea or tips?

Thank you so much.

Best Regards

### RE: Eigenvalue response to force injection with Ansys

Resonant frequencies don't need forcing, you are looking for the natural mode shapes which are not dependent on where they are excited. basically root(k/m).

Sorry can't help you with Ansys, but you are looking for a dynamic linear model. Be very careful with boundary conditions, very few joints are as stiff as you think they are. Rule of thumb on cars is 5000 N/mm.

Cheers

Greg Locock

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### RE: Eigenvalue response to force injection with Ansys

Are you seriously modeling the stiffness and response of a flashlight bracket?

If you have to model it in ANSYS, you already must have a design flaw. Not that I can see exactly what it is from the information provided, but if that's a heavy metal-cased flashlight and if the clips are aluminum then you definitely have a problem. Clips like that should be steel, preferably hardened, and make a "pling" noise when you tap them.

There are solutions in Roark Formulas for Stress and Strain that you can use to estimate the natural frequency of these clips. You'll have your answer in 15 minutes and then you can move on to solving whatever problem has happened. There's an even better book for your problem but I can't remember the title right now - it looks exactly like Roark's and it's focused on vibration analysis.

### RE: Eigenvalue response to force injection with Ansys

with you SW !! But you know how these things develop ... someone asks a question "what about vibration ?", and maybe the initial answer was "seriously ?", so then the question became "prove (sorry, Prove) to me there is no vibration problem".

I hope you have the airplane's frequency spectrum so you can compare ? Knowing the flashlight's resonant frequency (or the resonant frequency of the flashlight installed on those flexible (read isolating) brackets is "meaningless" without knowing the frequency spectrum that the installation is victim to. And this is presumably being installed on an interiors bulkhead.

My problem with this is the single fastener attaching each clip.

### RE: Eigenvalue response to force injection with Ansys

Blevins, that's the one, Greg.

RB: And this is presumably being installed on an interiors bulkhead.
Indeed, and just how much damping of *every* exciting frequency will that give you?

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