Natural Frequency Testing WIth an Impact Hammer

[TylerJO]

Hello All,

Quick question that I couldn't find a specific answer to on google. I'm doing some testing on a part to find it's natural frequency, my question is two fold:

1. The part is usually mounted on an engine, do I need to have it mounted as it normally would be to get a correct natural freq? Since the natural frequency is just a property of the part itself I would think it wouldn't change once bolted down, but I may be wrong. It would just save some time if I didn't have to mount it.

2. To find the natural freq. we're just mounting an accel on the part and then hitting it with an impact hammer. Resonance should then show up in the freq. spectrum. My question is there any reason we'd need to use any more than a 1 axis accelerometer? I occured to me that a 3 axis might be necessary since it's an odd shaped part but I can't remember if natural frequencies are directionally independent.

Thanks.

-Tyler

 

[Twoballcane]

1) Yes, more fastening points means higher Fn
2) You have to remember that where you place the accelerometers, you will be reading data at that point. If you place it on a lid on your UUT, you will read the lid’s Fn not the whole UUT. So place the accel on a place where it is ridged to make sure you get good data on the whole UUT. This is an art form in itself. Also, you should mount as many accels that would make sense, at least three, to see if most or all are reading the same, which means that the UUT is moving as a whole. If you can get your hands on ME Scope (

http://www.vibetech.com/go.cfm/en-us/content/mescope/x?),

your job would be easier.



Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”
"People get promoted when they provide value and when they build great relationships"

 

[IRstuff]

with the caveat that you take care not to alter the Fn with your sensors and installation.

TTFN
faq731-376

 

[electricpete]

The part is usually mounted on an engine, do I need to have it mounted as it normally would be to get a correct natural freq? Since the natural frequency is just a property of the part itself I would think it wouldn't change once bolted down, but I may be wrong. It would just save some time if I didn't have to mount it.

I would say that in general, the natural frequency(s) are a function of
1 - the part
2 - the method of attachment of part to the parent (engine)
3 - the mass/stiffness characteristics of the parent (engine)

It may or may may not be the case that 2 and 3 are irrelevant, depending on the system. Certainly resonant frequency of many parts will change if you change the tightness of mounting bolts for example.

When you say it depends on the part only, I'm not sure what you are saying... you want to test the part not mounted to anything? In most cases the lowest frequency will drastically change and that won't be representatative. As an example, just compare a beam: fixed/free vs free.

Sorry if I have misunderstood

=====================================
(2B)+(2B)' ?

 

[electricpete]

As a followup to my previous comments:

One way to attack the question would be make sure you understand the modes of interest (shape and frequencies).

An easier way would be just try a piece both ways and look at the difference.

Maybe you want to do both...the 2nd one is a good check on the first.

=====================================
(2B)+(2B)' ?

 

[Twoballcane]

If you can get ME Scope (

http://www.vibetech.com/go.cfm/en-us/content/mescope)

it will make your job easier.

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”
"People get promoted when they provide value and when they build great relationships"

 

[GregLocock]

You have three alternatives when testing. (A) free-free. this is great for the FEA guys, can be useful to the practical people. (B) in situ. This is a pain (that they need to cope with) for the FEA guys and useful for everybody. (C) bolt it to an infinitely stiff termination. This is OK for the FEA guys and somewhat useful for everybody else.

(B) is my preference. (C) is more trouble than it is worth. (A) can be misleading

Modes are directional, but the concept of x y and z is in the eye of the beholder, so it is unusual to get a mode that is strong in say x and non existent in y and z. But a triax is more informative.



Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[hydtools]

If you want to know only the frequency, listen to it.

Ted