Maximum displacement measure

[bcg444]

Hello everybody,

As a beginner in vibrations I am looking for some help.
I am going to perform the vibration test where the displacement measurement of the system at first natural frequency is one of the main tasks.
Therefore as a first step I am going to run the sine sweep to determine this frequency.
As a test that allows me to determine the displacement I am going to dwell at the natural frequency.
The PSD profile obtained from field measurements is the only data I have related to vibrations that the system will be subjected in reality.

My questions: what is the appropriate acceleration for the dwell test? Should it be 3 times G (rms) from PSD? Is this approach right, in general?

Thank you very much for help

 

[SomptingGuy]

If your system is lightly damped, you'll find it hard to "dwell" at the (I assume lowest) natural frequency. More likely you'll break your equipment! Be careful.

 

[IRstuff]

The maximum acceleration used in MIL-STD-167-1A for endurance testing is 1.28 g at 25 Hz:

http://assist.daps.dla.mil/eAccess/...689EBD&s_key=37E8F231E98B7BC0E6E55980875DDD62

Dwell does not have a direct linkage to your random PSD, since the resonance coupling is not equal for all frequencies. Moreover, since there is amplification at resonance, there is usually little need to pump in additional energy.

TTFN

 

[bcg444]

Thanks,
I would probably describe my goals in a different way:
Our system is mounted within the enclosure. To determine the minimum size of this enclosure I need to know the maximum displacement of the unit due to vibrations (the unit is mounted on vibration isolators that allow some movement).
Is there way of measuring or calculating the maximum displacement during the random vibration test? I was told that the only option to measure it is during a sine test.

 

[IRstuff]

Is the system subjected to shock? Usually, shock will get you sooner than vibration.

Is the isolation purely passive?

Is the vibration purely random?

If you overlay the PSD response with diagonal lines the represent displacement, you can find the point of largest displacement, which may or may not be the resonance point.

TTFN

 

[Twoballcane]

Can you program your shaker with the PSD profile? Is your PSD in hz^2/g vs hz?

Tobalcane
"If you avoid failure, you also avoid success."

 

[GregLocock]

"Is there way of measuring or calculating the maximum displacement during the random vibration test?"

Modelling clay. And, no you can't calculate it, not if you have true random inputs. The chances are that you don't have true random excitation.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bcg444]

Would the following approach be appropriate:
Having the random input profile and knowing natural frequency of my system, I can calculate the G(RMS) level of the narrow band around this frequency (lets say +/-5%).
Then multiplying it by 3 (3?) I can get the estimation of G at this frequency.
Thanks

 

[GregLocock]

No. If the signal is truly random then it can have an infinite amplitude at any one time, so the response will be infinite. You'll probably find the spec of your generator has a peak to RMS limit (if it is any good), and you will certainly find that your excitation system has limits. Both of these make the excitation non random to some extent.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bcg444]

Thanks for all the inputs. I'll try to figure out what is the best test procedure to follow in order to get some valuable results.