effect of mass on vibration response

[abakus2010]

If I have 2 installations on a helicopter floor, similar natural frequencies but 1 weighs 5 kg the other 500 kg, would the response of each to the same random vibration input (PSD) be the same ??

 

[GregLocock]

Yes, in theory

No, in practice, as the input PSD would change usually.

Cheers

Greg Locock


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[abakus2010]

Do you mean because of mass damping effects ?

 

[Twoballcane]

It depends where you measure your response. If you put your accelerometer on the helicopter frame that leads up to your devices, this would be your forced frequencies (maybe peaking with the RPM of the rotor) thus your PSD in. If you put your accel on top of the masses, I would say that your PSD out would be different from one mass to the other.

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

 

[IRstuff]

A helicopter is not a perfectly rigid object, so its vibration characteristics vary from location to location, helicopter to helicopter.

Nonetheless, MIL-STD-810 and MIL-E-5400 use envelope curves that encompass any installation location on any of the helicopters covered by the specs.

TTFN

FAQ731-376

 

[abakus2010]

Thanks guys - I'm using Mil-Std-810 to give my input PSD (and peaks due to rotor and BPF) - it just seems logical to me that the bigger mass would have a lower output g level, I'm just not sure how to quantify it !

 

[Twoballcane]

You have to be carful and not think sinusoidal. As in what your entire unit will do at a certain frequency. In random all of your frequencies will be excited. So your “box” may not see much but your board and small electronic parts like flat packs and such will see higher frequencies and may have higher G loads.

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

 

[IRstuff]

If I understand what you are saying, the answer is no. The input stimulus is independent of your UUT. You mount the UUT on the shaker table, and you apply the input specified in 810. The helicopter is way more massive than your UUT, so your UUT will shake like the helicopter, not the other way around, particularly for something that's 5 kg, or even 500 kg. A UH-60 has an unloaded mass 10x that of your largest UUT, so there will be minimal impact to the input levels or spectrum because of your UUT, particularly if there are no resonances within the spectral limits.

TTFN

FAQ731-376

 

[GregLocock]

I disagree, the modal mas of the structure at the mounting point will be much less than the total mass of the structure so in reality even quite a small mass sees 'mass damping' (horrible phrase). For instance in a car masses of the order of 5-15 kg, and tuned mass dampers of the order of 1 kg, are enough to significantly affect the vibration in the structure, which has a mass of the order of hundreds of kg.

This does not apply on a test rig type qualification test of course.

Cheers

Greg Locock


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