Acceleration (Vibration) Of source unit in multiples if G 2

[ninjaz]

Sometimes i observe , the vibration data of a unit is in terms of G. Why it is provided like that. Whether the accelerometer gives the reading in terms of G or some other facts behind that?

Your comments would be really appreciated.

 

[IRstuff]

power spectral density is in units of g/root(Hz) or g^2/Hz

TTFN
faq731-376

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[ninjaz]

Thank you IRstuff..can you explain with little more clarity.

 

[BlasMolero]

Dear Ninjaz,
G level is a "convenient" way to define an acceleration we use a lot in Dynamic analysis with NX NASTRAN, for instance in Response Spectrum Application usually the excitation curve is defined in G levels vs. frequency (Hz), or sometimes directly in real acceleration vs. frequency -- see the following image. Is also typical in Modal Frequency Response (SOL111) or Transient Dynamics (SOL112) with Enforced Base Motion with the structure being forced at a base node with a 1g sinusoidal (Seismic) loading condition.

But do not get confused, soon or later we enter in the FEA package the real value of acceleration used, ie, 1 G = 9.81 m/s2, (or the equivcalent value in the system of units used in the FE model). In the following picture we use Acceleration Scale Factor = 1.0 because the above excitation curve of response spectrum acceleration was defined in in/s2 vs. freq (Hz). Alternatively, you can define the excitation curve of acceleration in Gs vs. Freq. (Hz), and use an acceleration scale factor when defining the loading as 1G = 386.4 in/s2, then is up to you, we are talking the same all the time, OK?.

Best regards,
Blas.

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Blas Molero Hidalgo
Ingeniero Industrial
Director

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[GregLocock]

Vibration is often measured in terms of acceleration, and obviously needs some units. 1 g is an acceleration of 9.81 m/s/s or 32.2 fpss.

Confusingly for the statically minded loads are oftem mentioned as being in g, for example I'd refer to a 0.9g load on a wheel. That merely means the weight multiplied by 0.9.

Newton says that's OK.



Cheers

Greg Locock


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