Mechanical Engineering
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# Vibration

## Question

Explain about 'g', the unit used to measure vibration?

Edited by Bheemappa Bandikeri

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7 hours ago, Bheemappa Bandikeri said:

Explain about 'g', the unit used to measure vibration?

The historical reason for using g is that accelerometers [invented in 1920 McCollum/Peters] were typically the preferred measurement device to quantify vibration, and g normalizes acceleration. Early displacement sensors (LVDTS [Hoadly/1936], time-of- flight/doppler vibrometer lasers) were either range or velocity or band limited or inaccurate/inordinately expensive. Reporting accelerations (typically for constant angular velocity, rotating machinery where F=mω^2 r ) is preferred since it is (generally) independent of the added payload. This lets you estimate the base excitation/reaction/inertial forces in service conditions by simply multiplying the reported g values with the payload mass, m at the operational velocity.

Hope that’s helps !

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The use of the g unit simply expresses acceleration in multiples of the acceleration of gravity. This is true in regard to vibration, linear motion, etc. Thus if an acceleration of 10 m/s^2 is slightly over 1 g = 9.81 m/s^2.

DrD

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On 9/2/2019 at 2:26 AM, Anildo Dacruz Afonso said:

The historical reason for using g is that accelerometers [invented in 1920 McCollum/Peters] were typically the preferred measurement device to quantify vibration, and g normalizes acceleration. Early displacement sensors (LVDTS [Hoadly/1936], time-of- flight/doppler vibrometer lasers) were either range or velocity or band limited or inaccurate/inordinately expensive. Reporting accelerations (typically for constant angular velocity, rotating machinery where F=mω^2 r ) is preferred since it is (generally) independent of the added payload. This lets you estimate the base excitation/reaction/inertial forces in service conditions by simply multiplying the reported g values with the payload mass, m at the operational velocity.

Hope that’s helps !

If for example, the vibration measued as velocity, with value of 10mm/sec. How to convert this into 'g'?

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The short answer is you cannot. A velocity value alone does not tell you what the acceleration is. If on the other hand you know that the displacement is sinusoidal with constant frequency, then

y=A*sin(omega*t)

y-dot= A*omega*cos(omega*t)

y--ddtor=-A*omega^2*sin(omega*t)

Your data includes say the max value of y-dot (a velocity), then you can find A*omega. From there, it is not hard to find the acceleration, all subject to the assumption of sinusoidal motion.

DrD

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