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Vibration Information for rotating machinery applications.

 

Fundamentals of Vibration: Time Domain

Friday, May 20, 2011

Rotating machines such as fans, angle grinders, driveshafts, and compressors vibrate when they are operating. The vibration can be captured by a transducer (displacement, velocity, or acceleration). In order to understand vibration, we can start with a simplified problem. Let’s take an example of a fan that rotates on it’s axis. If we attach a weight on one of its blades and put a transducer on the bearing, we should see a waveform shown in the Figure below.

The waveform shown in the Figure is known as time domain. It is a pure sinusoidal waveform, which is generated by putting a weight on one of its blades or mass imbalance. The chart shows the magnitude of the vibration over time. Other information can be extracted from the chart:

Period

Period, T is the number of cycles per revolution. It tells us how many cycles in one revolution. The unit is given in second or s. In the case above, the waveform has one cycle in one revolution, T = 1s.

Frequency

Frequency, F is the number of occurrences or revolutions in one second. It is the reverse of period. The unit is usually given by Hertz or Hz. Other units that are commonly used are revolutions per minute or RPM and cycles per minute or CPM . The frequency of the vibration is, F = 1 Hz or 60 RPM.

Amplitude or peak (pk)

Amplitude or peak is the amount measured from 0 magnitude to the top peak. In this case the magnitude is 2 m/s. Amplitude is commonly used to indicate the severity of the vibration.

Peak-to-peak or (pk-pk)

pk-pk is the amount measured from the bottom of the trough to the top of the peak. For pure sinusoidal waveform, the value is equal to 2 times the peak value.

Root mean squared (rms)

RMS is measured by squaring the values, then averaging, and then the square root is obtained to arrived at the value. For pure sinusoidal waveform, the value is 0.707 x pk.

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