34.4 Vibration Measurement and Techniques

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34.4.1 Units of Measurement

The most commonly used units of measurement of vibration are as follows:

* Displacement (peak to peak): millimeter (mm) or micron (1026 m) in metric units, and thou or

mil (0.001 in) in imperial units

* Velocity (peak, RMS or “true peak”): mm/sec or m/sec in metric units, and in./sec in imperial

units

* Acceleration (RMS): g or m/sec2 in metric units, and g or in./sec2 in imperial units

* Frequency: hertz (Hz) or cycles/min (cpm) in both systems of units

* Phase angle: degrees in both systems of units

34.4.2 Measured Parameters and Methods

Under steady-state conditions, the vibration from a rotating machine is a periodic signal of a complex

waveform. During unstable operation or upset conditions, the signal may become random in nature. In

certain types of machines, transient signals that are nonperiodic could also be present due to internal

impacts and damping. Based on the simple spring – mass system and the mathematical Fourier analysis

procedure, all periodic complex waveforms can be reduced to the sum of a series of sinusoidal functions.

In the case of random signals, averaging techniques are used to reduce them to periodic signals for

convenience of analysis.

A quantitative assessment of the vibration can be made in terms of the amplitude, velocity,

acceleration, or the magnitude of force of the motion. Other key parameters such as frequency, phase

angle, and the time-varying nature of the signal are important in fully characterizing it. Because the true

signal is not purely sinusoidal, it is important to identify its magnitude as, peak, peak-to-peak or root

mean square (RMS). The preferred parameter of measurement varies throughout the industry and

depends on the nature, complexity, and type of machine, and the purpose for which it is measured. A

general classification of measured parameters and techniques based on industry and rotating machine

type is given in Table 34.4.

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