Vibration Measurement: The Complete Guide

As vibration isolation and reduction techniques have become an integral part of machine design, the need for accurate measurement and analysis of mechanical vibration has grown. Using accelerometers to convert vibratory motion into an electrical signal, the process of vibration measurement and analysis is ably performed by the versatile abilities of modern electronics.

CONTENTS

1. What is vibration?
2. Where does vibration come from?
3. Quantifying the vibration level
4. The Vibration Parameters: Acceleration, Velocity and Displacement

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VIBRATION MEASUREMENT
BY BRÜEL & KJÆR

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What Is Vibration?

A body is said to vibrate when it describes an oscillating motion about a reference position. The number of times a complete motion cycle takes place during the period of a second is called the frequency and is measured in hertz (Hz).

The motion can consist of a single component occurring at a single frequency, as with a tuning fork, or of several components occurring at different frequencies simultaneously, for example, with the piston motion of an internal combustion engine.

Vibration signals in practice usually consist of very many frequencies occurring simultaneously so that we cannot immediately see just by looking at the amplitude-time pattern, how many components there are, and at what frequencies they occur. These components can be revealed by plotting vibration amplitude against frequency. The breaking down of vibration signals into individual frequency components is called frequency analysis, a technique which may be considered the cornerstone of diagnostic vibration measurements.

The graph showing the vibration level as a function of frequency is called a frequency spectrogram.

When frequency analyzing machine vibrations, we normally find several prominent periodic frequency components that are directly related to the fundamental movements of various parts of the machine. With frequency analysis, we are therefore able to track down the source of undesirable vibration.

Where do Vibrations Come From?

In practice, it is very difficult to avoid vibration. It usually occurs because of the dynamic effects of manufacturing tolerances, clearances, rolling and rubbing contact between machine parts, and out-of-balance forces in rotating and reciprocating members. Often, small insignificant vibrations can excite the resonant frequencies of some other structural parts and be amplified into major vibration and noise sources.

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VIBRATION MEASUREMENT

Sometimes though, mechanical vibration performs a useful job. For example, we generate vibration intentionally in component feeders, concrete compactors, ultrasonic cleaning baths, rock drills, and pile drivers. Vibration testing machines are used extensively to impart a controlled level of vibration energy to products and sub-assemblies where it is required to examine their physical or functional response and ascertain their resistibility to vibration environments.

A fundamental requirement in all vibration work, whether it is in the design of machines that utilize its energies or in the creation and maintenance of smoothly running mechanical products, is the ability to obtain an accurate description of the vibration by measurement and analysis.

Quantifying the Vibration Level

The vibration amplitude, which is the characteristic that describes the severity of the vibration, can be quantified in several ways. On the diagram, the relationship between the peak-to-peak level, the peak level, the average level, and the RMS level of a sinewave is shown.

The peak-to-peak value is valuable in that it indicates the maximum excursion of the wave, a useful quantity where, for example, the vibratory displacement of a machine part is critical for maximum stress or mechanical clearance considerations.

The peak value is particularly valuable for indicating the level of short duration shocks etc. But, as can be seen from the drawing, peak values only indicate what maximum level has occurred, no account is taken of the time history of the wave.

The rectified average value, on the other hand, does take the time history of the wave into account but is considered of limited practical interest because it has no direct relationship with any useful physical quantity.

The RMS value is the most relevant measure of amplitude because it both takes the time history of the wave into account and gives an amplitude value that is directly related to the energy content, and therefore the destructive abilities of the vibration.