THEORY: Vibration Analysis

By Mark Lester

Aircraft Vibration Analysis

This section discusses making vibration measurements in aircraft. If you wish to have success in Vibration Analysis, you must take the time to understand how to collect measurements correctly. Making good, consistent measurements is the single must important step in good vibration analysis.

Vibration basics

To begin a discussion of vibration analysis, it is important to understand that the location of a vibrating body's surface varies with time. This motion of the body's surface is called a vibration or oscillation. The swinging of a pendulum and the motion of a plucked guitar string are typical examples of vibration. The theory of vibration deals with the study of oscillatory motions of bodies and the forces associated with them.

A vibratory system must, in general, include a means for storing potential energy (spring or elasticity), a means for storing kinetic energy (mass or inertia), and a means by which energy is gradually lost (damping or resistance).

Vibration measurement

A vibration transducer (sensor) is attached (or held by hand) to the vibrating surface during vibration measurement. The vibration sensor converts this mechanical motion into an electrical signal that corresponds to the body's motion in space. The vibration analyzer is then used to sample this electrical signal and make various calculations based on the electrical signal's properties.

Analysis modes

The signal values resulting from these calculations may be displayed in a number of different ways. Each different way of displaying the resulting data signal is called an Analysis Mode.

In some Analysis Modes, such as the Waveform Analysis Mode, signals are displayed in Time Domain. Time Domain readings are generally used to analyze impulsive or transient vibration signals.

Other modes such as the Spectrum Analysis Mode, utilize a display in the Frequency Domain. Frequency Domain readings are generally used to analyze repetitive signals such as those generated by rotating machinery. The entire signal or just a portion of it may be displayed depending on the analyzer settings chosen by the user.

Sometimes the user will choose to use an Analysis Mode such as the Overall Analysis Mode that computes a spectrum analysis and then combines the entire signal or portions of it into a single convenient numeric value.

A number of other analysis types are available to perform specialized signal analysis such as True RMS, True Peak and DC Scan Analysis Modes.

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