In the previous chapter, spectrum analysis was shown to be a powerful tool in determining the condition of various components inside a given machine. Vibration Measured on Non-Rotating Parts of a Machine (Absolute . Applying Spectra and Phase Analysis to the Diagnostics of Machine Faults. Spectrum Plots & Vibration Analysis - This is the most commonly used analysis tool and is usually sufficient protection for general speed machinery.


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If a rotor is operated at or near a critical speed, it will exhibit high vibration levels, and is likely to be damaged.

Spectrum Plots

Much rotating equipment is operated above its lowest critical speed, and this vibration spectrum analysis it should be accelerated relatively rapidly so as not to spend any appreciable time at a critical speed. Cross Correlation Cross correlation is a measure of the similarity in two time domain signals.

If the signals are identical, the cross correlation will be one, and if they are completely dissimilar, the cross correlation will be zero. Certain dual-channel FFT analyzers are able to measure cross correlation. Cycle One complete period of a periodic waveform is called a cycle.

The units for frequency vibration spectrum analysis to be called "cycles per second" until the ISO standardized on the term "hertz", in honor of Heinrich Hertz, the noted German scientist who was an early investigator of radio wave transmission.

Damped Natural Frequency If a resonant mechanical structure is set in motion and left to its own devices, it will continue to oscillate at a particular frequency known as its natural frequency, or "damped natural frequency". This will be a little lower in frequency than the resonant frequency, which is the frequency it vibration spectrum analysis assume if there were no damping.

The resonant frequency is also called the "undamped natural frequency".

Vibration Analysis Dictionary - Mobius Institute

Damping Damping is the dissipation of energy within a mechanical structure and its conversion ultimately into heat. There are several different mechanisms for damping, the most important two of which are coulomb damping and viscous damping.

Degree of Freedom In the description of the motion of structures or objects, a degree of freedom is one of several orthogonal components that can be used to completely characterize the motion.

For vibration spectrum analysis, a free object in space has six different degrees of freedom -- it can translate in three mutually perpendicular directions, and vibration spectrum analysis can rotate about the three mutually perpendicular axes.

Any motion of the object, no matter how complex, vibration spectrum analysis be resolved into these 6 basic motions. Some objects may not have all 6 degrees of freedom available to them; for instance an elevator in an elevator shaft is constrained to 1 degree of freedom.

Spectrum Plots & Vibration Analysis

When describing the motion of a complex structure, different parts may be constrained in different ways, and a great many degrees of freedom may be required to fully describe the overall motion of the structure. In performing modal analysis of a structure or in finite element modeling of a structure, it is not uncommon to consider hundreds of degrees of vibration spectrum analysis.

Demodulate, Demodulation Demodulation is the process of recovering the modulating signal from an amplitude modulated AM or frequency modulated FM. The demodulator is also called a detector.


In the field of vibration spectrum analysis analysis, it is sometimes found that certain signal components, such as 1X or run speed, will modulate other components such as gear mesh frequencies or bearing tones. A demodulator can be used to detect and recover these modulating signals.


See also Amplitude Modulation and Frequency Modulation. Detector An electronic circuit that determines the amplitude level of a signal in accordance with certain rules. The simplest type of detector consists simply of a resistor and capacitor, and vibration spectrum analysis measures the average value of a continuous fluctuating DC signal.

A more complex but must more useful detector is the RMS detector, which is almost always used in vibration analysis systems.