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The use of computers has in many cases replaced taping of the data. The use of high speed digital acquisition signals are stored directly in memory forfurther storing to a harddisk， and then processing it through a fast Fourier transform program.
.nterpretation o. Vibration Spectra
The spectrum analyzer correctly depicts the frequency content of each time-domain instant.however， the time-domain picture as well as its frequency-domain counterpart of a continuous signal change with time. Averaging is used to show which amplitudes predominate in a continuoussignal. For the most part， machinery vibrations result in ""stationary"" signals. A stationary signal has statistical properties that do not change with time. Inother words， the average of a set of time-history records is the same regard-less of when that average is taken. (A stationary signal is demonstrated by a machine running at constant speed and load. Averages are also used indiagnosing startups and load changes of machinery. In thisusage， averages of successive time intervals show the change in vibration levels and frequen-cies taking place.)
Averaging is a technique to improve the SjN ratio. Two or more succes-sive spectra made up of both periodic and random (noise) signals are added together and then averaged. This combination results in a spectrum with a periodic component that is much the same as when viewed in the instant-aneous signals but with random peaks of much less amplitude. This resultoccurs because the period peak stays at a fixed frequency in thespectrum， while the noise peak is fluctuating in frequency over the spectrum.
The fact that averaging removes noise-related signals is demonstrated by the instantaneous and averaged spectra shown in Figure 16-4a taken from the taped signal of a machine being diagnosed. A representation of the normal instantaneous spectra is shown in the second spectrum. An instantaneous signal clearly caused by noise was exhibited at one point in the tape and is shown in the upper spectrum. Note that the contribution of the instantaneous noise signal does not appear in the averaged signal. The large peak on the plots is the running frequency. .esser harmonics of the running speed also appear. The importance of the instantaneous signalshould not be overlooked. .uring startups， a long-term average may eliminateimportant parts of the spectra， which change because of the change in rpm.Also， nonperiodic impulses such as those caused by random impulsive loading may be masked by an average. Short averages can be used in ""waterfall"" graphs to show the growth of certain frequency patterns at run-up as shown in Figure 16-4b.
The frequencies of a spectrum can be divided into two parts: subharmonicand harmonic(i.e.， frequencies below and above the running speed). The subharmonic part of the spectrum may contain oil whirl in the .ournal bearings. Oil whirl is identifiable at about one-half the running speed (as are several components) due to structural resonances of the machine with the rest of the system in which it is operating and hydrodynamic instabilities in its .ournal bearings. Almost all subharmonic components are independent of the running speed.
The harmonic part of the spectrum may contain multiples of runningspeed， blade passage frequencies (given by number of blades times therunningspeed)， gear mesh frequencies (given by number of teeth times therunning speed)， andfinally， solid-disc resonant frequencies of the gear discs (independent of the running speed). Roller contact bearings may add another component based on the number of rolling elements present. Inaddition， a once-per-revolution or first harmonic frequency is caused by mechanical imbalance. Table 16-1 shows more of the ma.or diagnostics.To identify these frequencies with the various machine components， a base-line signature should be obtained.
Figure 16-4a. Noise attenuation by averaging
To be able to do effective trouble-shooting on any particular machine， it is necessary that the baseline signature of the machine be available and thoroughly analyzed. A baseline signature is the spectrum of machine vibra-tion when the machine is operating under ""normal conditions."" Generally， ""normal conditions"" are difficult to define and are .udgmental in nature.认hen a machine is first installed， or after it has undergone an overhaul， a vibration spectrum should be taken and stored to serve as a ""baseline"" for
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