Figure 16-8.Vibration spectrum (rpm =20，000， Pd = 1.20 psig)
amplitude can indicate problems such as unbalance. The spectrum showing this unbalance is in Figure 16-9. Misalignment problems can be also analyzed. Figure 16-1 shows a plot obtained from a casing-mounted pickupand the classical， high twice-per-revolution radial vibration. A high axial vibration also exists that is usually more prominent in diaphragm-type couplings. A high-speed machinery plot is shown in Figure 16-11. To deter-mine what the various frequency components represent， a detailed analysis of the machinery components must be known. This information consists of
Figure 16-1.. Gear bo. signature (low.fre.uency end)
the number of blades inthe impeller， the number of diffusers or nozzleblades， the number of gearteeth， the resonant frequencies of the blades orcasing (for antifriction bearings)， the number of balls orrollers， and (fortilting-pad hydrodynamic bearings)， the number of pads.
The use of accelerometers for diagnosing problems is very effective， since in many cases the high-frequency spectra give much more information than the low-frequency spectra obtained from proximity probes. An example canbe seen in Figure16-1卻， which shows that the two gear drives are in good mechanical condition. Figure 16-13 shows the high-frequency accelerometer signatures. These indicate a problem with gear A (a cracked or chipped tooth).
Accelerometers can also be used to detect problems with stator angles or tip stalls in axial-flow compressors. The analysis from proximity probesindicates that there is a high running-speed vibration， which can be accept-able. An analysis of the accelerometer spectrum (Figure 16-14) shows astrong frequency component of thefirst， second， and third harmonic of the fifth-stage stator blade. An inspection of the blades indicated cracks caused by low-stress high-cycle fatigue.
Figure 16-15 shows acoustic signatures of three .et engines of the same type installed in three different aircraft. The data were recorded with theaircraft at altitude， one engine at power and the other at flight idle. The top signature is the normal signature for this engine configuration. In the middle signature the once-per-revolution or unbalance components of the
Figure 16-1.. Gear bo. signature (high.fre.uency end)
Figure 16-14. A.ial.flow compressor spectrum showing blade passing fre.uency
fans on both engines areconsiderably greater than normal， indicating apoor fan balance. On the other hand， the once-per-revolution component of the gas generator at power is less than thenorm， indicating better balance. The bottom plot shows a third engine with a fan damaged by ingesting a bird on takeoff. The damaged fan has a large unbalance asshown by the size of the once-per-revolution component. Inaddition， the second-and third-order fan harmonics are very prominent compared to the two other signatures.
Obtaining baseline signatures is a very useful tool for detecting deterior-ation of an engine with time. Figure 16-16 compares the signatures of the machine when installed and after a couple of years of operation. Thespectrum shows an increase in level at the high-frequency range， indicating blade flutter problems. Inspection of the unit indicated a number of cracked blades. Another example (Figure 16-17) shows the increase over time of astator resonant frequency， indicating a high flutter of the blades. Inspection indicated cracks on that stage blade.
Spectrum analysis is a very useful tool in analyzing machinery problems. spectra in both subharmonic and high frequencies are needed to evaluate machinery problems fully.
Bibliography
Bickel，H.J.， "".alibrated Frequency .omain Measurements Using the Ubiqui-tous，"" SpectrumAnalyzer，F(xiàn)忡 忡r號(hào)lScJ忡ntJfJcMonogr號(hào)ph 2， January 197 .
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