10 CHAPTER 7.VIBRATION MONITORING Since the effect of cepstrum is to compress whole families of harmonic frequencies to a single quefrency and perhaps one or two rahmonics,it seems to be an ideal parameter for sideband growth(Kim,Shin Carlson,1991).Most gear faults give a combination of amplitude and frequency modulation which lead to sidebands in the spectrum.The increase in number and strength of such sidebands usually indicates deteriorating condi- tion.Cepstrum has also been effectively applied for bearing diagnostics(Tandon Nakra, 1992) Cepstrum analysis is generally used as a complementary technique to spectral analysis. It is seldom used on its own as a diagnostic tool as it tends to suppress information about the global shape of the spectrum.Furthermore,the derivation of the cepstrum is not a routine signal analysis procedure,and one needs to exercise a certain amount of care. High-frequency resonance technique Envelope detection or High-Frequency Resonance Technique(HFRT)is an important sig- nal processing technique that helps in the identification of bearing defects by extracting characteristic fault frequencies (which may not be present in the direct spectrum)from the vibration signal of faulty bearings.Each time a defect in a rolling element bearing makes contact with another surface in the bearing,an impulse is generated.The impact excites resonance of the bearing elements,bearing housing,machine structure or trans- ducer.These resonances are excited periodically at the particular ball-pass frequency related to the location of the defect and are amplitude modulated at the characteristic fault frequency.By demodulating one of these resonances,a signal indicative of bearing condition can be recovered.In practice the signal is bandpass filtered around one of the resonant frequencies thus eliminating most of the vibration generated by shft imbalance, gear meshing or vibration from other parts of the machine.This bandpass-filtered signal consists of a narrowband carrier at the resonant frequency,which is amplitude modulated at the characteristic frequency.The signal is then demodulated by an envelope detector in which the signal is rectified and smoothed to eliminate the remaining components at the carrier frequency and its harmonics.The spectrum of the envelope signal is then obtained to get the characteristic defect frequency of the bearing.The resonant frequency best suited for the envelope detection technique has to be found experimentally by trying the technique at some of these major frequencies (Tandon Nakra,1992). The success of the method has been demonstrated by several investigators.The method works well for damage on outer races of bearings but not as well for inner races.Problems are also experienced with advanced damage in which case the defect frequencies may become submerged in the rising level of the spectrum. 7.3.3 Acoustic measurements Sound and ultrasound Measurement of the acoustic noise of a bearing has the attraction that it reflects more closely the response of the human ear.The detectability of defects by sound measurement may be affected by sources other than bearing noise unless adequate precautions have been taken to isolate the latter.A partial or full acoustic enclosure lined with sound absorbing