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<p>i want to charecterise the type of faults and wants to distingush the fault basic frequency.</p>
Log scale is used when we want to see both low amplitude and high amplitude frequencies with same clarity. While using Log Scale, Low amplitude frequencies are amplified and high amplitude frequencies are compressed such that both are accommodated in same graph for visual interpretation.
Linear scale is normal scale which does not amplify or compress any signals. Advantage of this scale is that we can have correct interpretation of defect frequencies based on visual display. Only disadvantage is that small / very small defect frequencies are diminished when in presence of high amplitude signals
best to look at vibration spectra with a linear amplitude scale because that is a true representation of the actual measured vibration amplitude. Linear amplitude scaling makes the largest components in a spectrum very easy to see and to evaluate, but very small components may be overlooked completely, or are at best difficult to assign a magnitude to. The eye is able to see small components about 1/50th as large as the largest ones in the same spectrum, but anything smaller than this is essentially lost. In other words, the dynamic range of the eye is about 50 to 1
Linear scaling may be adequate in cases where the components are all about the same size, but in the case of machine vibration, beginning faults in such parts as bearings produce very small signal amplitudes. If we are to do a good job of trending the levels of these spectral components, it is best to plot the logarithm of the amplitude rather than the amplitude itself. In this way, we can easily display and visually interpret a dynamic range of at least 5000 to 1, or more than 100 times better than the linear scaling allows.
To illustrate different types of amplitude presentations, the same vibration signature will be shown in linear and two different types of logarithmic amplitude scales.
It might be said that the dynamic range of the eye, when looking at linear spectra, is about 34 dB.
Linear scale can be used when you concentrate your analysis on the fundamental frequencies
Log scale has to be used to interpret the low amplitude signals especially high frequency component and their harmonics generated by bearings, Y axis log scale can be useful in Sideband analysis of electric motor line frequency
It may seem to be best to look at vibration spectra with a linear amplitude scale because that is a true representation of the actual measured vibration amplitude. Linear amplitude scaling makes the largest components in a spectrum very easy to see and to evaluate, but very small components may be overlooked completely
Log scale is used to amplify the low amplitude vibration frequencies for visual interpretation.
Linear scale is normal scale for high amplitude and low amplitude vibration frequencies.
log scale is used when you want to see both the low amplitude frequencies in the presence of high amplitude peaks in the spectrum.The log scale amplifies the low amplitude frequency.
log scale helps to analyze side bands, especially when their amplitudes are too small to be viewed in linear scale.
It is used especially in the analysis of bearing frequencies, because the log scale very powerful to see the small frequencies in the appearance of big frequencies .
The bearing frequencies is very small in amplitude in comparison to 1x and VPF or any forcing frequency.
Regards.
Ibrahim Masoud
