Building the spectrum
Figure 1-59 shows a machine with 3 sources of vibration, motor speed, a bearing, and fan
blades. The waveform below the machine is the composite of the three. To the right of the
machine is a box with the individual frequencies overlaid on each other.
Figure 1-59 Three sources of vibration are combined in the composite waveform (left). They are
individually
overlaid in the box to the right of the machine.
The FFT process separates the individual sine waves from the composite waveform and displays
them according to their frequencies.
The spectrum is as if we are looking at those separated waveforms from the end. Notice how in
Figure 1-60 the individual waveforms are in a 3 dimensional box that is being rotated. Figure 1-61
shows the fully rotated box and the end view of each of the sine waves.
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Figure 1-60 The individual waveforms are shown in a 3 dimensional box that is partially rotated.
Figure 1-61 shows the 3 dimensional box rotated 90 degrees. The X axis of this view is
Frequency. The individual lines or peaks shown are the end view of each of the waveforms.
They are spaced apart according to their individual frequencies.
The tall peak on the left is the running speed of the pulley. The next peak is from the fan
bearing. The peak on the right is from the fan blades.
Figure 1-61 The 3 dimensional box is shown rotated 90 degrees revealing the end-view of the
waveforms. They
have been truncated so that nothing is shown below the zero line. The X axis is Frequency.
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Peaks relate to parts of the machine
This new view of the vibration called the spectrum is the key to seeing the condition of
machinery. The frequency tells the source of the vibration and the amplitude tells us about the
severity of the vibration.
Figure 1-62 Relating vibration frequencies to machine components
The concept described in Figure 1-62 is incredibly important in understanding vibration in
machines. What we have demonstrated here is that we can relate a particular machine
component to a particular frequency in the vibration spectrum. These are called “forcing
frequencies.” This is how we will know that we have a bearing problem as opposed to an
unbalance or misalignment problem. We will talk more about this in a moment.
an p
otor p
an la
arin
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Key points
? Students should understand what a vibration spectrum is.
? Students should understand the amplitude and frequency axes
o Frequency can be Hz or CPM
o Amplitude can be A,V or D
o Amplitude is pk, pk-pk or RMS
? Students should understand why we don’t simply analyze the time waveform.
? Students should understand the difference between the “time domain” and “frequency
domain”.
? Students should understand that the waveform is measured and that the spectrum is
calculated from the waveform using an algorithm called the FFT.
? The spectrum separates the vibration into frequencies.
? Different machine components will generate different frequencies
o This allows us to relate the vibration to the specific machine component