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5-18
Measurement tasks
Envelope spectrum for
roller bearing analysis
Envelope spectrum for
gearing analysis
Measurement setup
for envelope spectrum
Envelope spectrum and
appropriate time waveform
Envelope analysis
Envelope analysis is a proven method for detecting periodic
impacts in the vibration signal of a machine. It is mainly used for
the diagnosis of roller bearing damage and gearing damage and
is based on the demodulation of amplitude modulated vibration
signals.
Periodic impact sequences in a component excite high frequency
machine resonances that serve as a carrier signal of the low
frequency impact signal. To separate the low frequency modulation
from the high frequency resonance, the signal is filtered with a
high pass filter, rectified and then filtered using a low pass filter.
Consequently, the remaining signal consists of the modulation
signal only - the so-called 'envelope' - from which the FFT of the
envelope frequency spectrum can be calculated.
VIBXPERT saves the envelope time signal filtered last together with
the envelope frequency spectrum.
Note
To display the time signal, press the MENU key and select 'Info /
Time waveform' (see below).
VIBXPERT provides two methods for calculating the envelope: High
pass and low pass filtering is carried out on the basis of hardware
or software.
In the case of hardware filtering, the signal is processed in a filter
chip. This method is identical to that used in the VIBSCANNER FFT
data collector and, hence, is suitable for comparison measurements
with both devices.
Software filtering, on the other hand, calculates the envelope
with the aid of an algorithm. This method is more flexible that the
hardware-based method and offers more filter combinations.
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