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Enveloping is one of the most
utilized methods to diagnose bearings. This technique is based
on the constructive characteristics of the bearings and is able
to find shocks and friction even from the very beginning of a
flaw.
Bearing casing vibrations
The bearing case can vibrate for multiple reasons: imbalances,
misalignments, shocks, etc. Unlike the other vibration sources,
following the high speed forces variation, the shocks may excite
the bearing case at its resonance frequency.
Of interest in the bearing diagnosis are the occurrence
frequency and amplitude of such oscillations.
Although the resonance frequency manifests within a rather
narrow frequency band, the amplitude modulation process resulted
from those shocks and the variation of the transmission
conditions, calls for an analysis over a broader frequency band
centered on the resonance frequency.
The increasing of the analysis band is often limited by the
presence of the high frequency signals that overlap that band.
Fault frequencies
The occurrence frequencies of the shocks resulted from the
faults in the bearings are called fault frequencies. They depend
on speed and bearing geometry. Knowing the speed, we can get a
direct relation between the measured fault frequency and the
type of flaw.
Enveloping
The vibrations signal envelope is a
low frequency signal that follows the peaks of the filtered
input signal.
The envelope frequency spectrum
contains components with a frequency equal to the impact
occurrence rate and amplitude proportional to their energy.
The main steps in the vibrations
envelope analysis are:
-
Filtering
-
Enveloping
-
Decimation
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Spectrum analysis
Bearings analysis with DSA 500
The 100 kS/s/ch acquisition speed,
24-bit resolution, and the 524288-sample analysis buffer make
the DSA 500 analyzer be fit for high accuracy diagnosis.
The Bearing Diagnosis function
monitors a range of waveform and frequency spectrum parameters
that are specific to bearing diagnosis.
Fault frequencies are computed from
the input data on the bearing geometry, or loaded from a
user-customized database.
Cursors to identify the fault
frequencies, their harmonics and combined faults are available.
The vibrations signal may be
simultaneously process on multiple channels in order to monitor
both the overall machine status (overall vibrations, imbalance
faults and misalignments) and the status of one or more
bearings.
The analysis may be performed in the
frequency range for the constant speed regimes or order range
for the transient regimes.
Besides the specific bearing fault
noise, the vibrations signal contains certain signals of various
sources: imbalance, misalignment, gearing, and high frequency
noises.
Band-pass FIR filters remove these
components while keeping the position and amplitude of the
bearing’s faulty components unaltered.
The spectrum analysis of the input
signal and filtered signal leads to the overall assessment of
the machine and bearing status, and the optimization of the
filter.
The envelope frequency spectrum
shows the fault type and gravity.
The enveloping operation is
sufficient provided that the bearing fault signal is being as
centered as possible on the zero value. The presence of a
perturbation in the resonance band may vertically translates the
fault signal so that it could not pass through zero anymore.
The presence of a perturbation in
the resonance band makes the envelope’s frequency spectrum not
contain the components specific to the bearing fault.
Multiple band-pass filtering allows
maintaining a broad analysis band by selecting those bands that
are not affected by unwanted signals.
The signal obtained is available for
both the envelope and the waveform analysis.
Keeping the bearing fault energy by
using the multiple band-pass filter helps identifying it in the
envelope spectrum even from the beginning.
Frequency resolution is an important
factor in the identification of the bearing faults and accurate
determination of the bearing’s functional and constructive
characteristics.
Through the decimation operation and
spectrum analysis over 262,144 lines, DSA 500 provides a
milihertz resolution while the frequency domain is tens of
kilohertz.
The parameters of the input,
filtered and envelope signals are recorded in a proprietary,
Excel or Access format for monitoring and trending the overall
condition of the machines over time.
Non-linear trends generally are an
indication of impending problems. This warning enables the
maintenance department to schedule the necessary repairs before
an unexpected failure occurs, causing downtime and lost
productivity.
Machine status network monitoring
DSA 401 helps monitoring multiple
machines over a wireless or cable network. High-speed data are
being sent to the analysis system for complete and high-accuracy
diagnosis.
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