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Vibration &
Ultrasound Technologies: A Possible Integrated
Inspection Tool? by Stuart Courtney,
Senior Applications Engineer,
SKF Reliability Systems
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Introduction
The purpose
of this paper is to introduce condition monitoring and
reliability engineers to the principles of using
ultrasound for the assessment of machine condition.
Ultrasound can be a complimentary technology to
vibration, thermography and lubrication monitoring. It
must be emphasized that it is rarely successful as a
stand alone technology for effective machine condition
assessment and subsequent required maintenance
planning. This paper concentrates on the use of
airborne ultrasound as a complementary technique
particularly for machinery that may be inaccessible due
to guards or hazardous locations.
What is
Ultrasound?
Ultrasound
is defined as high-frequency sound waves, which are
above the range of human perception. Usually they start
at 20 kilohertz (kHz) and go up into the megahertz
range. Airborne/ Structure borne ultrasound covers
frequencies from 20 kHz up to 300 kHz.
How Does the
Instrumentation Work?
Airborne/structure-borne ultrasound instruments provide
information through multiple paths: qualitatively
through their ability to hear ultrasounds through a
noise-isolating headphone, and quantitatively via
incremental readings on a meter/display panel. Digital
instruments provide on-board data storage for data
logging and for viewing baseline data. Some newer
versions of the instruments also include on-board sound
recording for spectral analysis.
The
instruments allow inspectors to confirm a diagnosis on
the spot because they clearly discriminate among various
equipment sounds. An electronic process called
"heterodyning" accurately converts ultrasounds sensed by
the instrument into the audible range where users can
hear and recognize them through headphones. This
process enables users to record sound events through
conventional recording devices.
Most of the sounds sensed
by humans range between 20 Hertz and 20 kilohertz. (The
average high-end human threshold is16.5 kHz.) The
wavelength sizes of these frequencies tend to be
relatively large when compared with the sizes of sound
waves sensed by ultrasonic translators. The wavelength
of low-frequency sounds in the audible range are
approximately 1.9 cm (3/4") up to 17 m (56') in length
(when using the high-frequency average of 16.5 kHz),
whereas ultrasounds sensed by ultrasonic detectors are
only 0.3 cm (1/8") up to 1.6 cm (5/8") long. Since
ultrasound wavelengths are magnitudes smaller than those
in the audible range, they have characteristics that are
conducive to
condition analysis. One advantage is that the amplitude
of a generated ultrasound falls off exponentially from
the source, making the emission localized and easily
isolated for detection and analysis.
The AE
technique homes directly in on the high frequency (~ 100
kHz) component of the elastic waves being generated by
operating machinery. The resulting AE signal is very
strongly influenced by fault processes and has a much
reduced sensitivity to the effects of normal running
components. (ie good machines are much quieter at 100
kHz yet machine faults which result in deteriorating
contacting surface give rise to very loud signals).
Because of this it is possible to analyse the Ultrasound
signal
Connection to a data
collector
Most
Ultrasound systems have an output that can be connected
to a vibration data collector. The setup will be pretty
much the same for all and what you do with the signal
will also be pretty much the same.
Examples of
how to connect ultrasonic detectors to a data collector
can be obtained from equipment manufacturers. All
connect in a similar way and will give similar results.
Analysis
The output
of most ultrasonic detectors is a heterodyned signal so
you could look at the signal as a direct representation
and scale the values in an engineering unit such as
volts, Eus, or even Gs. The definition of the parameter
is arbitray as it is really has no meaning as an
amplitude value. This is purely used for a one shot
analysis function.
Much better
analysis can be performed by using a system of
enveloping which is somewhat similar to heterodyning.
Enveloping is a demodulation process that is used
predominantly to measure the incipient defects that are
present when a rolling element bearing starts to fail.
There are many different types of enveloping and many of
them can be applied to ultrasonic detectors. By using
the enveloping circuit of the data collector you
effectively double envelope the high frequency system.
The important thing to remember is that we are looking
at early deterioration of the rolling element bearing or
its lubrication film. The fluid film thickness in a
rolling element bearing is less than one micron, if
there is a breakdown of the film due to lack of
lubrication or parasitic loads
Detecting
bearing failure is relatively easy; careful choices need
to be made if detection of bearing problems is the
requirement. Ultrasound can be one method of determining
when bearings are suffering from poor lubrication or
subsurface fretting of the structure of the bearing
metals.
There are
some technologies that are direct reading from a sensor
these can be more easily used in a condition monitoring
regime as they are trend-able and scalable.
Examples
In order to
demonstrate the difference in monitoring a bearing using
a vibration sensor and airborne ultrasound a bearing was
monitored when it was first installed and then when it
was in its primary failure mode.
Pitfalls
It is
generally difficult to trend the output of ultrasonic
detectors, the units are not scalable and will vary
dependant how far the sensor is positioned from the
bearing being measured.
The
ultrasound detector is directional and care has to be
taken to ensure that the source of ultrasound id
effectively monitored.
Conclusions
Supplementing vibration testing with spectral analysis
of the output signal from the ultrasonic detector can be
a helpful tool when used for the analysis of bearing and
other types of mechanical faults. Some Ultrasonic
instruments detect frequencies centered at 40 kHz ± 20
kHz. Others have tunable filters, they are generally all
suitable for problem finding in a condition monitoring
based reliability regime. The ultrasonic signals are
demodulated to produce an audible signal, which are
heard using the headphone or viewed using the data
collector. The amplitudes of the spectrums not only
depend on the severity but also depend on the medium
that the signals are traveling through. As a result the
data is not easily scalable and therefore difficult to
trend. Nevertheless by using the data collector the
spectral data can be analyzed and the cause of the
machine problem can be determined. The bearing defect
frequencies can be heard and their spectrum viewed even
by taking the data as much as 10 feet away. Practically
the machine analyst could rapidly scan the machine with
the ultrasonic detector and the headset. When an unusual
sound from a bearing is detected, then a data collector
measurement will provide the analyst with useful data.
For the usual data collection including trending, the
data collector and an accelerometer is still the best
choice.
Mr. Courtney will also presenting a new paper at
IMC-2004 |
