State of the Art in Monitoring
Rotating Machinery-Part 1
Robert
B. Randall,
The
University of new South Wales, Sydney, Australia
Abstract
In the last thirty years there have been many
developments in the use of vibration measurement and analysis for
monitoring the condition of rotating machinery while in operation.
These have been in all three areas of interest, namely fault
detection, diagnosis and prognosis. Of these areas, diagnosis and
prognosis still require an expert to determine what analyses to
perform and to interpret the results. Currently much effort is being
put into automating fault diagnosis and prognosis. Major economic
benefits come from being able to predict with reasonable certainty
how much longer a machine can safely operate (often a matter of
several months from when incipient faults are first detected). This
article discusses the different requirements for detecting and
diagnosing faults, outlining a robust procedure for the former, and
then goes on to discuss a large number of signal processing
techniques that have been proposed for diagnosing both the type and
severity of the faults once detected. Change in the severity can of
course be used for prognostic purposes. Most procedures are
illustrated using actual signals from case histories.
Full Text (PDF)
Introduction
The vibrations measured externally on operating
machines contain much information about their condition, as machines
in normal condition have a characteristic “vibration signature,”
while most faults change this signature in a well-defined way. Thus,
vibration analysis is a way of getting information from the inside
of operating machines without having to shut them down. Another way
of getting information from operating machines is by analysis of the
lubricant, and “oil analysis” is useful in machine condition
monitoring. This article is concerned only with vibration analysis
techniques.
Machine vibrations are measured in two
fundamentally different ways – relative displacement of a shaft in
its bearings using so-called “proximity probes,” and absolute motion
of the casing (usually at the bearings) using absolute motion
transducers. Proximity probes must be designed into the machines and
are typically used on high speed turbomachines with fluid film
bearings. They are used for permanent monitoring of relatively
simple parameters such as peak-to-peak relative displacement and
shaft orbits (in the bearing) and are primarily used to protect
valuable and critical machines by shutting them down in the event of
excessive vibration. Only in a limited number of situations can long
term predictions be made. This is because incipient faults often
show up first at high frequencies, to which the relative
displacement measurements are not sensitive. Proximity probes have a
frequency range up to 10 kHz, but because of the natural reduction
of displacement amplitudes with frequency and the dynamic range
limitation of proximity probes to 30-40 dB, the limitation is really
of harmonic order (to about 10-12 harmonics). The dynamic range
limitation is determined mainly by electrical and mechanical runout,
i.e. the signal measured in the absence of vibration. The higher
dynamic range limit corresponds to the use of “runout subtraction,”
where the runout measured at low speed can be subtracted from other
measurements at high speed. This technique is somewhat dubious over
long periods of time where the originally measured runout may have
changed.
Since all vibrations represent an alternation
between potential energy (in the form of strain energy) and kinetic
energy, vibration velocity is the parameter most closely related to
stress, and is the parameter used to evaluate severity in most
vibration criteria. For the same reason, a velocity spectrum is
usually ‘flattest’ over a wide frequency range, requiring the
minimum dynamic range to represent all important components. By
comparison, vibration displacement tends to overemphasize low
frequencies (as for relative displacement) while vibration
acceleration tends to over-emphasize high frequencies. The latter
can sometimes be useful for faults, such as in rolling element
bearings, which show up first at high frequencies, but may disguise
changes at low frequencies. However, the best and most common
transducer for measuring absolute casing vibration is the
piezoelectric accelerometer which produces a signal proportional to
acceleration. Its dynamic range is so large (160 dB) that it can be
combined with electronic integration to give a velocity signal with
more than 60 dB dynamic range over three decades in frequency. This
cannot be achieved by typical velocity transducers with an upper
frequency limit of 1-2 kHz. For these reasons, the rest of this
article mostly assumes measurements made with accelerometers,
sometimes integrated to velocity. The meaning of “rotating machines”
has been taken to include reciprocating machines such as diesel
engines. Because of their importance and ubiquity, the measurement
of torsional vibration of the crankshaft is included as a
supplementary technique.
Fig 1:
Bearing fault
pulses with and without frequency fluctuation; (A, B, C) no
frequency fluctuation; (D, E, F) 0.75% random frequency fluctuation.
RESOURCES 