Your specialists always stress that the methods of vibration
diagnostics are divided into two main groups - high sensitive to the incipient
defects and effective to detect the before failure machine condition. What are
these methods and why not to confine ourselves, for example only to the methods
of the first group?
(The question was asked by the participants of the vibration diagnostics
seminar.)
The question is answered by
Natalia Barkova:
High sensitive methods of incipient defects detection are
based mainly on registration of new features of diagnosed objects that appear
only simultaneous with appearance of defects of certain types. As a rule, these
methods loose their sensitivity with appearance of a chain of defects because of
nonlinear influence of some features on the others.
In vibration diagnostics of rotating machines the following methods of incipient
defects detection are used most frequently:
a method of detection the high frequency vibration excited
by shock pulses (shock pulse method);
a method of detection the modulation of high frequency
vibration excited by friction forces;
a method of detection the modulation of vibration harmonic
components by periodical processes (method of analysis of the side
components of the blade, teeth and other vibration harmonics);
a method of detection the modulation of the rotor rotating
frequency (method of analysis of the speed pulsation);
a method of detection the incipient cavitation in the
fluid by the modulation of the random pressure pulsation (the envelope
method of the pressure pulsation).
Absolutely naturally that for the machines where the detected
features are present even without defects the corresponding methods of incipient
defects detection will not work. A typical example - the machines of reciprocal
type where the shock pulses are the necessary components of the operating
process itself. In this case the early machine diagnostics by the shock pulse
method, and not only this method, becomes inefficient.
The main positive peculiarity of the mentioned methods of incipient defects
detection is that they detect new periodical processes and to solve the
diagnostics problems the methods of increasing the sensitivity by accumulation
and averaging of measurement results can be used efficiently.
Correspondingly the main disadvantage of these methods is the decrease of their
accuracy after a significant defect development, especially after appearance of
a large group of developed defects due to the distortion of the periodicity of
the controlled parameters and nonlinear interaction of several defects in the
process of the oscillation forces and vibration forming. Namely, because of this
disadvantage in the monitoring systems it is necessary to use optimal
combination of incipient defect detection methods and methods of vibration
energetic parameters control such as overall vibration level, the level of most
intensive low frequency vibration components and so on.
Nowadays the methods of vibration energetic parameters control comprise the
basis of most vibration monitoring systems. To use them you have to construct a
sample of nondefective machine by all the controlled parameters. Such a sample
can be constructed by two main methods. The first one - by a group of similar
machines where these machines are manufactured. The second - by intermittent
measurements of controlled parameters on the beginning stage of the machine
operation, when there is no defect yet.
As a typical dispersion of the same energetic parameters of vibration for the
new similar machines is comparatively large (about 10 times, viz. 20 dB by the
level of the most powerful components), the second method has evident
advantages. And to decrease to minimum the probability of mistakes during
construction the samples on the beginning stage of machine operation its
condition can be controlled by the methods of incipient defects detection.
And the last consideration. The sensitivity of all the discussed incipient
defects detection methods decreases not only by the development of a chain of
defects but also when the vibration signal that is analyzed corresponds to a mix
of components that came to the measurement point from many machine units. To
keep the high sensitivity of the methods it is necessary, firstly, to select
correctly the vibration measurement points, and, secondly, to use such methods
of vibration analysis that make a preliminary division of the signal into
components that come from different machine units. There are no such means yet
in the most monitoring systems, so when you select the means of early defect
detection in the diagnosed objects it is expedient to consult preliminary with
corresponding specialists.
