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1. General aspects of monitoring and diagnostics.

Question 1.10.

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.

 

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