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1. General aspects of monitoring and diagnostics.

Question 1.11.

In your publications are discussed as a rule the methods and means for functional (operating) rotating machine diagnostics by vibration signal. Do you use the test diagnostic methods and if YES for what practical problems? What place do these methods occupy in your diagnostic systems?
(The question was asked by the participants of a seminar "VibroAcoustical Machine and Equipment Diagnostics" held in VAST, Inc.)

The question is answered by Alexej V. Barkov:

To answer this question it is necessary to define the problems that are most often solved by the functional or test diagnostics when the vibration signal is used. It is possible to discriminate several main groups of such problems.

By the test diagnostic methods are solved first of all such problems as:

  • working out technical and construction elements during designing new machines and equipment;

  • checking whether the machine or equipment operates correctly after manufacturing, repair or during maintenance;

  • testing the separate machine units during maintenance.

By the functional diagnostic methods are solved first of all such problems as:

  • emergency protection of a machine during operation;

  • machine and its units condition monitoring during operation without changing its mode of operation.

From the said above it is clear that the methods of test diagnostics during machine exploitation are used only during different types of maintenance, for example, the planned maintenance and with changing the mode of operation from its specified mode. The methods of functional diagnostics are used to prolong the work of the machine in the specified mode of operation by rejection the planned maintenance and hence are coordinated with test diagnostic methods with great difficulty.

If to look at the methods of functional and test diagnostics of machines only by vibration signal then it is necessary to take into account that these methods utilize different machine features. The methods of functional diagnostics are based on the results of the analysis, first of all, of the peculiarities of forming the oscillation forces, but not the results of measurements of the oscillation system's mechanical properties or the analysis of the vibration propagation peculiarities.

Most vibrodiagnostical test methods, first of all, use the measurement results of machine parameters as an oscillation system, in particular, its resonances and losses on resonance frequencies, and such methods as flaw detection - the peculiarities of vibration propagation in the mechanical system. Taking into account all that was described above the methods of test vibrodiagnostics can add the functional methods on the stage of machine and equipment operation.

So what vibrodiagnostical test methods and on what stages of machine service life can be recommended to be used?

Ultrasonic inspection as test diagnostic with the use of an external source of vibration most often is used for diagnostics of separate machine's details usually of a simple form before installing it in the machine on the stage of its manufacturing or maintenance. After such detail is installed in the machine the vibration inspection methods significantly loose their efficiency because of large active losses of the vibration energy in the attached constructions and the influence of the latter on the vibration propagation. In this case sometimes the operators use another methods such as electromagnetic methods, X-raying or other types of flaw detection. Naturally, the methods of ultrasonic inspection during machine maintenance without the machine dismantling cannot be recommended.

Such test method of vibration diagnostics as the investigation of machine or its details resonances, excited by external shocks, most often is used in machine outgoing test after its assemblage and during repair determination after machine dismantling. During machine maintenance it is used very seldom and as a rule to control the natural oscillation frequencies of the case constructions. Even more seldom for the excitation of natural frequencies are used the external sources of the harmonic oscillations, so called exciters, as their installation on the machine case or its details is very labor consuming operation. That is why the last method, as a rule, is not used together with the methods of functional diagnostics.

More often for machine units' natural oscillation frequencies analysis, and especially its rotor, the method of test diagnostics is used when the vibration is excited by the rotor itself on the harmonics of its rotation speed and the rotation speed is changing during machine speedup or running down. This method as a rule is used during machine service life for testing its units' condition.

One more method of machine mechanical characteristics analysis can be considered as a test method of vibration diagnostics. This is the method of comparative analysis of machine forced oscillations on the rotor rotation frequency before and after fixing trial weights during machine balancing in field. In this case, if during machine service life the rotor is balanced many times, this test diagnostic method is efficiently used in combination with machine functional vibration diagnostic methods.

Absolutely naturally, the design of our diagnostic systems allows the use of such test diagnostic methods as the vibration analysis during machine running out (speedup) and the method of analysis of natural oscillation frequencies of machine units vibration excited by single shocks. But these methods are not mandatory for the machine condition complex analysis mainly because of their complexity and labor consumption. So the automatic machine diagnostics during operation is conducted without the use of these methods.



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