Functional areas of vibration analysis techniques

In the life cycle of a machine or equipment at the division into groups according to the method used and the means of vibration diagnostics can be divided into three main stages:

1. design and study of prototype models;
2. production or repair of a batch production, its installation, and setup on-site;
3. condition monitoring in operation process between repairs.

Principal objective, which is solved by using means of vibration measuring and analyzing at the first stage, is the abatement of machine and equipment vibration by reducing the vibrational forces in the source and optimizing the mechanical properties of individual nodes and elements. Beside this the missions of vibration resistance optimization can be solved, that is ensuring the reliability of machines and equipment when operating in high vibration conditions just as activated by the object of research, and so acting on the object from the outside. At last one more objective can be solved is ensuring controllability of machines and equipment, that is the preparation of vibration control points to ensure obtaining the necessary diagnostic information at subsequent stages of the life of the object under control.

At this stage, methods of modal vibration analysis are mainly used, allowing identification of the most dangerous forms of vibrations of objects, especially at resonant frequencies and actions of the fundamental vibrational forces, such as the device rate speed.

At the second stage, that is in making (repair), installation and commissioning of commercial equipment using vibration analysis allow to solve a wide range of problems, the main of which are:

• step-by-step operation checking  of the parts and units manufacturing;
• incoming inspection of component parts and units;
• balancing of rotors in place of installation and setup;
• output vibration control;
• analyzing and eliminating the causes of excessive vibration.

Step-by-step operation checking of the machines and equipment parts components manufacturing is carried out by using a variety of control methods.  Vibrational methods may include ultrasonic inspection based on the analysis of the reflection and propagation loss of vibration wave, rendered by an external source. It is an independent area of nondestructive control.

The input control of component units using the methods and means of vibration analysis excited by these units is possible only in the presence of test benches to ensure the operation of these units in nominal or special modes. As a rule, methods of vibration analysis used in such control are determined by the quality of designing and manufacturing of test benches, which should not give vibration noise. For the majority of the input control units, and first of all roller bearings are used spectral analysis methods of their vibration and quite often spectral methods of the analysis bending around its high-frequency component.

For rotors balancing synchronous vibration analysis techniques are used, and above all vibration at a frequency of rotation of the rotor or the other of the balanced rotating unit. The amplitude of the vibration of the rotor speed and its phase relative to the reference signal from the sensor of the angular position of the shaft are entitled to be measured at each control point. The recommended accuracy of measurements of amplitude is about 5%, the accuracy of measurements of phase is about 2% (5-7 °). The main problems of balancing are associated with a possibility of an appearance on rotating speed of a rotor of the significant forces of different nature, which partially or completely can’t be lowered due to the centrifugal forces created by balancing masses. Detection and identification of these forces require an application of the whole arsenal of the vibration analysis methods used by diagnosticians.

The most difficult task from the nomenclature of the vibration analysis methods is a task of identification and elimination of the reasons for the increased vibration of the equipment after its production (repair) or assembly on-site. Specialists in vibration alignment are engaged in the solution of this task, and quality of the decision is in many respects determined by the depth of training of these specialists, as well as working experience in vibration alignment of specific types of machines and the equipment.

The main feature of work made with vibration alignment is that if there is a need it is necessary to research vibration of machines and the equipment in the different modes of functioning, including their partial disassembling. In the process of vibration alignment, all analysis types of the vibration form or its frequency analysis, including the synchronous and mutual frequency analysis, as well as the analysis of ranges bending beforehand the selected components of vibration of the different nature can be used.

At the third stage, during operation of machinery and equipment between overhauls, their vibration analysis can be conducted for the following tasks:

• emergency protection of critical equipment;
• monitoring and forecast of the state of the vibration (vibration monitoring);
• monitoring and forecast of the technical condition of machinery and equipment (vibration diagnostics);
• planning the deadline and the size of maintenance work;
• carrying out maintenance and quality control of their implementation;
• pre-repairment inspection of machinery and equipment.

Emergency protection systems of responsible machines and equipment typically have several parallel measuring channels, vibrating channels and enter to it as constituent parts. The required speed of decision made in such systems is very high (of a second), so deep vibration analysis on these channels is not required, but rather a continuous measurement of low-frequency vibration in a wide frequency band. The lower boundary frequency of this band is typically less than half the speed of the machine, while the upper most of the order of 1,000 Hz. The minimum response time for an abrupt increase in vibration in such a measuring channel is typically 2-3 rotor turns.

Rather often in vibration channels of emergency protection of machines with sleeve bearings approximators (measuring converters of relative movement of a shaft) which in parallel with protection provide the solution of problems of vibration monitoring on the basis of the analysis of orbits of the movement of a shaft in the bearing are used.

In the absence of sleeve bearings in channels of emergency protection piezoelectric accelerometers are, as a rule, used, also alongside with a filtration of a signal of vibration his integration for the purpose of measurement of the normalized vibration parameter, namely, the mean square value of vibration speed or vibration displacement is carried out.

The signal of vibration both from measuring converters of emergency protection and from additional converters can be used at the same time for the solution of problems of vibration monitoring. For this purpose, it is in parallel analyzed by technical means of systems of monitoring and if it is necessary, diagnostic aids.

Systems of vibration monitoring of critical machinery and equipment in accordance with the recommendations of existing standards for monitoring use mainly spectroscopic methods for the analysis of low-frequency vibration. There are also used methods of shape analysis of low-frequency oscillations (the orbit of shaft movement in the sliding bearings) or synchronous frequency and spectral analysis of vibration in the modes of start-up and running down of a machine in some systems.

As many spectral components of low-frequency vibration of machines grow at emergence of some defects long before initiating of an emergency, according to vibration monitoring sometimes it is possible to find a pre-accident condition of the machine, and in certain cases at creation of trends of its vibration state in time changes to predict time of approach of an emergency.

Two different problems can be solved as part of the vibration diagnostics of machines and equipment. The first problem is the identification of the reasons for the change of the vibration state found by monitors. Additional resources for measurement and the analysis of vibration are usually not required for its solution. Another problem is controlling and predicting the technical condition of the unit under test, which is impossible without detection of all main types of defects at an early stage of the development. You can determine only some of the developed defects by means of vibration measurement and analysis used for monitoring, that’s why a special section of preventive vibration diagnostics is involved during monitoring and predicting of the technical condition of machinery and equipment.

The aim of this diagnostics is the detection and identification of all major types of defects in its infancy, monitoring their progress and predicting on this basis, the technical state of the control object. Prevailing majority of defects in its infancy are found according to vibration signal only in rotary machines, without knots reciprocating type. Incipient defects due to the small magnitude of the vibrational forces may bring a noticeable vibration mainly at high frequencies, and only in the area nearest to the place of occurrence of the defect. Therefore, methods of analysis of low-frequency vibration, used in vibration monitoring tasks, do not provide detection of many possible defects in its infancy.

In the preventive diagnostic tasks, it is necessary to use the whole range of analyzes of high-frequency vibration, measured not in standard control points, but as close as possible to the point of occurrence of high-frequency vibrational forces and the places of their transmission to the stationary units of diagnosed machines.

The greatest distribution in preventive diagnostics was gained by methods of the analysis of a form of the high-frequency vibration excited by shock impulses in bearings, and methods of the spectral analysis of the bending-around high-frequency vibration excited by friction forces not only in bearings, but also in other units, for example driving wheels of pumps, turbines and other types of machines. It is natural that alongside with high-frequency vibration the vibration of machines at average and low frequencies is measured and analyzed. For its research, practically all types of the analysis of signals of the vibration excited in standard modes of behavior of machines are also used. The analysis of vibration at average and low frequencies allows to control more precisely development of the defects found on high-frequency vibration after they begin to actually influence a condition of the unit under test.

Planning of terms and amounts of works both on service, and on repair of machines and the equipment, demands full knowledge of their technical condition. The methods of preventive diagnostics using the analysis algorithms of diagnostic signals for detection of each of the arising defects are the cornerstone of such planning. However, in addition to detection of the defect it is necessary to evaluate the size and the degree of the danger and in order to do so, it is required to define threshold values for each of diagnostic parameters. This problem is solved with the help of the statistical analysis of results of independent diagnostic measurements or with a group of identical objects of control, or according to retrospective measurements of one (diagnosed) object of control.

The task of implementing the maintenance of machinery and equipment is an independent task according to used methods and algorithms for vibration analysis. The main operations of maintenance of rotating equipment are its alignment, balancing and replacement of lubricant in the bearings. Maintenance work can include replacement of defective parts, such as bearings. The most complete existing methods of vibration analysis are used at the stage of diagnostics of the machine before scheduling maintenance. However, vibration analysis is performed at the stage of balancing equipment, and the quality control of all kinds of maintenance work.

Thus, at the stage of balancing machines synchronous frequency analysis methods are used in order to measure all points of control amplitudes and phases of the machine vibrations at a frequency of rotation of its rotation, and under the control of the balancing results apply simple methods for measuring vibration level, regulated by existing standards.

The quality of the alignment of the rotors is checked using the methods of mutual spectral analysis of vibration, which allows to measure the shape of the shaft line vibrations in the rotor rotation frequency and its multiples harmonics followed by evaluation of the residual static and especially dynamic misalignment. Quality of grease in the bearings after its replacement is estimated using analytical methods mostly form high-frequency vibration in the shaft journal bearings.