The peculiarities of rolling element bearing vibroacoustic diagnostics for transportation applications

by

A.V. Barkov, N.A. Barkova, I.A. Yudin, and S.N. Rogov,
VAST,Inc. St. Petersburg, Russia

Abstract

When a certain method or instrument is chosen for the vibro diagnostics of machines used in transport applications, it is necessary to take into account the construction peculiarities of the machines and their operation conditions. First of all it is necessary to consider movement peculiarities the transportation vehicles and the influence of the extraneous noise on the diagnostic signal. A suitable method of analysis of the envelope of high frequency vibration for the diagnostics of the rolling element bearings is necessary. The advantages of this method are analyzed and the peculiarities of its implementation for the diagnostics of rolling element bearings on transport are presented.

The analysis of the bearing specific vibration in rotating machines shows that its high frequency components usually are excited by the forces that occur in the rolling element bearings of these units. In good rolling element bearings, the main type of high frequency oscillating forces is friction forces. When a defect develops in the bearing, there can also be found shock pulses due to the breaks in the lubrication layer between the friction surfaces. The method of diagnostics of rolling element bearings based on the analysis of high frequency vibration provides a number of advantages. Among them are the ability to locate the defective bearing and the simplicity of the vibration signal analysis because it does not contain any components from other units of the machine. These are the main reasons for the wide use of bearing diagnostics based on high frequency vibration analysis.

When a defect of mounting or wear of rolling surfaces appears, the friction forces are not uniform, but depend on the rotation angle of the rotating surfaces in the bearing causing the friction forces to be modulated by a periodic process. If cavities (spalls) or cracks appear in the bearing, periodic shock pulses also appear . It is possible to detect the presence of the friction forces modulation and the periodic shock pulses by the spectral analysis of the envelope of the random vibration excited by these processes [1]. When the friction forces are modulated by a periodic process, the harmonic component with the frequency fi can be found in the measured envelope spectrum (fig. 1a). The frequency fi is determined by the period of the modulating process. The modulation depth mfi is determined by the difference of the harmonic and random component levels L according to the expression:
(1)
where fa - the frequency range of the spectrum analyzer and
fn - the band width of the selective filter of the envelope detector.

Table 1. The main frequencies of defective bearings' vibration modulation

No.Type of defectMain frequencies of modulationComments
1Revolution around outer (fixed) racefr
2Nonuniform radial tension2fr
3Misalignment of outer race2fout
4Wear of outer racefout
5Cavities (cracks) on outer racekfout; k=1,2,3...
6Wear of inner racekfr; k=1,2,3...the decay of amplitudes with increase of k
7Cavities (cracks) on inner racekfin
8Wear of cage and rolling elementsfc or (fr-fc)
9Cavities, spallings on rolling elementskfrol
10Defects of several bearing surfacesfout + fin = zfr or
fout + fr
(fr-fc)(z+1)
no fout-fr or (fr-fc)(z-1)
11Slip of racekfr k=1,2,3...no other frequencies of modulation
12Defects of lubrication increase of high frequency vibration level

- rotor frequency
- cage frequency
- rolling element rotation frequency
- rolling element pass frequency, outer race
- rolling element pass frequency, inner race
z - number of rolling elements
drol- diameter of rolling element
dc- diameter of cage
- contact angle between balls and outer race

 When the shock pulses appear from the bearing, besides the main harmonic component, a set of higher harmonics in the envelope spectrum can be found (fig. 1b). The envelope spectra analysis of a number of a defective bearings shows that, according to the frequencies of harmonic components found in the envelope spectrum and their amplitude, it is possible to identify 11 type of defects in the rolling element bearings and, according to the overall vibration level, the defects of lubrication. The diagnostic symptoms for all 12 defects are presented in table1(above). The expressions for calculating the main modulation frequencies can also be found there. The levels for severe (dangerous), medium and incipient (initial) defects in terms of modulation depth (or vibration level for the defect of lubrication) have rather weak dependence on the rotation frequency, construction parameters or peculiarities of bearings operation. The severe defect level varies from 5%-10% of modulation for slowly rotating machines up to 20%-30% for high speed rotation machines [2].

This method of rolling element bearings diagnostics is implemented in the diagnostic systems developed by VAST, Inc. which are used in different industries for more than 3 years. Among the main advantages of these systems are the automatic use of defect identifying algorithms and long term condition prediction by a single vibration measurement. Difficulties in automatic detection and identification of the defects in the rolling element bearings can occur only when this bearing is influenced by non-uniform and shock loads from other parts of the machine. Thus the main diagnostic symptoms can be quite different when the radial load on the bearing rotates synchronously with the rotor. The reason for it can be, for example, a unbalance of the rotor or defects of joint couplings. An additional diagnostic criteria should be used in case of gearbox diagnostics because the gears can provide a pulsating load on the bearings, especially if the gear has a defect of manufacturing or nonuniform wear. We come across even more problems during the diagnostics of reciprocating mechanisms. The reason for this is that, when the mechanism changes the movement direction, the pulse load on the bearings is much higher that the static one.

And finally it is impossible to use the envelope method for diagnostics of such a machines where it is impossible to separate the random vibration component of the bearing within a band containing more powerful vibration components from other units of the machine. Such situations can occur when it is impossible to mount the transducer on the bearing shield.

The above mentioned restrictions often occur in the diagnostics of rolling element bearings in the transportation vehicles, for example wheel pair in the railway cars, aviation motors, escalators, etc. So, for example in the aviation jet engines the main problems are connected with the fact that it is impossible to mount accelerometers on a number of bearing units. To solve this problem there were conducted a lot of investigations aimed at finding the channels of high frequency signal propagation to the place where it is possible to mount the transducer, and the mode of motor operation when the influence of other units in the measurement point is minimal. The second problem with the aviation motors is a great number of gears in it. The defects of gears lead to increased dynamic loads on the diagnosed bearings which is why the gears and rolling element bearings should be diagnosed as a whole unit. Nows, the algorithms for simultaneous diagnostics are developed and tested. These algorithms can solve a number of practical problems that occur with the aviation motors diagnostics.

  Fig.1a(above) Example of an envelope spectrum of the high frequency vibration with one harmonic component ( wear of outer race)

Fig. 1b(above) Example of an envelope spectrum of the high frequency vibration with a set of harmonics (crack of the outer race).

The problems with wheel pairs diagnostics are connected to the dynamic loads on the box bearings during the train movement on the railroad. The most significant impacts occur on the rail joints, but all other irregularities of the railroad tracks and wheels lead to the similar dynamic loads. Such problems can be overcome by the use of special stands to rotate the wheels from an external drive. The important thing here is to obtain the same vertical static load on the box bearings. The high reliability of the diagnostics and condition prediction of the rolling element bearings in such conditions has been proved in a number of tests conducted on a number of enterprises in Russia and Eastern Europe. Also there are some cases, for example, on the escalators of the metro, when there is a need for the diagnostics of the rolling bearings that require fixed non-contact vibration or noise transducer. The solution of this problem can be obtained by using the narrow directed microphones that can measure the noise level transmitted by the bearing shield. The main difficulty here is that the minimum time required for measurements is about the 30-40 revolutions of the bearing. This requires special methods to control the directional pattern of the microphone during measurements.

Summary

The following problems can be solved by the analysis of the envelope spectrum of high frequency vibration:

The means of rolling element bearings diagnostics and condition prediction produced by the VAST, Inc. can be used to solve the majority of the practical problems in diagnostics without any adaptation to the customer equipment on transport. The same systems can be adapted to solve nontraditional problems in diagnostics of rolling element bearings on transport connected with the movement of the bearings near the means of measurement.

Bibliography

1. ALEXANDROV A., BARKOV A., BARKOVA N., SCHAFRANSKY V. - Vibration and Vibrodiagnostics of Electrical Equipment on Ships, - Sudostroenie, Leningrad, 1986.

2. AZOVTSEV Yu., BARKOV A., Yudin I. - "Automatic Diagnostics Of Rolling Element Bearings Using Enveloping Methods"- Proceedings of the 18TH Annual Meeting of the Vibration Institute, Hershey, PA, USA 1994.

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