fluid film bearing diagnostics using envelope spectra

8/22/2019 Fluid Film Bearing Diagnostics Using Envelope Spectra

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Fluid Film Bearing Diagnostics Using Envelope Spectra

by

Anton Azovtsev, Alexei Barkov, and Duncan Carter

Abstract

Spectrum analysis of the envelope of the high frequency vibration excited by

friction forces is becoming a common method for the diagnostics and condition

prediction of rolling element bearings. Measurement of the vibrations excited by

bearing friction forces present all the necessary information for diagnostics of

bearing condition including installation problems and the quality of lubrication.

Although not in common use, similar demodulation techniques, with

modification, are practical for the diagnostics of fluid film bearings. A number ofreasons exist that can limit the use of such techniques. First is an ambiguous

dependence between the features of friction forces and the defect severity due to

the strong dependency between friction forces and the thickness of the

lubrication layer. The thickness of lubrication varies with the type of bearing.

The second reason is the high probability and unpredictability of shaft oscillation

relative to the bearing. These oscillations, which depend on numerous parameters

of the machine, its lubrication system, etc., modulate the friction forces and make

the diagnostic process by the envelope spectra more complicated. Despite these

problems, a number of relationships between the defects of journal bearings and

the features of friction forces and the induced high frequency vibrations havebeen found which make diagnostics of journal bearings practical without the

necessity of installing displacement sensors in the bearings. These relationships

and comparisons with rolling element methods are discussed in the paper

together with several case studies of journal bearing diagnostics in different types

of machines.

I. The sources of random vibrations used for demodulation in rolling

element and fluid film bearings.

Spectrum analysis of the envelope of signals is becoming widely used in anumber of fields. If the content of the signal to be demodulated is limited to

representative signals of the physical phenomena of interest, extraction of

information containing numerous complex components having different features

can be accomplished with very accurate results[1,2,3] but this process may not be

adequately supported in most current vibration measurement equipment. The

main application of the envelope detection method is the analysis of low
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frequency oscillations of higher frequency signals. In the case of rolling element

bearing condition monitoring and diagnostics, it is used to extract the information

about the modulation of rolling friction forces and the power of the random

vibration extracted by these forces[4,5]. With modifications, similar

demodulation techniques may be used for the condition monitoring and

diagnostics of fluid film bearings.

The main sources of random vibration in rolling element bearings are rolling

friction and shock pulses. The friction forces can be considered as a combination

of great many micro-shock pulses randomly distributed in time that do not lead to

a breakdown of lubrication layer. Shock pulses that appear as a result of good

and defective friction surfaces interaction are much more rare, but their power

may exceed the power of these micro-pulses by several orders and break the

lubrication layer. The last feature is the reason for the fact that, in the high

frequency domain above 10KHz to 20 KHz, components of shock pulses can

prevail in the vibration signal.

In a fluid film bearing, the source of the random vibrations to be demodulated are

pressure pulsations in the lubrication flow between stationary and rotating

surfaces. There are two distinguishable types of such pulsations. The first type is

pressure pulsation in the border region of the laminar flow. Pulsations of this

kind occur when there are no significant vibration displacements of the shaft

relative to the bearing shell. The second type is the appearance of short term

turbulence in the lubrication flow due to the rapid changes in the lubrication layer

and, consequently, in the flow speed in the region of turbulence. This source of

pressure pulsation can be named, analogously to the rolling bearing situation, the

hydrodynamic "shock" pulse.

We can conclude that the amplitude of the rolling element bearing emitted

random vibrations, in cases of constant rotation speed and lubrication quality,

depends on the friction coefficient and bearing load. There are three main reasons

for possible dependency of the random vibration power on the friction surfaces

rotation angle. The first reason, which is the most frequent, is the dependency of

the friction coefficient ton the rotation angle due to non-uniform surface wear or

appearance of cracks (spalls). The second is connected with improper bearing

installation that may lead to the appearance of additional dynamic loads on the

bearing dependent on the rotation angle of friction elements. The third reason has

the same origin as the previous one. This is appearance of additional loads on the

proper mounted bearing that may change their direction or value because of

machine operation peculiarities of defects of other machine units. The most

typical example is shaft wobbling due to improper coupling of the shafts of two

or more machines.


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In the fluid film bearing case, the main reason for random vibration power

changes in time is shaft oscillations relative to the stationary bearing parts i.e.

changes of lubrication layer thickness and the consequent oscillation of flow

speed within the lubrication layer. A second reason can be the movement of the

oil wedge on the stationary friction surface. By the analysis of the modulation of

random vibration power, we can find out not only characteristics of shaftoscillations relative bearing shells, but also the oil wedge movement.

II. Detected Defects

As shown above, the analysis of a fluid film bearings high frequency vibration

envelope spectrum can be used to detect two main peculiarities of its operation,

the existence of vibration displacements between stationary and rotating surfaces

and appearance of hydrodynamic shock pulses. Both of these are determined by

the defects of either the bearing or of the shaft and other connected units of the

machine.

Parameters of friction surface oscillation such as frequency and amplitude can be

measured. Amplitude can be measured within a limited accuracy in the units

relative to the lubrication thickness. One difference between demodulation

techniques and displacement probes installed in the bearing is that the envelope

spectrum can not be used to determine the path of shaft axis oscillation-orbits.

For hydrodynamic shock pulses, their frequency, amplitude, and form (duration)

can be measured. The last parameter is closely connected with the number of

harmonics of the pulse frequency in the envelope spectrum. Note that sometimessingle hydrodynamic shock pulses randomly distributed in time can be observed

in fluid film bearings.

As a result of several years field experience in the condition diagnostics of

rotating machines with fluid film bearings using random vibration envelope

spectra and auto spectra measured on bearings, diagnostic symptoms were found

for the following group of defects:

shaft wobbling

misalignment of shell

wear of shell

self-sustained rotor oscillations

faults in lubrication layer operation


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The peculiarities of hydrodynamic shocks formation in the lubrication layer

allowed a division of the last defect into two groups:

appearance of hydrodynamic shocks in the lubrication layer

appearance of "dry" shocks in the bearing

The first one is an indication of the problems in lubrication functionality and the

second is an indication of a near breakdown situation.

The variety of defects identified by the limited number of symptoms in the

envelope spectrum of fluid film bearing high frequency vibration is enhanced by

the fact that these symptoms can be supplemented by the information derived by

the analysis of bearing vibration auto spectra. The peculiarities of hydrodynamic

shocks formation in the lubrication layer are such that their development does not

always change the parameters of both the low and high frequency domains of the

bearing vibration spectrum. That's why the algorithms of fluid film bearingsdiagnostics are based on the joint analysis of auto spectra and envelope spectra of

the bearing vibration.

III. Examples of Diagnostics

Our experience with diagnostics of rotating machines having fluid film bearings

in different industries in Russia showed that there are two basic groups of

machines, each having their own peculiarities. The first group is horizontal

machines with massive rotors. The second group is small, not very powerful

machines with limited loading of friction surfaces. The examples presentedbelow are typical for the machines mostly in first group.

Fig. 1a, 1b, 1c, 1d (below). Envelope and auto spectra of one of the supports of a

turbogenerator.

Fig. 1a(above). Envelope spectra of a turbogenerator before the defect was

detected.


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Fig. 1b(above). Auto spectra of a turbogenerator before the defect was detected.

Fig. 1c(above). Envelope spectra of a turbogenerator after the detection of

increased low frequency vibration.

Fig. 1d(above). Auto spectra of a turbogenerator after the detection of increased

low frequency vibration.

Consider figure 1. Here you can see auto spectra and envelope spectra measured

on one of the bearings of a 300 MW turbo-generator of a conventional power

plant in the city of Cherepovets, Russia. The vibration was measured on the

bearing of generator next to the turbine. The bearing has increased vibration

levels on the lower frequencies. The diagnostics showed that the reason for is

shaft wobbling because of the joint coupling defect.


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In the autospectrum, the shaft wobbling is indicated by the increase of rotation

frequency and, especially, by its higher harmonics and in the envelope spectrum,

by the presence of series of rotation speed harmonics that occur due to the

hydrodynamic shocks in the lubrication layer with the rotation frequency. Also,

there is no significant increase in the high frequency vibration levels which can

be indication of "dry" shocks in the bearing.

Fig. 2a, 2b, 2c, 2d (below). Envelope and auto spectra of two supports of a

turbogenerator after an increase of the temperature of a bearing.

Figure 2a(above). Envelope spectra of good bearing.

Figure 2b(above). Auto spectra of good bearing.

Figure 2c(above). Envelope spectra of overheated bearing.


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Figure 2d(above). Auto spectra of overheated bearing.

Figure 2 presents an example of different defect detection in a journal bearing.

This is a case history from one of 500 MW turbo-generators from a power plant

of St. Petersburg, Russia. The experts were invited by the maintenance staff

when, after repair of the machine, one of the bearings became overheated, but thevibration levels were within specified levels for a good bearing. The first

measurements of bearing high frequency envelope spectrum showed that the

bearing shell was installed mis-aligned. The diagnostic symptom in this case is

the second harmonic of rotation frequency that prevails over all other shaft

rotating frequency harmonics. The reason for this is that, due to the natural

movements, the shaft "contacted" the shell twice per revolution, though the

lubrication layer was not broken. After bearing disassembly, it was observed that

the shells were rather burned in just a few days of bearing operation and some

dangerous structural changes developed in the bearing. An autospectrum showed

no indications of the bearing defect.

Fig. 3a, 3b, 3c, 3d, 3e, and 3f(below). Envelope and auto spectra from periodic

condition monitoring and diagnostics of a turbogenerator exciter bearing.

Fig. 3a(above) Envelope spectra, initial state (February, 1995).


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Fig. 3b(above) Auto spectra, initial state (February, 1995).

Fig. 3c(above) Envelope spectra, medium wear of the bearing (August 1996), the

symptoms of the initial stage of shell spalls are present.

Fig. 3d(above) Auto spectra, medium wear of the bearing (August 1996), the

symptoms of the initial stage of shell spalls are present.


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Fig. 3e(above) Envelope spectra, bearing before repair (December 1996), the

symptoms of self-sustained rotor oscillations in the bearing and dry shocks exist.

Fig. 3f(above) Auto spectra, bearing before repair (December 1996), the

symptoms of self-sustained rotor oscillations in the bearing and dry shocks exist.

The third example, illustrated in figure 3, is the detection of a much more

dangerous defect that has similar diagnostic symptoms. This defect was found ona bearing of a turbo-generator exciter at one of power plants in Kiev, Ukraine.

During condition monitoring measurements of bearing vibration, an increase of

low - and high frequency components was detected in the vibration auto

spectrum simultaneous with the increase of harmonic components in high

frequency vibration envelope spectrum. In other words, all symptoms indicated

the presence of "dry" shocks in the bearing. The bearing was disassembled and

inspected. The shells had a number of spalls which caused the appearance of

shock pulses. With the analysis of data obtained by periodic condition

diagnostics, recommendations for bearing repair could be issued three months

before the final diagnostics, just after appearance of symptoms of severe bearingwear - randomly distributed in time hydrodynamic shock pulses.


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Figure 4(below).

Fig. 4. Envelope spectrum of a turbogenerator support in the mode of self-

sustained oscillations.

Figure 4 shows as additional example from journal bearing diagnostic practice -

detection of self-sustained shaft oscillations. Self-sustained shaft oscillations injournal bearings can occur in a single bearing and not necessarily in the most

worn one. Major problems can be caused by self-sustained oscillations during

rotor field balancing i.e. balancing in rotor's own supports. This situation is

typical for when an expert is called and, after significant financial investments

and labor efforts have been done in balancing, but appropriate results are not

attained. Figure 4 shows how self-sustained oscillations were detected. Before

that, balancing of the rotor in its own supports had been attempted for almost

four days with nearly zero results. After bearing shell replacement in two out of

eight lemon (elliptical) bearings of turbo-generator, the rotor was successfully

balanced in four runs.

IV. Peculiarities of Diagnostics

The main peculiarity of fluid film bearing diagnostics using the spectra of the

vibration envelope, as mentioned earlier in the examples from practice, is the use

of additional diagnostic symptoms from the auto spectra of bearing vibration.

The combination of diagnostic symptoms from auto spectra and envelope spectra

of bearing vibration is necessary for highly reliable identification of defect type

and severity.

The second peculiarity is connected with the diagnostics of the bearings of

machines with more than one shaft line connected with gear, belt or other

transmission devices. These transmissions may produce additional dynamic loads

on the bearing that introduce their own peculiarities of journal oscillation relative

stationary parts of the bearing. These peculiarities may cause significant changes


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in the process of pressure pulsations in the lubrication layer and fluid film

bearing vibration.

Figure 5(below).

Fig. 5. Envelope spectrum of a journal bearing in a one stage gearbox with

defects on both gears. Here Fz is a gear mesh frequency.

Figure 5 presents an envelope spectrum measured on the journal bearing from a

gearbox with wobbling of both driver and driven shafts together with defects of

the gearwheels of this transmission. For instance, the spectrum components on

the sub-harmonics of the shafts' rotation speed are the diagnostic symptoms, not

of the self-sustained shaft oscillations, but of the gear wear[6]. This peculiarity

demonstrates the peculiarity of the hydrodynamic shock pulses in the lubrication

layer on the combination frequencies when, due to existence of two or more

oscillation processes, the overall vibration exceeds the level of shock pulses

excitation.

As a result, the overall number of the defects identified as a defects of fluid film

bearings decrease. Their symptoms occur in conjunction with other symptoms for

the mechanical transmission defects. In particular, such defects as self-sustained

oscillation of the shaft or shell misalignment are not considered as a bearing

defects in mechanical transmissions, but are included in a defect group called

defects of gear wheels, belts, etc. At the same time, these defects are not missed

in the machine, but their identification requires significant complication of the

diagnostic measurements and processing algorithms.

One more peculiarity in diagnostics is connected with the choice of levels for the

defect detection by the envelope spectrum analysis. These levels are mostly

dependent on the lubrication layer thickness. In turn, the lubrication layer

thickness may differ greatly according to the bearing design. This results in the

need of defect levels adaptation for the different designs of the fluid film

bearings. A rather simple method was developed for this purpose. It binds the


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increase of the vibration components in the auto spectra and the levels used in the

analysis of vibration envelope spectra. Using this method, a customer easily

corrects the levels for defects on the adaptation stage of the automatic condition

diagnostic systems to his equipment.

Some differences in the choice of measurement points location for highfrequency vibration envelope measurements between rolling and fluid film

bearings should also be mentioned. Diagnostics is done by the high frequency

vibration excited by pulsations in lubrication layer which propagate through the

stationary bearing parts. When the bearing shells are mounted in a special way

that attenuates high frequency vibration while it propagates to the bearing shield,

it is impossible to measure the signals of interest without special means that

allow rigid connection between the accelerometer and the bearing shells.

Sometimes, a change in shell design is required.

A final main peculiarity of fluid film bearings diagnostics should be mentioned.In Russia, this problem postponed the wide introduction of condition diagnostics

systems for several years. This peculiarity is connected with the possibility of

fully automating the diagnostics process with the aim of replacing a highly

qualified expert with software for automatic condition diagnostics. Such an

approach is widely used in Russia where there is a lack of qualified experts in

vibration diagnostics. For the diagnostics of fluid film bearings, it is not

sufficient to analyze a single envelope spectrum. In addition, an autospectrum of

the bearing vibration should also be analyzed together with a number of envelope

spectra. The increased complexity of the diagnostics requires much more

computational power from the computers. After appearance of more powerful

personal computers, it became possible in Russia to develop and widely use

systems for automatic diagnostics of nearly all types of rotating machines units.

Conclusions

The analysis of results of development and practical application of condition

diagnostics methods for fluid film bearings without installation of special sensors

enables the following conclusions:

1. Envelope detection methods based on the spectrum analysis of high frequencyrandom vibration envelope that are well known to be successful in the

diagnostics of rolling element[7] bearings can be efficiently used for the fluid

film bearings condition diagnostics as well.


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2. Condition diagnostics of fluid film bearings in comparison to rolling element

bearings should be done by the joint analysis of auto spectra of bearing vibration

and envelope spectra of its high frequency components.

3. The probability of missing a dangerous situation using this type of diagnostics

is very low.

4. Automatic condition diagnostic systems based on the joint analysis of auto

spectra and envelope spectra require adaptation to the particular design of the

bearings i.e. the thickness of lubrication layer. The purpose for this is to

determine the levels for dangerous defects. This adaptation may be made by a

customer.

5. These systems are capable of automating the process of condition diagnostics

and safe operation time forecast. Tests of such systems in Russia demonstrated

their high efficiency when used by customers who had no special training indiagnostics.

References

1. Duncan L. Carter, U. S. Patent Number 5,477,730, "Rolling Element

Bearing Condition Testing Method and Apparatus" issued December 26,

1995.

2. Duncan L. Carter, "A New Method For Processing Rolling Element

Bearing Signals", presented at the 20th annual meeting of the Vibration

Institute, June, 1996.

3. Azovtsev A. Yu., Barkov A. V., Carter D. L., "Improving the accuracy of

Rolling Element Bearing Condition Assessment", Proceedings of the 20th

Annual Meeting of the Vibration Institute, Saint Louis, Missouri, USA,

1996, pp. 27-30.

4. Barkov A. V., Barkova N. A., Mitchell J. S., "Condition Assessment and

Life Prediction of Rolling Element Bearings", Sound & Vibration, 1995,

June pp.10-17, September, pp. 27-31.

5. A. A. Alexandrov, A. V. Barkov, N. A. Barkova, V. A.Shafransky, Vibration and Vibrodiagnostics of Electrical Equipment in

Ships, -Sudostroenie (Shipbuilding), Leningrad, 1986.

6. A.V. Barkov, N. A. Barkova, "Diagnostics of Gearings and Geared

Couplings Using Envelope Spectrum Methods", Proceedings of the 20th


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Annual Meeting of the Vibration Institute, Saint Louis, Missouri, USA,

1996, pp. 75-83

7. A. Azovtsev, A. Barkov, "Automatic computer system for roller bearings

diagnostics", Computers in Railways V , Proceedings of the COMPRAIL-

96 conference, 21-23 August 1996, Berlin, Germany, volume 2, pp. 543-550.

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fluid film bearing diagnostics using envelope spectra

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