The application of vibration signature analysis and acoustic emission source location to on-line condition monitoring of anti-friction bearings

L.M. Rogers*

The mechanical noise, vibration and heat generated by defects in fabrications, process plant and machinery are a measure of component condition, and once characterised allow on-line assessment of defect severity. Unexpected failures which might threaten the entire production process can thereby be avoided. There is an increasing need for reliable and robust condition monitoring instru- mentation with on-line data processing, particularly as the trend continues towards larger production units with little or no standby capacity for use in the event of unscheduled stoppages due to mechanical failure. This article describes the detection of incipient failure of rolling element bearings by kurtosis and the location of fatigue cracks in slowly rotating bearings by acoustic emission

Routine monitoring of the serviceability of machinery is not a statutory requirement, except where there is a high threat to personal safety or of gross pollution as a result of component failure. The main incentive for carrying out routine inspection, either manually or using automatic continuous monitoring instruments, is the material cost of allowing the 'spread in rot' to proceed unchecked. An unexpected failure at a 'bottle neck' in a large integrated process can bring production to a standstill. Such failures, which are often the result of a series of minor defects and errors, can be avoided if the defects are noticed and rectified early in their development.

Detecting and quantifying defects is only part of the story. The next problem is deciding how and when to undertake repairs with the incurrence of minimum cost and without jeopardising safety. It is important to analyse the benefits of condition monitoring against the cost of inspection, which can be high, particularly if 'total coverage' is required at regular intervals throughout the service life of the instal- lation.

For a number of years anxiety has existed regarding the per- formance and safe operation of the slewing cranes used on offshore production platforms (typically three per plat- form). These cranes are heavy rotating structures, mounted on a slew ring, a large diameter heavily loaded anti-friction bearing, at the top of a steel column or pedestal.

Fig 1 shows the crane on a gas production platform in the southern North Sea which was instrumented with acoustic emission sensors to evaluate this method for monitoring sub-critical fatigue crack propagation in the slew ring. Analysis of the vibration signature produced by the bear- ing in the frequency range 1 to 20 kHz and the low fre- quency dynamic response of such a crane using accelero- meters and strain gauges, has been of limited value in detect-

*The Unit Inspection Company, Sketty Hall, Swansea, UK

+ln collaboration with the Institute of Sound and Vibration Research, Southampton t3

Fig 1 Part o f a gas production platform showing the slewing crane which was instrumented with acoustic emission sensors

hag incipient damage. An alternative was to use acoustic emission monitoring with high sensitivity resonant trans- ducers and the fast data processing capability of a com- puter which can recognise and characterise the short duration acoustical events associated with incremental

0301-679X/79/02051-09 $02.00 © 1979 IPC Business Press TRIBOLOGY international April 1979 51

fatigue crack growth and grinding of metal fragments (debris) in the bearing.

The greater rotational speed of anti-friction bearings in fans, pumps, turbines, etc., results in much higher energy audio signals which can be readily detected using a flat response piezo-electric accelerometer with typical sensi- tivity of 10 mV/g. The vibration signatures are still com- plicated, but there are certain characteristic features of the waveform which can be recognised as indicative of damage when displayed on a CRT. These are the transient spikes produced when the rolling elements strike the damaged parts of the bearing. The signals have similar appearance to the acoustic emissions from propagating failure cracks, but are of regular recurrence frequency equal to one or other of the characteristic rotational frequencies of the different parts of the bearing. Because of this regular feature in the signature, the kurtosis value of the distri- bution of signal amplitudes at selected frequencies can be used to characterise the vibration signature in terms of the extent of damage (spalling) present. A portable analog instrument which provides a quick and reliable check of the condition of a bearing has been produced and evalu- ated.

This paper describes the slow speed application of acoustic emission to inservice monitoring of the integrity of off- shore production platform slewing cranes and the high speed application of kurtosis to monitoring the condition of rolling element bearings in medium to fast rotating machinery.

Vibration or sound?

The spectrum of vibration, sound and ultra-sound of in- terest to civil and mechanical engineers, spans eight orders of magnitude, from the very low frequency structural flexing of steel and concrete fabrications to the location and sizing of weld defects by ultrasonics (fig 2).

Vibration and stress analysis: examples of low frequency measurements (0.1 Hz to 100 Hz)

Dynamic analysis of structures and rotating machinery and the measurement of this behaviour using sensitive low fre- quency accelerometers, seismic velocity transducers and

strain gauges, have been used to check adequacy of design an d subsequently the detection of defects and the causes of such failures. Once the dynamic behaviour of a machine or structure is understood, deviations from this whether observed immediately after fabrication or during in-service operation, are a measure of the serviceability and integrity of the structure. The method is well established for studying the rotor dynamics of machinery in condition monitoring a and the integrity of steel fabrications) ,3

Acoustic emission, a high frequency measurement (40 kHz to 1 MHz)

Acoustic emission (AE) is the term applied to the spon- taneously generated elastic wave produced within a material under stress. Plastic deformation and the nucleation and growth of cracks are the primary sources of acoustic emis- sion in metals. These acoustic signals can be detected by remote sensors and their source can be located by compar- ing the arrival times at several sensors. So by 'listening' to the structure, crack growth can be detected, located and monitored, and a warning of incipient failure can be given.

As a non-destructive testing technique, acoustic emission has several unique features. In contrast with nearly all other NDT methods, the defect makes its own signal so that it can be detected by a remote, fixed sensor without needing to make an inch-by-inch search. It is also poten- tially less reliant on operator interpretation.

Acoustic emission techniques have been applied success- fully to the proof-testing of nuclear and petrochemical pressure vessels, 4,s leak detection in pipelines 6 and monitor- ing the structural integrity of bridges and aircraft. 7'8

The amplitude and recurrent frequency of the emissions depend on the material composition and microstructure, the mechanism of crack growth, the loading and crack size (stress intensity). For example, hydrogen induced inter- granular and transgranular cracking in high strength steels 9 is a particularly 'noisy' process and as a general rule of thumb, the energy content of the emissions decreases with decreasing material strength. However, stress corrosion crack- ing and corrosion fatigue cracking in welded structural steels, for example BS 4360, grade 50D, used as a fabricating material in platform jackets and process plant, can also pro- duce high amplitude primary and secondary acoustic emis-

Fig 2 Spectrum of vibration and sound used for inspection and testing

52 TRIBOLOGY international April 1979

sions. ~° This is due to the different micro-structure in the weld and heat-affected zone where cracking originates. The secondary emissions produced by, for example, rubbing of the crack faces, or the crushing of corrosion products in the crack and cavitation, may also be of sufticienily high amplitude to permit location of cracks in so called 'quiet materials', n

Acoustic emission instrumentation Dunegan Endevco 3000 series acoustic emission source location equipment was used to make the wave propagation and signal characterisation measurements on the slew ring. Briefly, the equipment used accepted analog inputs from four sensors (type S140 with 40 dB type 1801 preampli- tiers). The signal conditioning amplifiers provided a further 60dB of gain. A one-volt threshold was used for recognising 'events' and measuring the difference in the arrival times of the signal at two sensors (At), see Fig 3. The emission source was computed automatically and the events versus distance displayed in real-time on a CRT. In addition to measuring At's, the amplitude, rise time, duration and ring-down counts of the signal were logged and could be displayed on the CRT in real-time in a variety of forms, for example events of amplitude A versus A histogram or log (sum events) versus A.

The arrival time was measured with a resolution of 0.1 microsecond and the dynamic range for amplitude measure- ment was 100 dB. In this case 0 dB was equivalent to 1/aV at the sensor when using a 40 dB preamplifier. The back- ground electronic noise level was typically 15 dB. The gain on each channel could be adjusted in 1 dB steps from 40 to 100 dB, and it was usual to set the gain such that the background noise was a few dB below the one-volt thres- hold for event detection.

Included was the facility for 'gating' of the analog signals via a 'parametric input', such as load. This permitted data acceptance during the part of the loading cycle when back- ground interference was minimal. Alternatively, the same facility could be used to 'guard' the sensors from remote sources of background mechanical noise.

Acoustic emission could, therefore, be detected amid high levels of noise using 'spatial' and 'parametric' filter- ing, i.e. emissions would only be counted if:- (a) their source lay within the area determined by linear location between two sensors (spatial filtering) (b) the emissions occurred when the background noise, as determined by the voltage controlled gate, was minimal (parametric filtering).

Data processing occurred at approximately 100 events per second.

Bearing condition monitoring Intermediate frequency analysis of vibration using kurtosis A number of bearing monitoring systems have been instal- led in British Steel Corporation Works to give warning of incipient damage and advanced damage based on event (spike) counting above a pre-set threshold level and root- mean-square acceleration monitoring respectively. The optimum operating frequency has been found to be 1 kHz to 10 kHz. ~2 The setting of the threshold level for event counting depends on the background 'base' level corres-

Fig 3 The relationship between events and counts

ponding to the undamaged state which can vary consider- ably from machine to machine, depending on the size, load, operating speed and general condition (alignment and balance). For optimum sensitivity of detection, each machine would have its own threshold level for detecting early and advanced damage conditions.

An instrument which avoids the necessity for setting thres- hold levels for event counting, has been developed by the Corporate Engineering Laboratories of the British Steel Corporation. + It has been found that kurtosis (for defini- tion see appendix) is sensitive to the spiky character of vibration signatures and can be used to monitor bearing condition. The principal advantage of the technique is that the measurement is insensitive to the amplitude and recurrent frequency of events, i.e. loading and shaft speed. It is primarily a measure of the wave shape. The kurtosis value for a Gaussian distribution of amplitudes (charac- teristic of the random amplitude noise generated by a new bearing) is 3. For a spiky signal, typical of a slightly damaged bearing, its value ranges between 3 and about 20. By studying the frequency dependence of kurtosis, early and advanced damage conditions can be resolved. Kurtosis is, therefore, a method of allotting numbers to wave patterns, independent of the signal amplitude. A photograph of the prototype instrument is shown in Fig 4. It has a dynamic range of 0.02g to 100g and four pri- mary operating frequencies, each 5 kHz band pass cover- ing the range 0 to 20 kHz. Studs (of the type shown in the photograph) are attached to the bearing housing, prefer- ably at a point having a direct metallic path to the outer race of the bearing. The stud allows rapid attachment of the accelerometer using an SKF adaptor and ensures that the operator returns to the same place on the machine for successive measurements. A typical set of readings and corresponding vibration signatures for a bearing in the undamaged and slightly spawled states are given in Fig 5. The amplitude modulation of the signal is due to imbalance.

If the signal amplitude in the different frequency bands drops below O.02g (rms) or the recurrent frequency of the acoustic events (spikes) originating from the damaged parts of the bearing, is low and irregular, kurtosis monitor-

TRIBOLOGY international April 1979 53

ing using the present instrument is no longer practical unless the integration is done over a sufficiently long period of time. Such signals are characteristic of large slowly rotating bearings, such as crane slew rings, LD converter trunnion bearings, concast turrets and/he bearings of large dish-type micro-wave and radio-wave receivers. In this case, the detection of bearing defects, in particular fatigue crack propagation in the inner and outer races, requires a different approach.

Fig 4 Prototype kurtosis meter, designed by Corporate Engineering Laboratory, BSC

Detection of propagating fatigue cracks in offshore crane slew ring bearings by acoustic emission

Platform cranes have failed catastrophically due to the development of fatigue cracks in the vicinity of the rolling faces of the slew ring. The application of linear elastic fracture mechanics has indicated a critical crack depth of around 5mm for a typical installation. It is extremely difficult to inspect slew ring bearings for cracks and a method for in-service diagnosis of condition, without the need even for part dismantling, is desirable. Because of the slow rotational speed of the crane, application of conventional vibration analysis (0 to 20 kHz) is of limited value for on-line condition monitoring. This is also true of strain gauging, xa Encouraging results have recently been obtained using acoustic emission resonant sensors, operat- ing at frequences between 100 and 300 kHz. In addition to the primary emissions from propagating fatigue cracks, the secondary emissions produced by rubbing of the crack faces, grinding of metal fragments in the bearing and im- pacts between the rolling elements and the damaged parts of the bearing in the load zone, will be a function of the overall condition of the bearing.

A recent publication in 'Engineering Materials and Design "s reviews the design features of the Taperex cross-roll slew ring, as used in the Sea Lion crane*. The purpose of the slew ring

*There is no implication in this paper that the Taperex slew ring has performed unsatisfactorily

20 mV

20 mV

I0 msec

20

I0

"~ 6 0

4

x Damaged

O Undornoged

X

I I I I I I 1 I I I

--t-

m

v

2o

g cJ

x Domoged

O Undomoged

I l u ~ ' ~ - - - - t ' t ~ - - ~ - _ - . b I 5 IO 15 20 < 20

Frequency, kHz

Fig 5 Vibration signature for undamaged and damaged rolling element bearings o f the same type on a machine running at 1425 rpm. Also shown are the corresponding values o f kurtosis {~2) and rrns acceleration for six frequency bands

54 T R I B O L O G Y internat ional Apr i l 1979

is to support the upper rotating part of the structure on the lower, fixed base. The drive for the rotating part is housed on a geared ring acting on the fixed race of the slew ring itself. Fig 6 shows such a gear in the inner ring version, similar to the slew ring of the platform crane on which the acoustic emission measurements reported below were made.

Nitrile rubber

P r e ~ 1 [ shlms

Induction hardened race

~ Geor teeth

p

Fig 6 Taperex cross-roll slew ring

Slew ring g e o r ~

Flange

=A

SI

=A S e c t i o n A-A

/ N

SI

/ • , Crone mot ion

-s2 /

' ~ P i n i o n drive

O- 180"

Fig 7Sketch o f pedestal with elevation and plan showing transducer positions relative to slew ring and twin pinion drives

Results

An exploratory exercise was undertaken on a platform crane to determine the background noise level during slew- ing under different loading conditions and the suitability of using a simple linear array for emission source location. The sensors, $1 and $2, were positioned diametrically opposite to each other on the fixed inside face of the slew ring 2cm below the gear teeth (Fig 7).

Large amplitude emissions were observed when the crane was in motion. Fig 8a shows the distribution of signal amplitudes on sensor $1 for a sample time of 2½ min, during which time the crane was rotated through 180 ° and back to its starting point. A total of 10 000 events were recorded, of which ten had amplitudes in excess of 25 dB above the electronic noise level.

There was little difference between the amplitude distri- butions recorded when the crane was off load and when it carried a four tonne load (boom at 56 degrees to vertical). Jolting the load periodically had little effect on the ampli-

I000

g

IO0

IOO

I: I'J ',

Electronic noise

lOs burst of pulser at maximum output I l

60 dB IOOdB Amplitude (A)

b

50

I I I [ [ I SI $2 O IOO

Distance ( x )

I I I

Fig 8 {a) Amplitude distribution of signals on Sx obtained while crane rotating slowly of f load. Sample time: 2.5 min (b) Results o f emission source location measurements between sensors $I and $2 while crane rotating of f load. Sample time: 3 min

TRIBOLOGY international April 1979 55

tude of the emissions. Linear location measurements made while the crane was swept through five complete half-circle cycles, both on and off load, showed no prominent build- up of emissions at any point on the circumference of the slew ring, see Fig 8b, the emission sources being uniformly distributed around the circumference of the ring. These emissions were possibly due to friction between the rolling elements and the inner ring, and gear meshing noise produced as the twin gear drive engaged the ring.

On this occasion the slew ring was not suspected of being damaged, and therefore the measurements form a good basis for comparing results for undamaged and damaged slew rings. Had the slew ring contained active fatigue cracks during the test, these would have been expected to result in a higher density of emission at specific points in the amplitude distribution plot at high amplitudes. In addition, the energy of the emissions from each active source would be expected to increase with increasing load.

Defect Severity Criteria

The American Society for Testing and Materials has pro- duced a code of practice for acoustic emission monitoring during proof testing. This gives a method of measurement and criteria for defect severity assessment. 16 In the light of the limited field trials done to date, this procedure is meant to serve as a guide only. It avoids reference to absolute values. The gradient of the acoustic activity with respect to stimulus is used to define three levels of severity.

Grade A: Inactive source. Gradient is fairly constant or decreasing with increasing stimulus. Event count is low.

Grade B: Active source. Event count increases steadily with increasing stimulus. Gradient is fairly con- stant.

Grade C: Critically active source. Event count is high. Gradient increases with increasing stimulus or increases with time under constant stimulus. Source should be evaluated by other NDT methods.

Conclusions The high sensitivity of the acoustic emission measurement and the good accessibility to the slew ring favour this tech- nique for detecting fatigue crack growth in the geared race and rolling elements of the bearing. The structure may be monitored during a periodically applied proof test in which it would be loaded beyond the normal working level. Alternatively, it may be continuously monitored during normal working.

The observation of strong signals is in itself encouraging. These can only be originating from the moving parts of the slew ring. Simply integrating the .ring-down counts or event rate during one complete rotational cycle of the crane under constant loading, is likely to be a measure of the general conditions of the moving parts of the bearing. This is a simple procedure analogous to the method of event counting, already used to monitor the condition of anti-friction bearings on vibrating screens and steel strip mills. 1'x2 If the background noise proves too excessive, then spatial and parametric filtering, as described above, can be used to resolve the acoustic emissions produced by propagating fatigue cracks.

As with all condition monitoring, the ultimate reward is improved design and performance, made possible by

reference to the heat, noise, vibration and strain charac- teristics recorded periodically during the campaign life of previous generation plant.

References 1. Rogers L.M. Diagnosis of plant condition from measurements

of temperature vibration and strain with emphasis on pattern recognition. Proc. Soc. Chem. Ind. Conf. On-line Surveillance and Monitoring o f Process Plant, City University, London, pp. 16.1 -16.16 (1977)

2. MeUing T.H. Transducers will test Claymore's Integrity. Off- shore Engineer (August 1977)

3. Cook S. O.S.O. Funds three electronic systems for monitor- ing the integrity of platforms, Offshore Engineer Supplement pp. 8 - 9 (July 1978)

4. Kelly M.P. and Bell R.L. Detection and location of flaw growth in the EBOR nuclear reactor vessel Dunegan/Endevco, San Juan Capistrano, California, DE-73-4

5. Parry D.L. Acoustic Emission Analysis in oil and chemical industry, Exxon Nuclear Company, Richland, Washington, USA (April 1972)

6. Lord A.E.J., Deisher J.N. and Koerner R.M. Attenuation of elastic waves in pipelines as applied to acoustic emission leak detection. Materials Evaluation, pp. 49 - 54 (November 1977)

7. Pollock A.A. and Smith B. Stress wave emission monitoring of a military bridge. Non-Destructive Testing pp. 348 - 353 (December 1972)

8. Bailey C.D. and Pless W.M. Acoustic emission used to non- destructively determine crack locations in aircraft structure fatigue specimens, 9th Symposium on Non-Destructive Testing, San Antonia, Texas (April 1973)

9. Green G., Maclntyre P., Walker E.F. and May MJ. Application of the acoustic emission technique to the characterisation of crack propagation processes in steels, British Steel Corpor- ation SheffieM Laboratories Report No. PT/7418/2/77/8 (February 1977)

10. Harris D., Maclntyre P. and Walker E.F. Acoustic emission during nitrate-induced stress corrosion cracking in welded BS 4360, Grade 50D structural steel. British Steel Corpor- ation Sheffield Laboratories Technical Note PT/EM/4 7/48 (August 1978)

11. Webborn T.J.C. and Rogers L.M. The detection and loca- tion of fatigue cracks in tubular welded joints using acoustic emission. British Steel Corporation Report 019/15/77 (1977)

12. Rush A.A., Dalton B.L. and Knoyle J. Monitoring bearings on the Llanwern Works 1.8m Hot $1 rip Mill, British'Steel Corporation, Corporate Engineering Laboratory Report No. CFL/PH/43/73: enquiries to 140 Battersea Park Road, London, SW11 (December 1973)

13. Dyer D. and Stewart R.M. Detection of rolling element bearing damage by statistical vibration analysis. Proc. Soc. Chem. Ind. Conf. On-Line Surveillance and Monitoring of Process Plant, City University, London, pp. 3.1 - 3.13 (1977)

14. Wyon A.S. and Mackinnon J.A. Dynamic Response of North Sea Cranes. Proc. Brit. Soc. Strain Measurement Conf. on Measurement in the Offshore Industry, Heriot-Watt Univer- sity, Edinburgh (September 1975)

15. Anon Design Evaluation of the Taperex Cross-RoU Slew Ring. Engineering Materials and Design. pp. 42 - 44 (Jan- uary 1978)

16. American Socie W of Testing and Materials, Designation E569, Standard recommended practice for acoustic emission monitoring of structures during controlled simulation, 1916 Race Street, Philadelphia, Pa, 19103 (1976)

56 TRIBOLOGY international April 1979

Appendix - Definition of Kurtosis

Vibration signals may be considered to be statistical in nature and from the acceleration signals it is possible to derive a probability density of the instantaneous ampli- tudes. This value indicates the probability of occurrence of an acceleration of a particular amplitude. Rather than examining the actual probability density curve, it is more informative to examine the statistical moments of the data, from which the type of distrib.ution may be asses- sed. The statistical moments o f a set o f data (xi) which are of most use in defining the distribution are the first four:-

N 1 st moment = ~

/=1

i.e. the mean value x

N ( x i - x-) 2_ o2 2nd moment = i=1 N

i.e. the variance and higher moments following the general form

N (X i _ ~ ) / ]th moment =

i=1 N

The third and fourth moments are generally non-dimen- sionalised by means of the standard deviation, a, to give the coefficient of skewness/31 and the kurtosis/~2 as follows:-

1 N ( x i _ ~ ) 3 = - - ~ /31 o 3 '=1 N

and 1 N (x i _ ~-)4

/ 3 2 = ~ ~ 1 X

The skewness, as the name implies, indicates the degree of the probability density curve. The kurtosis indicates the peakiness of the data. Kurtosis values for wave forms with zero skewness are:-

square wave 132 = I sine wave /3: = 1.5 Gaussian distribution /32 = 3

In general vibration signals from a bearing in a good con- dition have a Gaussian distribution and thus have a kurtosis value of 3 which is independent of size and speed of the bearing.

Kurtosis meters tested

Use of kurtosis for monitoring rolling element bearing condition has been studied by the British Steel Corpora- t ion*. Encouraging results have been obtained in both laboratory tests and field trials,

Laboratory tests

Tests were performed on a modified Timken taper roller bearing endurance test machine (Fig 1), of the type used

Thermocouple \ Accelerometers

Axial clearance \ adjustment plate ~k

. ~ \ \ ~ ~ ' ~ Variable ///'J, dd I,.",Y////;?~,] P"r/~7"A D ~ / / / / / k " ] . speed

/ / / / ~ le~t shaft

~ C a g e speed

~ pick-up

Toper roller J ~ -~ - -~Lood ing piston bearing ~ ~ - ~ Hydro ulic

pressure

Fig i Bearing endurance test machine

to establish the LBlo life of bearings: four identical bearings are equally loaded on a common shaft. To shorten the time to failure, the bearings were run at constant speed but at twice the recommended load. Lubricating oil was chosen so that the viscosity was corrected for this increased load, and fed to the bearings at constant temperature.

Minor damage detected

In a test lasting a total of 84h (1.6LB10), it was apparent after 74h that one of the centre bearings had begun to deteriorate. At 84h it was judged that the damage was beginning to propagate and so the test was terminated. Inspection showed that one roller in bearing 2 had suffered fatigue damage, and several adjacent rollers had small pitting marks, presumably caused by wear debris.

Fig 2 shows the variation in kurtosis, peak level, and rms values of the acceleration waveforms. The kurtosis value rose above 3 at about 1.3LBl.o, while the rms level did not increase significantly until 1.5LBlo. The spread of values of the three parameters with load and speed varia- tions are shown in Fig 2. Load variations of 0 -1 lkN and speed variations of 800-2700 rev/min gave peak acceleration variations of-+ 65% and rms value variations of +50%: the kurtosis value varied by +8%.

3O

20

to m "~ 6.0

3.0 2.0

o ~o

/, -+8% /o

-~_-------_-Q_-+~_ - 2 - W_-~_ °

I i I i ¢ I 8 i 0 40 60

Test time ( h )

~ 3.0

2 0

"~ 1.0 ~ o

+65% T °

i. o_o l ,o --t' o o

I

t50% i o

o-LJ

~ 0.4

1~0.3

" I T --'-~ - - I | I i

40 60 .80 Test time ( h )

' ' i ' d . . . . 0 20 0 0 0 60 80

Test time (h)

Fig 2 The variation o f kurtosis, peak level, and rms values o f the acceleration waveforms with load and speed

TRIBOLOGY international Apri l 1979 57

Severe d a m a g e

In a test lasting a total of 658h, incipient damage was detected after 444h. At this stage the bearing was re- moved and inspected: an inner race fatigue crack 3ram long was found. The bearing was then reassembled and the test continued for a further 214h.

Severe damage was sustained by the original bearing (Figs 3 and 4) and this damage caused primary and secondary damage on the other three bearings. Fig 5 again highlights the early warning capabil i ty of kurtosis, while Fig 6 shows the variation of kurtosis in various frequency bands as damage propagates. In general,

Figs 3 and 4 Damage on rollers after 675h o f running

"~ ~ ^ | , Test stopped Test completedJ E °uI- J, to allow examination Ib "1 °

/ v of Dearing b / 24 . . " . t ' , ,, :o, _46,

/ 4', <Z O I n ~ . - - c , - n m ~ o . o ~ O O O . . , o - - c J ~ I L I 2

300 400 500 600 700 Test ( h )

Fig 5 Variation o f rms and kurtosis values with time during the second life test (Centre bearing 3Hz -5kHz)

init ial signs of damage are indicated by an increase in kurtosis values in the low frequency band: as damage becomes more severe, the kurtosis values increase in the higher frequency bands. When damage becomes very advanced, kurtosis values peak above 20 kHz but fall in the lower frequency bands: it is the distr ibution, not the absolute value of kurtosis, that is important.

F ie ld t r ia ls

Several hundred bearings were monitored during the field evaluation of the kurtosis meter. Bearing types monitored included: plain, taper, and spherical roller; deep groove, angular contact, and double self-aligning ball; and Cooper split bearings. A variety of load, size and speed condit ions were used, giving D m.N values between 104 a n d 4 x 105 .

20

IO

6 D

o a

a b x C

o ~ - -o

2 I I I I I I 2 5 4 5

Fig 6 Kurtosis profiles at various stages through the second life test f l = 3Hz-5kHz, a = 329h; f2 = 5 - 1 0 kHz, b = 454h; f3 = 10-15kHz, c = 637h; f4 = 1 5 - 2 0 kHz, d = 657h

20

IO 8

6

4

5

2 1.5 0.3 0.2 2 0,3 g 5.5 13 14 17 7.5 16 K

/ o P

o ~ / / J /

/ /

o

4.7 3 1.7 5.5 3.5 3.7 13.8 15

/ °

- / 0 ~ 0

2 I I I I I I I I I I 2 3 4 5 6 I 2 3 4

a f b f

7.8 7,5 g 8 13.8 K

P / /

/ /

I I 5 6

36 31 3.1 30 K

O ~ O ' 0 0

I I I I I 2 3 4

C f

Fig 7 Kurtosis profiles f l = 3 H z - 5 kHz, f2 = 5 -10kHz , f3 = 1 O- 15kHz, f4 = 15-20kHz, f s = 20kHz, f6 = 20kHz

58 TRIBOLOGY international Apri l 1979

The only bearing type which gave cause for concern was the Cooper split bearing, which can generate impulsive- ness as the rolling element passes over the split in the race. Kurtosis evaluation of this type of bearing was only partially successful and further information is being obtained to establish the usefulness of the technique for this type of bearing.

The instrument successfully predicted damage on a wide variety of bearing types. The rms values were found use- ful in diagnosing very advanced damage and the combina- tion of kurtosis and rms values give a success rate of 96% when the predicted damage was compared with visual inspection of the bearings.

Three examples are presented in Fig 7. Fig 7(a) shows the kurtosis profile of a 55mm double self-aligning ball bearing supporting a cooling fan running at 1400 rev/min. There had been no increases in rms values, but the kurtosis profile indicated medium to advanced damage. Inspection revealed that the balls were pitted from corrosion, with one ball in particular being badly spalled. The inner track was discoloured and fretting had com- menced. Fig 7(b) shows the kurtosis profile from an 85ram double row spherical bearing supporting an exhaust fan running at 1500 rev/min. The bearing was running hot, but there was no significant increase in the rms levels. The kurtosis profile, however, indicated medium to advanced damage: inspection showed that the cage was severely deformed, but there was no roller damage. Fig 7(c) is for a bearing in a similar application. This kurtosis profile would not normally warrant removal of the bearing. In this case, however, it was convenient to remove the bearing and close examination revealed a fine crack across the contact surface of one of the rollers, confirming the kurtosis prediction of early damage.

*This data has been abstracted from an article by A.A. Rush published . in the February 1979 issue o f lron and Steel lnter- national {pp 23-27). The article gives a full definition o f kurtosis, a description o f the kurtosis meter, fuller data, detail on accelero- meter attachment, etc. lron and Steel International is published by IPC Seience and Technology Press

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