A LABVIEW miniexpert to identify bearing defects automatically

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The rolling-element expert vi

This is an example how advanced data processing of shocks produced by surface defects leads to a robust miniexpert telling if a bearing surface defect is present on one or some of the components like inner races, outer races of rollers (balls) even at very slow rotational speeds. This limits the rate of false alarms characterizing simpler methods like SPM, HF, etc.. The diagnosis is displayed by a miniexpert vi in a straightforward fashion, thus bypassing the need of a vibration expert to analyze the bearing signatures.

Test rig

The rig features variable speed from 60 to 1550 rpm, radial loads from 0 to 10 Tons, completely separable roller or ball bearings on which calibrated defects are introduced on races and/or rollers (balls). These defects have increased severities with widths ranging from 0.1 to 0.4 mm. Bearing tested are SKF22NU15EC and FAG 1215TV

The test rigs with hydraulic jack, the test bearings and the variable speed drive.
Not shown: a hydraulic unit for lube and high-pressure jack (100bars).

One of the bearings tested FAG 1215TV test bearing: completely separable to introduce calibrated defects on races and balls. Inner bores are 75 mm. Shocks were measured with resonant accelerometers at first (fres at ca 5 KHz). Later normal off-shelf general-purpose accelerometers were used instead to use standard accelerometers from conventional monitoring system. Their response were digitally high-passed to eliminate low-frequency contributions of vibrations and get true shock signatures.

Left-hand side: The test rigs with hydraulic jack, the test bearings and the variable speed drive. Not shown: a hydraulic unit for lube and high-pressure jack (100bars). One of the bearings tested FAG 1215TV test bearing. Inner bores are 75 mm. Shocks were measured with resonant accelerometers at first (fres at ca 5 KHz). Later normal off-shelf general-purpose accelerometers were used instead to use standard accelerometers from conventional monitoring system. Their response were digitally high-passed to eliminate low-frequency contributions of vibrations and get true shock signatures.

Raw defect signatures: see Table below along with miniexpert diagnoses

Raw signatures :Sound bearing: typically gaussian noise. Surface roughnesses in bearings exhibit very little spatial correlation. As a result, sound bearings tend to produce acoustic responses that are almost gaussian noise. Such a noise is characterized by a Kurtosis (random variable) close to 3.
Outer race defect: Succession of shocks with about the same amplitude whose spacing can be derived from the B(all)P(assing)F(requency)O(uter) race characteristic frequency of the bearing, depending on the pitch diameter, the ball (roller) diameter, the number of the rollers (balls), the angle of contact (if any) and the shaft rpm. Rotational speed is 1500 rpm in this example obtained with a calibrated defect on the outer race of a SKF22NU15EC As all other responses they are digitally high-filtered from 2 KHz on.
Inner race defect: Succession of shocks modulated over a period corresponding to a shaft rev. Shock spacing corresponds to a B(all)P(assing)F(requency)I(inner) race that can be computed from standard formulae, knowing the pitch diameter, the ball diameter, the speed and the contact angle. Shaft rpm is again 1500 rpm like in all following signatures.
Roller (roller)surface defect: Same remark as before except that shock spacing corresponds to twice the characteristic B(all)S(pin)F(requency) and is modulated with a period corresponding to one cage rev.

Expert system as a LABVIEW vi: Basic idea behind data processing

Suppose that a surface defect appears at the surface of a ball (roller) as shown below. This defect will cause successive shocks as it encounters either the inner or the outer race. If the load carried by the bearing is vertical downward, such shocks tend to increase with the pressure between roller and races. This pressure reaches a maximum when the contact between the defect and the races are aligned with the load zone. That is the case when the defect is below. As the positions of the contacts depart from the vertical, the shocks they produce decrease in amplitude.. Shocks altogether disappear when the ball is located in the upper part of the outer race since there is no longer any pressure between the ball and either race. In this reasoning, the outer race is supposed fixed and the bearing is not preconstrained (look at suffix C which is C3 in the test bearings).
That is what one conveys in the left-hand-side representation of the bearing for one cage revolution. Blue circles indicate the positions of the ball defect with the inner race, whereas red circles have the same meaning with the outer race. Circle radii are proportional to shock intensities. They depend on some distribution of the load zone. In order to account for the attenuation of shock waves through the roller as sensed by an outboard accelerometer, one could further reduce the intensities of the blue circles, although this was not really observed with the experimental results from the test rig. The right-hand side bearing shows the position of contacts of the defects with the races over several cage revs.

Now the main red circle can be viewed as the source of the acoustic wave generated by the defect and the subsequent blue and red circles like echoes of this main shock waves. These are not real echoes as one encounters in the propagation of acoustic shock waves. The latter would be much closer spaced. The former are pseudo echoes caused by the dynamic and kinematics of bearings and their surface defects.

Attenuated echo-like shock waves lend themselves to cepstral techniques when one wishes to separate them from the main incident shock wave and reach a high data compression ratio. To this aim, one must exert some care to obtain sound cepstra according to L.F. Pau [Failure Diagnosis and Performance Testing Series Control and Systems Theory vol. 11, Marcel Dekker]:

1) It is important to start the data acquisition when the defect enters its load zone and terminates it when leaving the zone. Therefore one samples the acoustic response over a time span extending over several load zones for ball defects (the longest of all corresponding to successive cage revolutions). One then narrows the time window to correspond to a maximum energy in its center and extend over half a cage revolution. AS shown in the picture below, one samples the shocks over a few cage revs and then extract a shorter part of the data acquisition window to center it over half a cage rev.

) one then applies a double exponential window w(t) simulating an attenuation of shock waves when departing from the window center. Like other defects, outer race defects lacking specific load zones now generate attenuated echoes.

The vi expert.

The bearing miniexepert is the LABVIEWvirtual instrument whose panel is shown below for slow-rotating bearings. It features:.

the control window to select the number of averages to be used for identification
a “bearing characteristic window” to enter the geometric data of the bearing, to compute the characteristic frequencies from the rotation speed:
These can be obtained with standard programs like SKF ATLAS for most standard bearings.
the “file saving window” to chose between on-line or delayed analysis
a raw signal window gives a graphical presentation of the time waveform
the cepstrum window gives the cepstrum waveform and draws the cursors corresponding to the characteristic bearing defect quefrencies.
The “result window” displays the automatic diagnosis in a clear fashion:
default not on bearing, inner race default, outer race default or ball default.
The other windows are used to check the variations of the speed during the test or to follow the evolution of parameters like Kurtosis, Crest factor, ...

Conclusions

Using signatures obtained with the test rig with bearings fitted with various types of surface defects and increasing defect severities, the miniexpert consistently identified the defects correctly even at such low speeds as 60 rpm. Bearings were either of the roller type (SKF22NU15EC) or of the ball type (double row spherical FAG1215TV). These bearings share the interesting property that one can easily implement calibrated defects on the surface of all elements (races and rollers or balls) because one can separate their components easily meccano-style without breaking any of their components.
One ran out of luck to establish severity charts based on shock amplitudes. Remember that Kurtosis and cesptra (through the log transform) are adimensional criteria. They perform extremely well to identify bearing defects despite somewhat "rotten" acoustic paths between the source of the shocks and the outboard accelerometer and/or despite very low rotational speeds (down to 60 rpm and tiny 0.2mm wide defects, it never failed). They are great for a boolean type of diagnosis. They become somewhat useless to assess fault severities. The latter must rely on the amplitudes of the shock waves as measured by the outboard sensor, eventually with notions like carpet, peak levels as in SPM or other methods.
Severity charts like from SPM performed well to follow defects with increasing widths from 0.2 to 0.4 mm. That is fine for trending. However, some sound bearings were declared as faulty by SPM right from the start, thus causing a false alarm. With SPM, faulty bearings systematically went unnoticed when rotating below 400 rpm.
It would be nice to merge reliable severity charts, if any exist, in the above LABVIEW vi to reach an unsurpassed bearing expert that, for sure, tells that the bearing is faulty (done without false alarms with the present vi) and by how much (not done). In order to eliminate a small inconvenience, one could link the above bearing miniexpert to data banks like SKF ATLAS to save the user the transcript of the bearing geometries.
The miniexpert requires to measure the rotational speed rather precisely. This was done with keyphasors and the 24-bit counters of National Instruments AT-MIO cards. Another vi version allows to enter the speed manually if it is known by other means. Accelerometers were standard products as are available in common vibration monitoring systems.

Mail us if you want more information on this product or wish to use the test rig!

Responsible Editor: G.D'Ans, Research engineer at Laborelec and industrial collaborator of ULB, tel 32 2 650 25 15 (ULB) or 32 2 382 0 568 (Laborelec)