E-Journal of Advanced Maintenance Vol.2 (2010) 43-49
Japan Society of Maintenology

Electromagnetic Nondestructive Evaluation Method for Discriminating
and Sizing of Defects in Bearings
Shingo MAWATARI 1,*, Ryo KAYATA2, Stéphane PERRIN 2, and Kazuo YASHIRO1,†1

Japan Nuclear Fuel Limited，4-108，Aza Okitsuki，Oaza Obuchi Rokkasho-mura，Kamikita-gun，
Aomori-ken 039-3212，Japan2IIU, 2-7-17-7F Ikenohata, Taito-ku, Tokyo 110-0008, Japan
ABSTRACTIn the present paper, a new electromagnetic nondestructive evaluation technique is proposed to detect and size
defects on the inner and outer rings of ball bearings. A permanent magnet is used to generate a static magnetic
field and the eddy currents induced inside the rotating ball bearing are detected by a coil wrapped around themagnet. A signal processing method that makes use of the autocorrelation is proposed to detect the defects inball bearings and to evaluate their size. Experimental validations are carried out using common ball bearings.
The comparison through the use of the proposed method of healthy bearings with bearings with defects allows
detecting, discriminating and sizing the defects, in spite of the presence of a metallic casing.
KEYWORDS

ARTICLE INFORMATION

electromagnetic nondestructive evaluation, signal processing, ball bearings,eddy currents, autocorrelation.

Article history:
Received 25 January 2010
Accepted 2 August 2010

1.

Introduction

In the frame of the maintenance of nuclear facilities and to insure the reliability of theequipments, different suitable maintenance methods are applied, such as Time-Based- Maintenance(TBM) or Condition-Based- Maintenance (CBM).
In order to optimize the current CBM techniques, a way to monitor the condition of theequipment with accuracy is necessary. Currently, in-situ maintenance in nuclear facilities and theidentification of the abnormal condition of rotating machines is performed by the mature technique ofmeasuring vibrations. There is a long history of monitoring machines through the measurement ofvibrations and large databases have been accumulated. Nonetheless, there are cases where theabnormal condition of a part of a rotating machine is not reflected into the vibration measurements,
where radioactive contamination prevents to assess the condition before maintenance, or where better
performance cannot be achieved. In order to address these issues, the development of a new condition
monitoring technique is desired.
Electromagnetic evaluation methods [1-5] have already been applied to the condition
monitoring of rotating machines. In particular, it has been successfully applied to the condition
monitoring of the main cause of the degradation of rotating machines, the bearings.
In this study, it is confirmed that nondestructive electromagnetic condition monitoring is possible
through the application of an electromagnetic evaluation method to the bearings of rotating machineswith simulated defects on the inner and outer rings. Moreover, a relation between the electromagnetic
signal and the size of a defect is established through the application of the electromagnetic evaluation
technique that makes use of the autocorrelation.

*Corresponding author, Tel: (+81) 175-71-2335, E-mail: shingo.mawatari@jnfl.co.jp.
Present address: Tokyo Electric Power Company, 1-3 Uchisaiwai-cho 1-chome, Chiyoda-ku, Tokyo 110-8560,
Japan†ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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S. MAWATARI, et al./ Electromagnetic Nondestructive Evaluation Method
for Discriminating and Sizing of Defects in Bearings

2.

Principles of the electromagnetic evaluation method

When a conductor passes through a static magnetic field, eddy currents are induced on its surface.
In particular, when a static magnetic field is applied from the outside to a rotating machine, eddy
currents are induced on the surface of any conductive rotating body such as the bearings (Figure 1).
These eddy currents vary depending on the movements or vibrations of the rotating parts. Because
these variations of the eddy currents can be measured by the magnetic sensor, evaluation of thecondition of the bearings is possible.
Eddy current

StaticMagnetic
Field

MagnetFig. 1. Principle of measurement

Fig. 2. Signal Processing
Vibration
sensorTable 1 Experimental device specifications

ElectromagneticSensor

Bearing

Bearing specifications
Bearing typeSelf-aligning ball bearing(1204S)
Inner diameter
20 mm (0.79 in.)
Outer diameter
47 mm (1.85 in.)
Bearing width
14 mm (0.55 in.)
Rollers per row
12Rows2
Bearing holder specification
Length300 mm (11.81 in.)Width
500 mm (19.69 in.)Depth
250 mm (9.84 in.)
Material
S45C

Fig. 3. Positioning of the sensor.

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S. MAWATARI, et al./ Electromagnetic Nondestructive Evaluation Methodfor Discriminating and Sizing of Defects in Bearings

3.

Evaluation of bearing defects

3.1 Signal processing
The processing of the signals for the electro- magnetic evaluation of the condition of the bearings
is now presented. Figure 2 shows the flowchart of the steps of the signal processing procedure. At step
1, the time-domain signal is acquired and its spectrum is computed and analyzed. Then, a band-passfilter is applied in order to keep only the spectral components related to the presence of a defect. As
the band-pass filter depends on the type of the bearing, the size and mass of the machine, the material
properties etc., an in-depth analysis is necessary. At step 2, the envelope of the filtered signal and its
spectrum are computed. The spectrum is analyzed to identify periodic high frequency components andspectral components related to the rotation frequency or defects of the bearing. Depending on the typeof the defect, inner or outer, different frequency components will appear on the spectrum of theenvelop signal. Because these spectral components are not present in the case of a bearing without
defect, it is possible to discriminate between the presence or not of a defect and to identify thelocation of the defect (inner or outer ring), depending on their value (see section 4.1).
3.2 Autocorrelation method
By computing the autocorrelation coefficients on the envelope of the filtered signal, theuncorrelated noise is reduced, the periodic high frequency components are more easily identified andthe correlation of the high frequency components is evaluated. In the case of the presence of a defect,
local maximums of the autocorrelation coefficients are observed on each side of the peakcorresponding to the characteristic frequency of the defect and are used to evaluate its width. Thewider is the defect the larger is the time interval between the peak and the local maximums.

4.

Experimental validation

4.1 Bearings and electromagnetic sensor
The bearings on which measurements were performed are self-aligning ball bearings model
1204S. Table 1 shows the experimental device specifications and Figure 3 shows the positioning of
the sensor. The main characteristics of the bearings are an outer race diameter of 47 mm, inner race
diameter of 20 mm, height of 14 mm and two rows of 12 balls. The distance between the bearing andthe sensor, filled by the bearing holder, is approximately 5 mm (see Fig.3).
The material of the bearing holder is S45C. For a frequency of rotation n, the frequency of
passage of the balls at a fixed point on the inner ring is fi = 7.13n. The frequency of passage of theballs at a fixed point on the outer ring is fo = 4.87n. The coefficient such as 7.13 and 4.87, which
decides by the part size of the bearing and the number of rollers, is calculated from the following
formula (rin , rout : Radius of inner ring and outer ring , N : Number of rollers).
Character frequency of outer ring defect:

fo =rinN ⋅nrin + rout(1)

fi =routN ⋅nrin + rout(2)

Character frequency of inner ring defect:

Figure 4 and Table 2 shows a picture of the electromagnetic sensor and its specifications. Theelectromagnetic sensor is constituted of a permanent magnet (neodymium magnet) that creates thestatic magnetic field and of a pick-up coil wrapped around the magnet that measures the eddy currents
induced on the surface of the rotating parts.

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S. MAWATARI, et al./ Electromagnetic Nondestructive Evaluation Method
for Discriminating and Sizing of Defects in Bearings

Table 2 Electromagnetic sensor specifications
DiameterHeightMagnet type
Coil turnSurface magnetic
flux density

φ20 mm(0.79 in.)
20 mm(0.79 in.)Neodymium magnet
500
0.5 TFig. 4. Picture of the sensor4.2 Experimental conditions
Measurements were performed with a computer through an analog-digital converter (ADC) and asignal conditioner. The signal conditioner amplifies the signal by 100 and filters it with a low-passfilter of cutting frequency 10 kHz. The ADC acquires the signal at 20 kS/s with a resolution of 16bits
over the range +/-5 V. Measurements were performed on self-aligning ball bearings with slits on theinner or outer rings to simulate defects. Figure 5 shows a schema of an outer ring defect as well as a
picture of the outer ring defect no. 2 and Figure 6 the schema of an inner ring defect and a picture of
the inner ring defect no. 2. As seen on Fig. 5 and 6, the slits that were made on the inner and outerrings extend only on a single race.
Table 3 gives the running condition and the numbering of the bearings with a defect on the inneror outer ring as well as the width of the opening of the defects. Outer and inner rings defects of threedifferent sizes were prepared. The defects were manufactured using a grinder with a diamond bit. Thewidth of the defects is the width measured near the middle of the surface where the balls go throughthe defect. The test bearings were designed to simulate the process of degradation of the bearings byflaking due to inner and outer ring defects, to experimentally identify inner and outer ring defects andto evaluate the width of the inner and outer ring defects at the location where the balls are passing
through. The frequency of rotation is n=20 Hz.
The inner ring defects are shift at the rotation of the inner ring that is the rotation frequency n.
The outer ring defects are fixed and always located diametrically opposed to the location of the sensor(the farthest location from the sensor).

Fig. 5. Schema of an outer ring defect and picture of the outer ring defect no. 246

S. MAWATARI, et al./ Electromagnetic Nondestructive Evaluation Method
for Discriminating and Sizing of Defects in Bearings

Fig. 6. Schema of an inner ring defect and picture of the inner ring defect no. 2

Table 3 Experimental condition
Running condition
Speed
LubricantLoading condition

1200 rpm (20 Hz)
GreeceNo-loadWidth of Outer ring defect
No.1
No.2
No.3
Width of Inner ring defect
No.1
No.2
No.3

0.4 mm2.1 mm
3.7 mm
0.4 mm
2.1 mm
3.8 mm

4.3 Results
Figure 7 and Figure 8 show the results of time-domain signal after the band-pass filter [1 kHz - 5kHz] in the case of inner and outer ring defects. The rollers of bearing have produced a periodic
change of the signal by passing a defect.
Figure 9 and Figure 10 show the spectrum of the envelope signal for an inner and an outer ring
defect. A frequency component of 97.3 Hz (approximately 4.87*20 Hz) in the case of an outer ring
defect and of 143 Hz (approximately 7.13*20 Hz) in the case of an inner ring defect is identified.
Figure 11 and Figure 12 show the results of the computation of the autocorrelation in the case of
inner and outer ring defects. In the case of an outer ring defect, at the delay time corresponding to97.3 Hz, a peak is identified in the autocorrelation (see Fig.11). The location of the local maximums
on each side of this peak are use to evaluate the width of the defect. The circumference at the surfaceof the outer race is 124.72 mm (approximately 47mm * π ) that is covered by each ball at thefrequency of rotation of 8.11 Hz (approximately 0.405*20 Hz). In the case of the outer race defect no.
3, the analysis of the time interval between the main peak and the local maximums gives the value ofapproximately 3.7 ms. (see Fig. 11), that is a distance of about 3.74 mm. This is in good agreementwith the real width of the defect (3.7 mm, see Table 3).

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S. MAWATARI, et al./ Electromagnetic Nondestructive Evaluation Method
for Discriminating and Sizing of Defects in Bearings

Fig. 7. Filtered signal (outer ring defect)

Fig. 8. Filtered signal (inner ring defect)

Fig. 9. Envelope (outer ring defect)

Fig. 10. Envelope (inner ring defect)

Fig. 11. Analysis of the autocorrelation (outerring defects and healthy bearing)

Fig. 12. Analysis of the autocorrelation (inner ringdefects and healthy bearing)

The same analysis can be performed on the other outer ring defects. The results are given inTable 4. In the case of the outer ring defect no. 1, no local maximums are observed. The possible
reasons are that the computation of the envelope and the autocorrelation coefficients buried the signalof interest into noise, or there is the possibility that the small width of the defect makes that it has noinfluence, such as vibration, on the balls. Nonetheless, the presence of a main peak that does notappear in the case of a healthy bearing is an indication than the defect is detected and that its width isless than 2.1 mm.

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S. MAWATARI, et al./ Electromagnetic Nondestructive Evaluation Methodfor Discriminating and Sizing of Defects in Bearings

Table 4

Evaluated and real width of the defects
Real width

Evaluated width

No.1
No.2
No.30.4 mm
2.1 mm
3.7 mm−1.87 mm
3.74 mm

No.1
No.2
No.3

0.4 mm
2.1 mm
3.8 mm−1.92 mm
3.53 mm

Outer ring defect

Inner ring defectIn the case of the inner ring defects, the same analysis is conducted (see Fig. 12). Nonetheless,the main peak that is considered is now located at 7.13 times the period of an inner ring defect (1/143Hz). This is because the inner ring, and the defect, is rotating at the rotation speed and the highest
signal due to the defect is measured when a ball passes over the defect at the time it is located exactly
under the sensor. The results are given in Table 4.
As it can be seen, a very good agreement between the evaluation of the width of the defects(inner or outer rings) and the actual width of the defects is obtained. Only in the case of the smallest
defect (0.4 mm on the inner or outer rings) the width cannot be evaluated even if it is detected.

5.

Conclusions

Thanks to the in-depth analysis of the autocorrelation coefficients of the envelope of the filteredsignal, it is possible to evaluate the width of a defect through the effect of the passage of the balls overthe defect. For inner or outer defects of width larger than 2 mm, very good quantitative evaluation of
the width was performed. For smaller defects, detection and discrimination is possible while sizing ofdefects is not. It is nonetheless possible to give an upper limit for the width (2 mm) of the defect.
The autocorrelation method presents a significant improvement of usually applied methods, withelectromagnetic or vibration sensors, by allowing the quantitative evaluation of the size of the defects
in addition to their detection and discrimination.

References[1] H. Huang, K. Miya, N. Yusa and D. Kosaka. Electromagnetic detection of rotating machines defects. 10thsymposium of the Japanese Nondestructive Evaluation Association, Tokyo, 25-26 Jan. 2007.
[2] D. Kosaka, H. Huang and N. Yusa, Investigation of the electromagnetic nondestructive evaluation methods
for rotating machines, 4th Annual Conference of the Japan Society of Maintenology, Fukui University, 2-3
July 2007.
[3] D. Kosaka, H. Huang, N. Yusa and K. Miya. Electromagnetic nondestructive evaluation of rotating blades.
The 13th International Symposium on Applied Electromagnetics and Mechanics, Michigan State University,USA, 9-12 Sep. 2007.
[4] D. Kosaka, H. Huang, N. Yusa and K. Miya. Electromagnetic nondestructive evaluation of rotating blades.
Science and Technology of Maintenance (under review).
[5] R. Kayata, H. Huang and N. Yusa. Electromagnetic evaluation method for the measurement of bearings withparticles, 5th Annual Conference of the Japan Society of Maintenology, Mito, 10-12 July 2008.

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