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AN INNOVATIVE JIG TO TEST MECHANICAL BEARINGS
EXPOSED TO HIGH VOLTAGE ELECTRICAL CURRENT
DISCHARGES
Nicolaas Steenekamp
Gauteng Department of Infrastructure Development, Impophoma House,
Johannesburg, (South Africa).
E-mail: nicolaas.steenekamp@gauteng.gov.za ORCID: http://orcid.org/0000-0001-6858-4207
Arthur James Swart
Department of Electrical, Electronic and Computer Engineering, Central University of Technology,
Bloemfontein, (South Africa).
E-mail: aswart@cut.ac.za ORCID: http://orcid.org/0000-0001-5906-2896
Recepción:
27/01/2020
Aceptación:
06/04/2020
Publicación:
30/04/2020
Citación sugerida Suggested citation
Steenekamp, N., y Swart, A. J. (2020). An innovative jig to test mechanical bearings exposed to high
voltage electrical current discharges. 3C Tecnología. Glosas de innovación aplicadas a la pyme. Edición Especial,
Abril 2020, 195-215. http://doi.org/10.17993/3ctecno.2020.specialissue5.195-215
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ABSTRACT
Premature bearing failures due to Electrical Current Discharge (ECD) has been recognised
for almost a century. The purpose of this paper is to present an innovative jig that may be
used to expose mechanical bearings to ECD, in order to clarify its associated eects on the
bearing that need to be understood before any mitigating techniques can be proposed. An
experimental design is used in this study. A method is presented using an ignition coil wiring
harness of a vehicle to safely induce ECD across a specic bearing. Three samples were
used and analysed with an optical and electron scanning microscope. The used ball bearing
exposed to ECD showed micro-cratering, a result of electric current passage. Micro arching
marks on the raceway surface of this bearing was also visible, and especially near the groove
of the synthetic rubber seal and steel plate slinger. Surface pits were observed which were
produced by electrical arching. A few deep scratches and indentations were observed on the
raceway surface. This is due to abrasive wear particles embedded in the raceway surface
sliding between the major bearing components. The aspect of electrical pitting wear
and debris found in the lubricating oil are unknown and deserve further research from a
tribological point of view. A recommendation is made to use this innovative jig to test the
impact of ECD on bearings from other suppliers.
KEYWORDS
Electrical Current Discharges, Micro Arching, Micro Cratering, Ball Bearing, LOM, SEM.
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1. INTRODUCTION
The global bearings market was valued at approximately USD 92.81 billion in 2017 (Bizwit
Research & Consulting LLP, 2019). Ball bearings are a common component in machinery
that nds widespread use in numerous industrial applications. These include air, water and
land transport, agriculture, construction, manufacturing and mining industries. Premature
bearing failures is one of the main reasons for machinery down time (Jacobs et al., 2016). The
failure mechanisms of bearings have been well researched and documented i.e. Brinelling,
Contamination, Corrosion, Fatigue, Fit, Lubrication, Misalignment and Overloading
(Massi et al., 2010; Bhadeshia, 2012; Upadhyay, Kumaraswamidhas, & Azam, 2013).
However, bearings may also fail due to Electrical Current Discharges (ECD) that may
originate with lightning, high voltage spikes or high potential dierences. The cause of
bearing failure due to electric current passage has been recognised for almost a century (Liu,
2014). As a matter of fact, electric potential dierence exists between shafts and bearing
housings in machinery equipment due to the asymmetric eects of the magnetic elds,
magnetized shaft, and electrostatic eects, etc. (Chiou, Lee, & Lin, 2009). Some practical
solutions to mitigate bearing currents has worked eectively for sinusoidal alternating
currents i.e. shaft grounding to bypass current, ceramic-coated bearings and hybrid insulated
bearings. Some of these solutions are not as eective against the fast switching Pulse Width
Modulation (PWM) inverter technology that causes high frequency non-sinusoidal bearing
current (Liu, 2014). A diversity of condition monitoring techniques exist that can be used
to identify developmental bearing failure.
The purpose of this paper is to present an innovative jig that may be used to expose
mechanical bearings to ECD, in order to clarify its associated eects on the bearing that
need to be understood before any mitigating techniques can be proposed. Firstly, an
overview of the construction and operation of dierent types of key bearings is presented.
Secondly, analysis techniques are presented to separate a single failure mechanism from the
complex mechanisms. Thirdly, a method is presented using an ignition coil wiring harness
of a vehicle to safely induce ECD across a specic bearing. Fourthly, the obtained results are
discussed. Finally, concluding remarks and recommendations are presented.
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2. MAIN BEARING CLASSIFICATIONS
An organogram outlining the classication of commonly found bearings is as shown Figure
1 (NTN Corporation, 2015). A rolling bearing consists of four major components: an inner
and outer raceway, rolling elements and a cage that maintains equally spaced intervals
between the rolling elements. The rolling elements are situated between the inner and
outer raceways to translate motion. Rolling bearings are grouped into two main rolling
element classications, ball bearings and roller bearings. Ball bearings are classied by the
raceway type: deep groove or angular contact. Roller bearings are classied by the roller
type: cylindrical, needle, tapered, and spherical (NTN Corporation, 2015). Roller bearings
typically have greater load carrying capacities because of the greater contact area of the
roller bearings to the adjacent raceway surfaces (Bhadeshia, n.d.).
Furthermore, rolling bearings can also be classied by the load type: radial or thrust.
Radial bearings support radial loads and thrust bearings support axial loads. Most roller
bearings can simultaneously support radial and thrust loads (NTN Corporation, 2015).
Rolling bearings are also supplied in multiples of separable and non-separable rolling rows,
for example single, double and quadruplet congurations. The choice of a bearing and
conguration depends on the stiness and load requirements of the application. A brief
discussion on some of these bearings now follows.
Duplex angular contact ball bearings are typically selected to increase stiness and load
carrying capacity of the support ends of upright and overhung shafts and screw drives
(SKF Group, n.d.a). Angular contact ball bearings have inner and outer ring raceways that
are displaced relative to each other in the direction of the bearing axis. The congurations
are typically back-to-back, face-to-face and tandem. Duplex angular contact ball bearings
are most commonly found in centrifugal pumps (SKF Group, 2012a).
Self-aligning ball bearings are typically selected for industrial applications where low
friction is preferred over high load carrying capacity to accommodate misalignment, shaft
deections and thermal expansion (SKF Group, n.d.b). Furthermore, the self-aligning ball
bearing has the lowest friction of all rolling bearings which allows them to operate at higher
speeds and cooler temperatures (SKF Group, 2018). Self-aligning ball bearings have two
raceways on the inner ring and has a single spherical raceway on the outer ring that can
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counteract up to three degrees of misalignment. Tapered bore ball bearings induce bearing
preload and are typically secured with adapter sleeves to smooth or stepped shafts. They
are typically found in paper mills (SKF Group, 2018) and the textile industry (SKF Group,
2014).
Like duplex angular contact ball bearings, high-speed duplex angular contact ball bearings
are typically selected for applications that demand high reliability and superior accuracy,
for example a machine tool spindle in CNC turning, machining centre and milling
machine (SKF Group, 2012b). High-speed duplex bearings are usually sealed to eliminate
contamination to prevent premature bearing failures.
Radial needle roller bearings are typically selected for their stiness and high load carrying
capacity (SKF Group, n.d.c). The diameter of a roller element of a needle roller bearing
is relatively small in relation to its length. Needle roller bearings are used in applications
where space is limited. Typically, a needle roller is combined with a shaft or housing bore
to serve as a raceway. They are used, for example, in the universal joint of a drive shaft and
rocker-arm pivot of a vehicle (SKF Group, n.d.d).
Spherical thrust roller bearings are well suited for heavy-duty applications where axial
and or combined axial and radial loads needs to be accommodated (SKF Group, 2010).
Spherical thrust roller bearings are like self-aligning ball bearings and are typically found
in a cooling water pump for a thermal power plant and marine thrusters (“SKF explorer
spherical roller thrust bearings boost design options”, 2003).
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Figure 1. Classication of rolling bearings. Source: (NTN Corporation, 2015).
For this research work, a radial ball bearing for rolling bearing units, as shown in Figure
2, is selected as it is cost eective and has a basic mounting interface that can be used
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between the surface of the jig and the pulley shaft system. This type of bearing is based
on a sealed deep groove ball bearing. The ring of the outer raceway is convex to allow for
shaft miss-alignment by tilting in the rolling bearing unit, and the ring of the inner raceway
is extended with a locking device to enabling quick and easy mounting onto shafts (SKF
Group, n.d.e). The structural diagram of the radial ball bearing for rolling bearing unit
used is shown in Figure 3.
Figure 2. Radial ball bearing for rolling bearing unit. Source: (NTN Corporation, 2018).
Figure 3. Structural diagram of the radial ball bearing for rolling bearing unit. Source: (ETK Bearing Company,
n.d.).
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Rolling bearings for special applications such as journal bearings, linear actuators, and
linear motion products are excluded from the scope of this work, as the main objective
is to initially verify the operation of the jig along with its results. Various techniques exist
to analyse these bearings for damage after they have been used, as discussed in the next
subsection.
3. BEARING ANALYSIS TECHNIQUES
Various techniques exist to analyse bearings after they have been damaged by ECD. Typical
evaluations include Solid particle analysis, Fourier Transform Infrared Spectroscopy (FTIR),
Light optical microscope (LOM), Scanning Electron Microscope (SEM) and Chemical
Analysis.
Solid particle analysis is an excellent technique to analyse debris found in the lubrication
of machinery. The morphological results of the wear debris of the components are key in
determining commonalities. A particle separating disk is used to separate the solid debris
particles from the lubrication for viewing under a microscope. Adequate magnication and
lighting are required for viewing and analysis of the lter patches (Raadnui, 2012).
FTIR is used to examine the degradation of lubrication (Aditya, Amarnath, & Kankar,
2014). The FTIR analyser is used to record the transmittance spectrum of new and used
lubrication. The Nitration Index and Oxidation Index are widely applied for quantifying
the oil degradation in used oil analysis. Nitration products have a characteristic absorbance
between the wavenumber range of 1650 cm
-1
and 1600 cm
-1
, the region immediately below
that of the oxidation products. Oil oxidation occurs in the carbonyl (C=O) region between
the wavenumber range of 1800 cm
-1
and 1670 cm
-1
(Robinson, n.d.).
LOM analysis is popular as it illuminates and magnies small samples that are dicult to
clearly observe with the naked eye. Purely digital microscopes are now available that directly
display images on a computer screen (Gianfrancesco, 2017). One of the disadvantages of
LOM is the relatively large resolution limit. The resolution limit is controlled by diraction,
which in turn is controlled by the numerical aperture of the optical system and the
wavelength of the light used. Another disadvantage of LOM is the poor contrast produced
when light is reected o surfaces with a high degree of reectivity (Bergström, 2015).
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The SEM overcomes these challenges. The magnication power of the electron microscope
is continually improving, with modern eld emission scanning electron microscopy (FESEM)
providing magnications of up to 550 000 times and resolutions down to 0.5 nm (Ingham,
2013). SEM instrumentation is equipped with energy dispersive spectroscopy (EDS)
system that allows for the study of the topography, morphology, chemical composition and
crystallographic Information (Scimeca et al., 2018). The techniques as shown in Table 1 will
be applied to the used samples bearings to analyse ECD damage.
Table 1. Bearing analysis techniques.
No
Analysis
technique
Equipment model number Purpose of analysis technique
1 LOM analysis Celestron 44302 LOM will provide initial images of any damage
2
SEM/EDS
analysis
Jeol JSM 6610
SEM will verify the images and provide chemical
composition
4. THE INNOVATIVE JIG
This section presents the innovative jig that may be used to expose mechanical bearings to
ECD and is shown in Figure 4. The labels are presented in Table 2 with a short description
and purpose.
Figure 4. The innovative jig.
The innovative jig comprises readily available components in South Africa. Some of the
components required minimal alterations to t into the jig, without compromising its
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functionality. The separation of the carbon brush holder from the voltage regulator of
an automotive alternator was challenging due to the poor stiness of the material and to
perform workpiece clamping of the carbon brush holder. A variety of prying tools were
successfully used to separate the carbon brush holder from the voltage regulator.
Another challenge encountered was in reducing the shank circumference of the ignition
distributor to t through the wide inner ring of the rolling bearing unit for rigid mounting
purposes. The internal cavity of the ignition distributor seems to have been pre-cast
asymmetrical, causing the tooling to rupture through the thinned wall sections along the
shank. The shank circumference at the drive gear end of the ignition distributor has an
internal plain bearing which oered enough clamping support for rigid mounting purposes
that overcame this challenge. The overall cost of the project was approximately USD 336.
Table 2. List of components of the innovative jig and their purpose.
No Description Purpose
1 Ignition control module
Electronic switch to control timing between discharges of the ignition
coil
2 Bench Lathe
220V electrical motor providing rotational motion to the lathe V-belt
and pulley
3 Lathe V-belt and pulley Driving pulley, providing 850 revolutions per minute (rpm)
4
12 Volt electronic ignition
coil
The ignition coil steps up the 12 V input to a 30 000 V output
5
Distributor timing belt and
pulley
Providing rotational motion to the Hall effect sensor of the ignition
distributor
6 Clamping screw
Angular alignment of timing belt and timing pulley of the ignition
distributor
7 Ignition distributor
Hall effect sensor provides an input signal to the ignition control
module
8
Crocodile clip, cathode
terminal
Battery cathode terminal connected to the carbon brush holder
9 Carbon brush
A spring-loaded carbon brush contact interface between the cathode
terminal and the shaft mounted radial ball bearing sample
10 Ignition spark plug lead
Connecting lead to transfer high voltage pulses between the ignition
coil and radial ball bearing sample; this becomes the anode terminal
11 Circuit breaker
A circuit breaker is used to switch the power supplied from the battery
to the ignition coil wiring harness on or off
12 12 V 12 Ah battery Direct current power supply to the electrical components
13 Thread cutting tap
A thread cutting tap is screwed in to the grease nipple hole to create a
spark gap of two-millimetres between the outer raceway of the radial
ball bearing sample and the anode terminal
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No Description Purpose
14 Insulated G-clamp
Rotational restraint and grounding isolator for the radial ball bearing
sample
15 Radial ball bearing sample
Radial ball bearing sample xed to the shaft with two hollow set
screws as shown in Figure 3
16 Turnbuckle hook to eye
A screw mechanism used to adjust the contact depth of the spring-
loaded carbon brush to the linear shaft
17 Jig Timing belt and pulley Parallel alignment of timing belt and driving timing pulley
18 Jig mounting screw
The jig attaches to the cross slide of the lathe. The V-belt is tensioned
between the lathe end driving pulley and jig end driven pulley
19 Shaft bearing supports Typical bearing support ends for the shaft to allow rotational motion
20 Jig V-belt and pulley Driven pulley output shaft speed, 850 revolutions per minute (rpm)
21 Base plate of jig Mounting interface for selected components
5. METHODOLOGY
This section presents the method to safely induce ECD across a radial ball bearing for
rolling bearing units, in order to clarify its associated eects on the bearing that need to be
understood before any mitigating techniques can be proposed. An experimental design is
used which may be considered as a detailed investigational plan to obtain the maximum
amount of information specic to the objectives (McIntosh & Pontius, 2017). The detailed
investigational plan in this study starts with:
• Assembly procedure of the innovative jig as shown in Figure 4 and Table 2;
• The location and weather conditions of the research site as shown in Table 3;
• Test parameters for the radial ball bearing for rolling bearing unit samples as shown
in Table 4;
• To cut material samples from each of the tested radial ball bearing for rolling bearing
units for LOM and SEM analyses.
Table 3. The location and weather conditions of the research site.
Latitude Longitude Elevation Season Temperature Wind Humidity
25°47’34.74”S 28°19’5.77”E 1418m Spring 22 °C 13 km/h 53%
Table 4. Test parameters for the radial ball bearing for rolling bearing unit samples.
Sample Time Shaft Speed ECD
1 0 minutes 0 rpm No
2 15 minutes 850 rpm No
3 15 minutes 850 rpm Yes
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An ignition coil wiring harness of a vehicle was used to induce ECD across a radial ball
bearing. This product was selected because it is widely used in the automotive industry, and
readily available over the counter. The test parameters for the radial ball bearing samples
are shown in Table 4. The experiment was done on 31 October 2019 at 19h00. Sample 1
was kept unused as new with the purpose of being a control sample with which to compare
and evaluate the other two samples. Sample 2 and 3 were both exposed to a shaft rotational
speed of 850 rpm for fteen-minutes.
Only sample 3 was exposed to ECD. Sample 2 and 3 were rigidly mounted with two hollow
set screws onto a linear shaft section of the innovative jig, as shown in Figure 4. An insulated
G-clamp was mounted on each sample as a rotational restraint and grounding isolator. For
sample 3, the cathode terminal was connected to the carbon brush holder. A thread cutting
tap was screwed into the grease nipple hole of the bearing unit of sample 3 to create a spark
gap of two millimetres between the outer raceway anode terminal. The bench lathe was
switched on and brought up to a speed of 850 rpm. The power supply circuit breaker was
activated, and the fteen-minute countdown started. Samples were cut from each of the
tested radial ball bearing for rolling bearing units. The samples 1, 2 and 3 were mounted
for LOM and SEM to analyse for damage as shown in Table 1.
6. TEST RESULTS AND DISCUSSION
The section presents the results that were obtained by using the methodology as presented in
the previous section. The results are presented as samples 1, 2 and 3. The SEM photographs
of ball bearings retrieved from samples 1, 2 and 3 are as shown in Figure 5, Figure 6 and
Figure 7 respectively. The Jeol JSM 6610 SEM had a magnication factor of 60 and a size
scale of 200 µm.
Figure 5. SEM photograph of a ball bearing: Sample 1 – Unused bearing.
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Figure 6. SEM photograph of a ball bearing: Sample 2 – Used with no ECD.
Figure 7. SEM photograph of a ball bearing: Sample 3 – Used with ECD.
Sample 1 (see Figure 5) shows the surface of an unused ball bearing which served as the
control sample. Bearing manufacturers polishes rolling elements to an average surface
roughness of 0.05 µm (Jacobs, 2014). The irregular scratch marks on the surface could be
due the unavoidable ball to ball collisions in the polishing process (Pattabhiraman et al.,
2010). Sample 2 (see Figure 6) shows a polished surface of a ball bearing. During run in, a
mild degree of polishing occurred between contacting asperities and were worn down that
produced ne abrasive particles. The polishing results in a satisfactory contact between
surfaces which is considered acceptable wear and tear (“Polishing – Bearing failure”, n.d.).
Sample 3 (see Figure 7) shows a dull surface appearance that is characterized by small
craters of a few microns in diameter. Micro-cratering is a result of electric current passage
in the bearing (SKF Group, 2006). The LOM photographs of the outer raceway of samples
1, 2 and 3 are as shown in Figure 8, Figure 9 and Figure 10 respectively. The Celestron
44302 LOM had a magnication factor of 3.
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Figure 8. LOM photograph of the outer race: Sample 1 – Unused bearing.
Figure 9. LOM photograph of the outer race: Sample 2 – Used with no ECD.
Figure 10. LOM photograph of the outer race: Sample 3 – Used with ECD.
Sample 1 (see Figure 8) shows the unused surface of an outer raceway which served as
the control sample. The surface showed no observable damage. Sample 2 (see Figure 9)
showed scratch marks on the raceway surface, parallel to the rolling direction. A higher
magnication of the localized areas of samples 1 and 2 were sought with SEM (see Figure
11 and Figure 12). The Jeol JSM 6610 SEM had a magnication factor of 130 and a size
scale of 100 µm. Similar to the results obtained from the ball bearing samples 1 and 2
(see Figure 5 and Figure 6), the raceway samples 1 and 2 had a mild degree of polishing
that occurred between contacting asperities that worn down that produced ne abrasive
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particles. For abrasive wear to occur, the ne particles, or one of the contacting surfaces,
needs to be substantially harder than the abraded surface. The abrasive wear process leads
to a characteristic surface topography of long grooves running in the sliding direction
(Jiménez & Bermúdez, 2011). Sample 3 (see Figure 10) showed localised damage on the
outer raceway surface (circle shown in Figure 10). The damage was observed to be adjacent
to the anode terminal (see Figure 4 and Table 2). A higher magnication of the localized
area of samples 3 was sought with SEM (see Figure 13).
Figure 11. SEM photograph of the outer race: Sample 1 – Unused bearing.
Figure 12. SEM photograph of the outer race: Sample 2 – Used with no ECD.
The SEM photograph of the outer raceway of sample 3 is as shown in Figure 13. The Jeol
JSM 6610 SEM had a magnication factor of 11 and a size scale of 1 mm. Sample 3 (see
Figure 13) shows micro arching marks on the raceway surface leading towards the groove of
the synthetic rubber seal and steel plate slinger (see Figure 3). The arcing marks are typical
proof of electric current passage in the bearing (Liu, 2014). A higher magnication of the
localized area of samples 3 (circle shown in Figure 13) was sought with SEM. The Jeol JSM
6610 SEM had a magnication factor of 35 and a size scale of 500 µm was used as shown
in Figure 14.
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Figure 13. SEM photograph of the outer race: Sample 3 – Used with ECD.
Figure 14. SEM photograph of the outer race: sample 3 – Used with ECD.
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Sample 3 (see Figure 14) shows many surface pits which were produced by electrical arching
(Raadnui, 2012). A few deep scratches and indentations are observed on the raceway
surface. This is due to the abrasive wear particles embedded in the raceway surface and
ploughed away (Aditya, Amarnath, & Kankar, 2014). Any rolling bearing has some degree
of sliding due to the dierence in the internal geometry and loading conditions of the
bearing (Morales-Espejel, 2019).
7. CONCLUSION
The purpose of this paper was to present an innovative jig that may be used to expose
mechanical bearings to ECD, in order to clarify its associated eects on the bearing that
need to be understood before any mitigating techniques can be proposed. The ball bearing
exposed to ECD showed a dull surface appearance that is characterized by small craters
of a few microns in diameter. Micro-cratering is a result of electric current passage in the
bearing. Micro-arching marks were observed on the outer raceway surface. The arcing
marks are typical proof of electric current passage in the bearing. A few deep scratches
and indentations are observed on the raceway surface. This is due to the abrasive wear
particles embedded in the raceway surface and ploughed away due to the dierence in the
internal geometry and loading conditions of the bearing. The aspect of electrical pitting
wear and debris found in the lubricating oil deserve further research from a tribological
point of view. Some practical solutions to mitigate bearing currents has worked eectively
for sinusoidal alternating currents. Some of the solutions are not as eective against the fast
switching Pulse Width Modulation (PWM) inverter technology that causes high frequency
non-sinusoidal bearing current. The foremost limitation of this article is that neither the
voltage, nor the current values of ECD are known. A recommendation is made to conduct
qualitative research with the innovative jig to test the impact of ECD on bearings from
other suppliers. The obtained primary data will warrant similarities or dierences present
in used ball bearings exposed to ECD.
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