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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).