Electric Discharges in Ball Bearings and Damages
Kenji Matsumoto, STLE Member
Honda R & D Co. Ltd., Utsunomiya, Japan
Akira Sasaki, STLE Fellow
Maintek Consultant, Yokohama, Japan
Naoki Yoshida
Kyushu University, Fukuoka, Japan
ABSTRACT
Recently it has been reported that bearing damages of gear boxes for wind power
generators are often caused by wear and failure due to electric discharges. Electric
discharges also occur in gear boxes for automobiles but do not end up in failure.
Therefore, electric discharge simulation tests were carried on the surfaces of
bearing balls that had been operated well for a long time. The test results showed
that the brown film on the surface of used bearing balls was non-crystalline carbon
film and that the film protected the surface metal structure from melting and
damaging when electric discharge was caused. The voltage from sliding was
estimated as roughly 0.05 kV, however, this suppression was observed at 1.5 kV,
which far exceeded this estimation. Depositing this non-crystalline carbon film or
similar diamond-like carbon (DLC) film on the sliding surface would protect the
surface from damage by electric discharge.
Keywords: Roller bearings, Gear box, Electric discharges SEM, TEM, DLC
INTRODUCTION
bearings has increased. A bearing maker
The studies on bearing failures due to
electric current is not new (1), (2). Prashad
made extensive studies on bearing failure
due to electric current (3) - (8). For energy
saving, use of electric motors which use
reports that the use of Variable Frequency
Drives (VFDs) in industry has increased
for reducing electricity costs since 1990’s
and that more premature bearing failures
have occurred due to electrical erosion (9).
Another maker reports that bearing
damages occur due to current passage in
the use of roller bearings in wheel sets and
traction motors in rail vehicles, direct
current and alternating current motors,
and generators of wind power (10).
Damages by electric discharges also occur
in sliding parts of automobile parts.
Although many studies have been
conducted on bearing failures, there are
little metallurgical studies on the damaged
surface of an inner race of a bearing used
for a gear box. There are black continuous
craters at the center of the image that
appear to be caused by electric discharge.
parts by spark discharges of electricity.
changed except the color of the surface
changed from the original glossy silver to
glossy brown.
We paid attention to this finding and
caused electric discharge on the surface of
these bearing balls. The damaged surface
by electric discharge was carefully
observed using transmission electron
microscopy (TEM) and SEM. We aim to
understand the possible suppression of
INVESTIGATION OF THE SURFACE OF
A BEARING INNER RACE
Figure 1 is a scanning electron microscopy
(SEM) image of crater damage apparently
caused by electric discharge on the
surfaces of inner race of a bearing of a gear
box.
The diameter of these spots is 2 to 10 m
(11).
These craters are not found in the surface
of bearing balls at the principal axis that
have been used for a long time.
The surface of the bearing balls have not
damages of the surfaces by electric
discharge based on these experiments．
EXPERIMENTAL METHODS
A device with an electric circuit shown in
Figure 2 is prepared for electric discharge
tests. Two voltages, namely 1.5 kV or 0.05
kV, are applied instantaneously between
the probe and the test balls to generate
electric discharge. The resulting damage is
observed.
Figure 1 Black spots were observed in the
sliding of used driving transmission parts.
(SEM image)
Detailed observations were made on the
Electric discharge experiments are carried
out after coating balls with gear oil thinly
and uniformly. A photo of the ball bearing
used in experiments is shown in Figure 4.
Figure 2 Circuit for electric discharge
tests.
Figure 3 Photo of the electric discharge
device.
Figure 3 is a picture of the electric
discharge device. Bearing balls that did
not show any damage in the inner and
outer races after driving for 95,000 km is
used as the test piece (12).
Figure 4 Photo of the examined ball
bearing (after driving for 95,000 km).
EXPERIMENTAL RESULTS
To understand what caused the brown
color of the ball bearing surface, the rolling
contact surface of a bearing ball was cut
out using a focused ion beam (FIB) and
TEM measurements were conducted
Figure 5 shows TEM images immediately
below the rolling contact surface before
and after use.
The bright-field image of a used ball
reveals an incrustation layer at the surface.
Electron diffraction (ED) patterns indicate
that this has an amorphous-like
composition with a thickness of 60 – 100
nm. This covering layer is considered to be
solidified oil film.
Many micro particles of around 100 nm in
size are found within the incrustation
layer according to dark-field image
observation. Therefore, detailed analysis
are carried out on these micro particles.
Figure 5. Cross sectional TEM image just
under the surface (bearing-ball)
Figure 6 and 7 shows detailed analysis of
ED patterns.
Figure .7 Detailed analysis of ED patterns
Detailed analysis of ED patterns shows
that the micro particles are BCC
structured Fe, magnetite (Fe3O4), and
wustite (FeO).
Electric discharge tests were carried out
on the used balls and new balls.
Figure 8 shows photos taken during
electric discharge tests.
Figure 6 The non-crystalline layer where
ED patterns are analyzed.
Figure 8 Pictures taken during electric
discharge tests.
Sparks could be observed with naked eyes
when the discharge voltage was 1.5 kV but
could not be observed when it was 0.05 kV.
Therefore, electric discharge is confirmed
by waveform measurement using a digital
oscilloscope. Figure 9 shows SEM images
of bearing ball surfaces after electric
discharge．
using FIB.
Figure 11. SEM images of sections cut out
using FIB.
Figure 9. SEM images of bearing ball
surface after electric discharge.
(Left: a new ball, right: the used ball)
The parts of samples indicated in red
boxes were cut out using FIB. Figure 12
shows a TEM image after electric
discharge of a new ball
One electric discharge was given to each
sample (bearing) ball and many craters
were formed. Figure 10 shows
enlargements of SEM images of the
surface after electric discharge
Figure 12 TEM image after electric
discharge of a new ball.
The surface damages were larger on new
Electric discharge instantly increased
temperature and melt a part of the ball
surface to form a rim. Furthermore, rough
texture as shown within the yellow frame
was formed near the crater.
Figure 13 shows a TEM image after
balls although the applied voltage was the
same. Therefore, the parts near craters
were cut out using FIB and TEM
observations were carried out to
investigate the craters in detail. Figure 11
shows SEM images of sections cut out
electric discharge of the used ball.
There is non-crystalline film remained on
the rolling contact surface that formed by
usage for a long time. Furthermore, the
metal structure retains crystal grains that
formed during heat treatment at time of
Figure 10 Enlargement of SEM images of
the surface after 1.5kV electric discharge.
(Left: a new ball, right: the used ball)
production. Therefore, there appears to be
no effect of damage from electric
discharge.
High-angle annular dark-field scanning
transmission electron microscopy
(HAADF-STEM) was hereby used to
observe in detail.
penetrate. The damage from electric
discharge is very small and it affects down
to 0.8m in depth.
Figure 13 TEM image after electric
discharge of a ball used for a long time.
Figure 14 shows STEM images after
electric discharge of the used ball. The
non-crystalline film appears in white in
the bright-field (BF) image because
electron beams are more likely to
penetrate. In contrast, this structure
appears in black in the HAADF image
because there are less electron beams
compared to the surrounding structure.
The original metal structure lies below
the non-crystalline film, which is seen as
white regions in the film in the BF image.
SUMMARY AND CONCLUSION
1. Brown film that forms on the surface of
bearing balls used for a long time is
non-crystalline carbon film, and
This shows cracks formed in the metal
structure by electric discharge, and here
some iron crystals with large atomic
number are lost and replaced by
accumulation of light elements. As
electron beams are more likely to
nanometer sized particles of oxides of
iron scatter in the film.
2. Melting and removal of the structure is
suppressed when simulated electric
discharge is applied through this film.
3. The voltage from sliding is estimated at
Figure 14 STEM image after electric
discharge of the used ball.
around 0.05kV, however, the
suppression is observed at a
significantly higher voltage of 1.5 kV.
(5)
POSTSCRIPT
(6)
This investigation suggests that there is
some possibility that non-crystalline
carbon film, such as DLC film, on the
surface of bearing balls may have some
possibility of suppressing damage by
electric discharge. We will continue to
study the effect of non-crystalline carbon
films including DLC films on the possible
suppression of damages by electric
discharges.
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