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Eye on the ball
EV drivetrains are placing new demands on components such as bearings,
however advanced bearing technology and materials can overcome this issue
Bearings, although located
deep within the vehicle, are
one of the most critical components
in contributing to drivetrain
e iciencies and delivering reliability
for automotive manufacturers.
Breakthroughs in technologies
such as batteries, power
management electronics and
electric motors have paved the way
for the dramatic emergence of
electric mobility, but every aspect
of the EV drivetrain is being
advanced in the push for e iciency
and reliability.
The demands on bearings and
the push for e iciency throughout
the drivetrain is prompting OEMs to
explore new low-friction approaches
rather than the conventional tapered
roller bearings that are found in
gearboxes and di erentials.
Another challenge is the high
speed of the electric motors in EVs
up to 30,000 rpm speeds, far higher
than those found in most industrial
applications, are needed to ensure
the drivetrain works e iciently. The
speed acceleration and
temperatures generated by these
machines place enormous strain
on bearings and their components
such as cages and rolling elements.
Bearings running at such high
speeds also need e ective
lubrication and cooling. Without this,
there is a high risk of rapid
degradation of the bearing
components. If not, both races
and rollers may overheat, causing
premature failure and the need for
early replacement.
In addition to high rotational
speeds, electric motors can also
present further challenges for
bearings through electrical
discharges. High-frequency voltage
switching of the motor inverter can
result in current leakage. This
current can be conducted through
the bearings causing arcs in the
rolling contacts and may result in
surface fatigue and erosion.
Manufacturers are also
developing special lubricants with
lower viscosity and special additives
in order to reduce power losses from
friction and support operations at far
higher speeds. However, lower
viscosity lubrication can also have
an impact on the wear and fatigue
characteristics of bearings.
Accelerated wear can result in
catastrophic failure, having an
obvious impact on reliability.
In response to these new
demands, manufacturers such as
SKF are investing in new
technological developments and
adapting or redesigning
conventional bearings. By
redesigning elements such as the
polymer cage, rolling components
and using special lubricating grease
for example, customized and hybrid
bearings are already solving some of
the fundamental issues that can
a ect reliability in electric vehicles.
In terms of electrical discharge,
ceramic ball bearings are currently
the most attractive solution given
that ceramics are typically
electrically insulating materials.
164 // January 2020 // www.electrichybridvehicletechnology.com
While using ceramic rolling
elements to create a so-called
hybrid bearing resolves this issue, it
also has the benefit of having
roughly half the density of their steel
equivalents, ceramics are lighter and
therefore tend to run at lower
temperatures. In addition, ceramic
bearings are much harder than steel
so last up to 10 times longer,
typically requiring less lubrication.
Although currently only around
5% of bearings in EV applications
are hybrids featuring ceramic
elements, this is beginning to
change. The value that hybrid units
o er in reliability and extended
service life is becoming increasingly
recognized and acknowledged.
Another solution that is being
explored is carbonitriding bearing
raceways for applications where
bearings are exposed to
contamination or poor lubricants.
Carbonitriding increases the surface
hardness and consequently, as with
ceramics, increases the in-service
lifetime. As the global EV market
continues to expand, many more
advanced ways of dealing with all
the specific conditions required for
EVs are also under development.
These include special greases that
maintain a stable viscosity at higher
speeds and temperatures.
Given the new constraints linked
to EV technology development, the
bearing contribution to the overall
system behaviors should not be
underestimated. Manufacturers
such as SKF are creating bearings
that are fit for the future with
improvements in performance,
e iciency and reliability.
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To learn more about SKF,
visit: www.magupdate.co.uk/PEHV
Bearings designed to meet the
demands of future vehicles help
performance and reliability
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