AROUND the world, the electrification of passenger transport is picking up momentum. As part of their commitments to the Paris agreement on climate change, a growing number of countries are set to phase out the production of new combustion-engine vehicles over the next two decades.
Electric vehicle powertrains may be mechanically simpler than their combustion-engine counterparts. But, according to Hugh Zhu, Manager Product Development DGBB at SKF, building an efficient, reliable and enjoyable electric drivetrain is extremely challenging.
“Motors used in automotive applications are designed to operate at very high speeds. For example, traction motors used in EVs rotate today at up to 20,000 RPM, and are expected to reach up to 25-30,000 RPM in future designs, as engineers strive to improve performance, whilst at the same time minimising the size and weight of powertrain components,” said Zhu.
This race for higher rotational speed ratio prompts challenging and demanding conditions on the bearings used in EV transmissions.
Although the number of bearings is about the same in an E-axle compared to a traditional one, the technical challenge is higher than before. To withstand high centrifugal forces, and minimize self-heating generated by rapid rotation, these bearings require specialized cage designs, internal geometry and lubricant characteristics.
Manufacturers also need them to operate reliably for the design-life of the vehicle, around ten years or 300,000 km today, and up to 500,000 km or even more in a near future.
Speed isn’t the only challenge, though, said Zhu. High and fast switching voltages of the electric motor inverter bring a greatly increased risk of stray currents in powertrain components. If they pass through conventional steel bearings, such currents can damage their surfaces, leading to higher friction, increased vibration and early failure.
That’s where ceramic bearings come in.
Zhu said many OEMs in Asia, Europe and North America alike have selected SKF bearings as part of their electric drivetrain set-up.
The company’s hybrid Deep Groove Ball Bearings (DGBB), for example, use ceramic rolling elements and steel rings. According to Zhu, bearings of this type offer improved high-speed performance and best-in-class electrical insulation characteristics, making them the premium choice for high performance EV powertrains.
In addition to its product development work, the SKF technical centre also supports car makers in application-specific engineering. One area of particular interest for this market is noise and vibration.
“An EV is much quieter than a conventional car, so even small amounts of noise from the powertrain become noticeable,” said Zhu. Our expert teams work with leading international automakers to identify sources of undesirable noise and develop ways to address them at both bearing and system level to ultimately improve the experience of the end-user.”
Carmakers are set to launch around 450 new battery and plug-in-hybrid vehicle models over the next two years. Zhu said SKF has the technical, manufacturing and supply chain capabilities needed to support the sector’s accelerating growth.