The need for speed
SKF is accelerating the development of electric vehicles. With its new High Speed Ball Bearing 1.8 for e-Drive, the company is taking the lead in the race for higher rotational speed – a key component for improving power density in EV powertrains.
The number of electric and hybrid vehicles on the roads is rising rapidly. By 2030 more than half of all passenger vehicles sold will be electrified, market analyst Bloomberg predicted in its 2020 outlook. Technical development will attract new buyers and lead to more competitive pricing. In addition, according to the outlook, battery prices will fall and charging times and infrastructure will see major improvements. Just as important in terms of cost reduction and sustainable development of the EV market, power density will improve.
In collaboration with SKF, Austrian automotive research and consulting firm AVL has invested in high-speed technology to develop a cost-effective drive solution for EV applications. The new high-speed e-axle is fitted with SKF’s newly launched HSBB 1.8 – High Speed Ball Bearing 1.8 for eDrive – and delivers 30,000 rpm on the input shaft.
“That’s quite an achievement,” says Mathias Deiml, responsible for system development for electro mobility at AVL in Regensburg, Germany. “Especially since today’s electric cars run at about 12,000 to 14,000 rpm. The next generation will run at 20,000 rpm. Machines running at 30,000 rpm – some said that would be impossible to achieve, at least with a greased steel ball bearing.”
Increased efficiency and greater mileage
Thanks to the higher speeds enabled by SKF HSBB 1.8 for automotive electric motors, AVL can offer a more compact and cost-effective motor with increased efficiency and greater mileage – something Deiml hopes will increase the number of electric cars on the roads.
“Electro mobility is at the edge of getting greater volumes,” he says. “Governmental incentives are put in place to promote the use of electric vehicles, and car manufacturers are pressured to bring out models that are attractive range-wise, charging-wise and also cost-wise, but for most people electric cars are still too expensive.”
Batteries are still very costly. The same applies to the electric drivetrain. “As we are active on the powertrain side, we made it our mission to look at the price of the electric drive system,” says Deiml. “This is how we came up with the idea of making the motor more compact while still getting the same power output, and we do that by increasing the rotational speed.”
The power generated by the electric motor is calculated by torque (Nm) x speed (rpm) = power (kW). If the speed is increased, the torque can be lowered while keeping the same power output. “By lowering the torque of an electric motor, you can decrease the motor volume, which increases the power density of the entire electric car,” explains Deiml. “By reducing the mass of the motor from 70 to 35 kilograms [150 to 75 pounds], you can reduce the use of critical metals such as those in the motor’s high-powered magnets, which also makes the production more sustainable.”
Building an efficient and reliable drivetrain
Electric vehicle powertrains are mechanically simpler than their combustion-engine counterparts. But building an efficient and reliable electric drivetrain is extremely challenging. The race for a higher rotational speed ratio places 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 with 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, from 10 to 15 years or 185,000 miles today to 300,000 miles or more in the near future.
SKF has proved that the bearing can withstand the load and speed cycle that we have defined.
Mathias Deiml, responsible for system development for electro mobility, AVL
About two years ago, when AVL set out to develop the new high-speed e-axle, the first mission was to find the right bearing, designed to withstand the forces brought on by the higher rotational speed. To achieve this AVL needed a partner willing to develop the next-generation bearing. From the beginning of the EV revolution SKF had worked closely with the automotive industry to meet the new challenges, so the company was an obvious match.
“Working with SKF has been very positive,” says Deiml. “The frequent exchange of information, test results and analyses has been really valuable, and SKF has proved that the bearing can withstand the load and speed cycle that we have defined.”
Close cooperation with SKF
Anthony Simonin, technical portfolio manager, works at SKF’s EV & HEV Application Competence Center in Saint-Cyr-sur-Loire, France. In close cooperation with SKF’s product development and engineering teams in China, France and Italy, this team of international experts has been working on the stimulating – and sometimes also tedious – task of developing the new SKF HSBB 1.8 for the EV market.
“The first limitation to tackle was the cage,” says Simonin. “We worked a lot on improving the design of the cage, to increase the stiffness to sustain the increased acceleration and the deceleration.”
It took several months of simulation testing of different concepts before there was a short list of prototypes ready for the test benches at AVL in Regensburg.
“The second major task was finding the right grease,” Simonin explains. “We had grease for high speeds and we had grease for high temperatures, but we lacked the combination. With the support from a grease supplier we finally found a solution that met all the requirements.”
Improved high-speed performance
While developing the new EV bearing, speed wasn’t the only challenge. High and fast-switching voltages of the electric motor inverter bring an increased risk of parasitic 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. To offer improved high-speed performance and best-in-class electrical insulation characteristics, the SKF hybrid SKF HSBB 1.8 uses ceramic rolling elements and steel rings.
“Our newly launched HSBB 1.8 will allow all our customers to achieve very high rotation speeds, up to a 1.8M NDm* speed factor, on their electric powertrain,” says Simonin. “It will enable higher efficiency and improved vehicle mileage at a lower cost than by increasing the battery pack size.”
Automotive industry shows interest
Interest from the automotive industry has been huge, Simonin says. Only days after the release he had multiple requests for a quotation. “SKF has developed a reliable solution to manage high rotational speed in EV applications,” he explains. “Of course, I am pleased and keen to see the bearing brought into play in the automotive industry.”
The AVL high-performance e-axle with double motors has left the test benches and proven its capabilities in a demo vehicle, the Tesla Model S90. As an electric vehicle is much quieter than a conventional car, even small amounts of noise or vibration from the powertrain become noticeable. According to Deiml, the new e-drive passed this test as well.
“We are ready for mass production,” says Deiml. “The next step is to find an EV manufacturer who believes in this new technology, which is so far ahead that’s it’s close to unbelievable. If you drive the car and feel the acceleration and then see how small the motor actually is, it is truly amazing.”
*N is the speed limit of the bearing in rpm; Dm is the median diameter of the bearing in millimeters.