Customised and Hybrid Bearings Solve Fundamental Issues in Electric Vehicles by SKF

The high speed of the electric motors used in EV&HEV powertrains requires the use of customised bearings. Ceramic ball bearings are most capable of dealing with all the specific conditions, says Anthony Simonin, EV and HEV Competence Centre Manager at SKF.

Gothenburg, Sweden, 17 May 2019: On the surface, there appears little difference between an electric and hybrid vehicle (EV&HEV) and the conventional car, powered by an internal combustion engine, that it will increasingly replace. 

However, beneath the chassis, the gulf is huge, with the most fundamental shift being in the drivetrain. Here, powerful electric motors provide the power, and the way in which they work, and how they connect to the rest of the car, requires critical components such as bearings to be redesigned or adapted to work in this new environment.

For instance, the motor itself must run at very high speed, far faster than it would in a conventional industrial application, meaning that bearings within it must be able to cope without failing. In general, this has been solved by adapting conventional steel ball bearings, such as by redesigning the cage and using special lubricating grease.

A typical EV uses up to four motors, if each is connected to a separate wheel. These run at up to 30,000rpm, which is around three times the speed of a typical industrial motor. This puts an enormous strain on the internal bearings, so these will need to be replaced or adapted.

One way is to redesign the bearing and the polymer cage, adapting the fundamental design. This ensures that the overall bearing configuration can withstand the higher speed, acceleration and temperature that we can see in these applications.

In addition, the higher speeds require more efficient lubrication. Here, special greases are required that can keep their viscosity and so continue to be effective lubricants at these higher speeds and temperatures.

While the electric motor, in combination with an inverter, ensures high efficiency, there is one feature that can affect conventional steel bearings. The high-frequency voltage switching of the inverter, which in turn causes current leakage. The current can end up being conducted through the bearings. This can cause problems such as surface pitting, and subsequent catastrophic failure.

In a sense, the problem is caused by running the motor in its most efficient way: increasing motor speed boosts power and efficiency, without having to increase current or torque. However, increasing the speed also increases voltage frequency, which leads to electrical discharge.

The sensible answer is to use a ball bearing that is naturally insulating and will not conduct a current.