July/August 2019

n TRANSPORTATION 44 July/August 2019 www.drivesncontrols.com Custom and hybrid bearings help to solve EV issues E lectric and hybrid vehicles (EVs and HEVs) may look superficially like conventional cars powered by internal combustion engines but, beneath the chassis, the gulf is huge, with the most fundamental shift being in the drivetrain. Powerful electric motors provide the power, and the way they work and connect to the rest of the car, requires critical components such as bearings to be redesigned or adapted. For instance, EV motors can run at up to 30,000 rpm – about three times faster than a typical industrial motor. This puts an enormous strain on the internal bearings. This has been solved by redesigning the cages of conventional steel ball bearings and by using special lubricating greases. Redesigning the bearings and their polymer cages can help them to withstand the higher speeds, acceleration and temperatures encountered in EV applications. In addition, the higher speeds need more efficient lubrication using special greases that keep their viscosity and continue to operate effectively at the higher speeds and temperatures. While electric motors, in combination with inverters, ensure high operating efficiency, their performance can be affected if they rely on conventional steel bearings. The high- frequency switching of the inverter can cause leakage currents which can be conducted through the bearings, resulting in problems such as surface pitting – and, potentially, in catastrophic failures. In a sense, the problem is caused by running the motor with utmost efficiency: increasing the motor speed boosts power and efficiency, without having to increase current or torque. However, increasing the speed also increases the voltage frequency, leading to the problem of electrical discharges. One answer is to use a ball bearing that is naturally insulating and will not conduct currents. Switching from conventional steel balls to ceramics – creating“hybrid”bearings – can solve the problem, because the ceramic balls are natural insulators. However, due mainly to the high cost of the hybrid bearings, few EV designers are using them so far. At SKF, we estimate that only around 5% of bearings in EV applications are hybrids, and these are typically specified only after the earlier failure of steel bearings. The main reason that designers are not specifying hybrid bearings from the outset is that they cannot justify the extra cost. They prefer to run conventional bearings until they fail. However, some larger EV manufacturers have been using sophisticated simulation software to model the exact conditions experienced by bearings in EVs. They can see and quantify the amount of leakage current, and its likely effect on the bearings. If it makes financial sense, they will replace steel bearings with hybrid ones. The use of hybrid bearings is therefore becoming more common, and is likely to increase further. Making such replacements is usually easy because the hybrid bearings have the same dimensions as conventional steel bearings. In addition to their insulating properties, ceramic balls have other advantages: n they are 40% less dense than steel equivalents, which means they can run at lower temperatures; n their life expectancies are up to ten times longer; and n they need less lubrication. As well as resisting the effects of arcing, hybrid bearings can improve efficiency, due to their lighter weight and greater hardness, helping EVs to travel further on a charge. But, there is no escaping the fact that hybrid bearings are expensive. SKF is therefore looking at ways of solving the problem of electrical damage to bearings, but without having to resort to ceramic balls. One approach, still under development, is to create another path for the current, so that it does not pass through the bearing itself. Despite their advantages, ceramic balls cannot solve the problem of current leakage completely. While they resist the effects of surface damage, currents can still work their way to a vehicle’s gearbox and cause damage there – by destroying the spline connection, for example. We expect our new development, which is due to be launched by the end of this year, to solve this problem, while also costing less than hybrid bearings. We are not married to any particular technology. Right now, our hybrid bearings can help overcome problems of current leakage in EVs; soon, we will have this new technology that promises to do the same, but at a lower price. In the future, EV manufacturers may be able to overcome the problem of current leakage by“designing out”the problem at its source. Electric vehicles are the way ahead for transport. Although they currently account for a relatively small part of the market, making them as efficient and reliable as possible will help them to take a larger share of the market. n The high speed of the electric motors used in electric and hybrid vehicle powertrains requires the use of customised bearings. Ceramic ball bearings can help to tackle some of the problems, reports Anthony Simonin, manager of SKF’s EV and HEV Competence Centre, but they are expensive and alternative designs are being developed. Hybrid bearings containing ceramic balls can tackle the problem of damage caused by leakage currents – but at a price that not all electric vehicle manufacturers are prepared to pay