Conductive lubricants protect electric motors

June 12, 2014 // By Christoph Hammerschmidt
In future electric and hybrid electric vehicles with higher supply voltages, electric discharge in the motor bearings becomes a serious issue: The discharges can cause significant wear, which reduces the life time of the motors. In a joint research project led by Robert Bosch GmbH, a group of companies has developed a conductive lubricant that helps to avoid the discharge-induced wear.

Higher voltages are associated stronger alternating electric fields, explains Bosch researcher Gerd Dornhoefer who oversaw the SchmiRmaL project. SchmiRmal stands for "Switchable intelligent tribo systems with minimum friction loss and maximum lifetime". Strong electromagnetic fields however can trigger electric discharge in the ball bearings of motors and generators. Sparks cause tiny regions of the metal to melt, creating microcraters in the bearings raceway. This effect is known as electro pitting. The consequence: The bearings first start to generate more noise, and over time they tend to fail prematurely.

Since today's lubricants are isolators, static and magnetic fields can cause voltages to build up between shaft and casing of electric motors. Lubricants typically used in ball bearings separate balls and raceway from each other - an effect that increases with rotational speed. This mechanical effect is desirable since it avoids wear. However the bearings become charged - and if the voltage gets high enough it can create a spark, with the results described above.

The research group identified additives to the lubricants that prevent discharges in that they turn the lubricant into an electric conductor. For this purpose the researchers added carbon and ionic liquids - molecules that carry an electric charge to the lubricant. According to Dornhoefer, the researchers succeeded in reducing the electric resistance by a factor of ten millions - good enough to prevent electric discharge.

The next step in the project: The researchers now have to make sure that the additives do not affect the lubricant's properties at extremely high or low temperatures, and that it does not reduce the anti-corrosive effect of that lubricant. The research group now hopes that their achievement helps to speed the volume production of electric vehicles.

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