Test methodology for assessing the performance of lubricants for gears in e-mobility applications
Technical University Munich
Prof. Dr.-Ing. K. Stahl, Dr.-Ing. T. Tobie, M. Geitner, M.Sc.
The use of electric motors instead of combustion engines in the drive train of vehicles offers great potential for environmentally friendly mobility. In addition, by operating at high speeds and saving mass at the same time, the efficiency and power density of electric drives can be increased. For this reason, current developments in the automotive industry show a strong trend towards the use of electrified powertrains instead of the previously used combustion engines.
Different drive concepts such as central motors or wheel-mounted motors in purely electrically powered vehicles continue to necessitate the use of gearboxes in the drive train. The tribological load-bearing capacity of the gear teeth in the transmission is ensured by using a suitable transmission lubricant. Lubricants often contain performance additives whose effect is strongly dependent on the operating conditions, in particular the lubricant temperature. Due to the complex physical-chemical interactions between the lubricant components and the gear material, the performance of gear lubricants cannot yet be sufficiently estimated theoretically and must be verified and proven in lubricant tests.
In standard test methods, state-of-the-art lubricants are examined at speeds of n = 2,250 min-1 up to a maximum of n = 4,500 min-1, while speeds of up to 20,000 min-1 currently occur in e-mobility. In future developments, even significantly higher speeds are to be expected. Due to the greatly increased speed compared to the standard test methods, lubricants used in e-mobility will therefore be used under significantly different operating conditions than can currently be tested.
Initial investigations already show influences with regard to the micropitting and pitting load carrying capacity and, in particular, with regard to the scuffing load carrying capacity at higher speeds. These initial results underline the relevance of further investigations into the load-bearing behaviour of lubricants under e-mobility conditions compared to the previous standard oil test methods.
The research project will therefore examine whether the current test procedures for assessing the load-bearing capacity of lubricants with regard to the relevant damage mechanisms (fretting, micropitting and pitting) are meaningful for the operating conditions in e-mobility applications. If the standard test procedures are not representative, a modified test methodology for assessing the performance of lubricants for gears in e-mobility applications or at high circumferential speeds is to be developed.
Against this background, comprehensive investigations into the influence of operating conditions close to e-mobility on the load-bearing capacity of lubricants, particularly at different circumferential speeds, are planned as part of the research project. Based on this, recommendations for the testing and reliable assessment of e-mobility lubricants will be derived. The following results are expected after completion of the project:
- Test methodology for comparative evaluation and characterization of e-mobility lubricants
- Reduction of the risk of tooth flank damage in industrial applications (e-mobility) and thus reduction of costs due to repair failures and downtimes
- Basics for lubricant development in E-Mobility
- Findings for transmission development in e-mobility