24th International Colloquium Tribology - January 2024 97 The Effects of the Lubricant Properties and Surface Finish Characteristics on the Tribology of High-Speed Gears for EV Transmissions Boris Zhmud 1* , Morteza Najjari 2 , and Boris Brodmann 3 1 Tribonex AB, Uppsala, Sweden 2 Xtrapid Innovations, Detroit, USA 3 Optosurf, Ettlingen, Germany * Corresponding author: boris.zhmud@tribonex.com 1. Introduction As more and more electric cars are coming to the market, some new unique tribological challenges for electric vehicle transmissions become apparent. Proper selection of fluids for EV transmissions is critical yet complicated by a wide diversity of EV hardware. In general, EV transmission fluids call for a different spectrum of properties compared to conventional ATFs. High-speed gear drives also pose higher demands on gear accuracy and surface finish quality. At the moment, automatic transmission fluids (ATFs) are often used in EV reduction transmissions. Common ATFs such as MERCON ® LV, DEXRON ® VI, Toyota T4, Honda DW-1, etc have KV100 in the range 6 to 8 cSt. Ultralow viscosity (ULV) ATFs go down to KV100 3.5 to 4.5 cSt and are formulated using synthetic base oils, such as poly alpha olefins (PAO), esters and oil-soluble polyalkylene glycols (OSP). The market demand for ULV ATFs has been very low so far. At the same time, viscosity wise, ULV ATFs are a better fit for high-speed EV reduction transmissions. Even though being called “high-speed,” gear in an EV transmission operate at variable speeds from zero to the maximum speed of the motor, with a high torque instantly available already at low speeds. This complicates the lubricant selection as the lubricant should be able to provide both adequate gear protection against scuffing and wear at low speeds and good efficiency and heat removal at high speeds. Wear on the teeth can be a limiting factor at low speeds. Wear particles may affect other transmission components, such as bearings and actuators. To understand the wear behavior, a whole system approach is essential. In general, EV transmission fluids call for a different spectrum of properties compared to conventional ATF: some properties being universally important for both: efficiency, durability, seal compatibility, wide operating range, environment, health and safety profile, with some other properties such as oxidation stability, copper corrosion and electrical conductivity gaining increased significance. When it comes to improving gear tribology specifically for EVs, everything basically boils down to proper gear design and geometric optimization, which includes selecting right materials and methods for gear manufacture and post-processing. There is a delicate balance between gear accuracy and surface finish quality. In practice, we can never get perfect gears - good enough is the best. Gear accuracy is regulated by ISO 1328 and a number of national standards. The adequate accuracy for gears used in electric vehicles is around ISO 1328 Grade 6, but high-speed gears rated for speeds over 20,000-rpm have higher quality requirements. Assuming that gears have been machined with desired accuracy - which is normally accomplished by conventional grinding - additional surface finishing techniques can be applied in order to further optimize the surface roughness and waviness profiles. These include a variety of abrasive and non-abrasive processes, such as shaving, lapping, honing, abrasive flow machining, turbo-abrasive machining, stream finishing, accelerated surface finishing, electropolishing, burnishing, etc. Recently developed mechanochemical surface finishing methods such as Triboconditioning ® CG can be used as the final finishing operation bringing about a triad of effects: (i) surface roughness profile optimization, (ii) compressive stress buildup, and (iii) tribofilm priming, which greatly improves the tribological and NVH behavior of gears [1]. In the present communication, we demonstrate the effects of different lubricants and surface finishing technologies on the tribology of gears using tribological tests and advanced thermal elastohydrodynamic simulations. Important roles of lubricity additives and surface finish optimization are highlighted in conjunction with a move towards ultralow viscosity fluids. 2. Results and Discussion Transmission fluids used in this study were blended by using a mixture of API Group II 600N and PAO3.5. Additional experiments were carried out using sustainable while oils produced from waste plastic instead of PAO. A commercial ATF additive package was used with the same treat level for all viscosity grades, see Table 1. For scuffing resistance evaluation, an FZG back-to-back gear test rig with FZG type A gears was used according to the standard ASTM D5182 procedure (A/ 8.3/ 90). Low-speed wear measurements were carried out according to ASTM D4998. For efficiency studies, an FZG back-to-back gear test rig with FZG type C gears with tip relief was used. Some gear pairs were additionally superfinished using the Triboconditioning ® CG method. 98 24th International Colloquium Tribology - January 2024 The Effects of the Lubricant Properties and Surface Finish Characteristics on the Tribology of High-Speed Gears for EV Transmissions Table 1: The properties of gear oils in study Viscosity grade KV40, cSt KV100, cSt Density, g-cm-3 ISO VG 100 98.1 11.2 0.87 ISO VG 46 46.2 7.0 0.85 ISO VG 22 23.4 5.0 0.83 ISO VG 15 14.5 3.5 0.82 The surface of gears was characterized using using a Form Talysurf Intra stylus-type surface metrology system and an Optosurf OS500 scattered light system. In the FZG A/ 8.3/ 90 scuffing test, all four oils managed to reach load stage 12 without scuffing as was expected based on the add-pack specifications. The low-speed wear data show relatively large variations, but in general, mechanochemically finished gears demonstrate a significant reduction in wear, 3 to 5 times on the average, see Figure 1. Figure 1: The effects of surface finishing technique and lubricant viscosity on pinion wear The total losses for the four oil viscosity grades in study are shown in Figure 2. The lowest viscosity grade is associated with the lowest loss. Figure 2: Total friction loss measured for different oil viscosity grades (LS 9, 60- o C) It should be noted that, as high-speed gear applications are concerned, PLVs in excess of 100-m/ s can be encountered in extreme cases. Such conditions cannot be emulated by the current FZG efficiency test, highlighting the need for a dedicated high-speed test rig. One would expect that a reduction in surface roughness should automatically lead to a reduction in friction torque. However, in practice, this is not always the case as there is an intimate interplay between the gear microgeometry and the surface roughness. With mass-finishing techniques, it is virtually impossible to modify surface roughness without incurring some subtle modifications of the microgeometry. The surface roughness profiles of conventionally finished gears and mechanochemically finished gears are compared in Figure 3. Figure 3: Typical surface roughness profiles of conventionally ground (GR) and mechanochemically finished (MC) gears The TEHD simulations indicate a significant reduction in friction shear stress and the asperity contact ratio, which is beneficial for gear tribology. The type of the base oil and the addpack have a significant effect on the result. For instance, polyalkylene glycols (PAG) have lower asperity-asperity friction than conventional hydrocarbon-based products. Figure 4: Calculated friction torque for conventionally ground and mechanochemically finished low-loss helical gears with VG-15 and VG-46 lubricants References [1] B. Zhmud, M. Najjari, B. Brodmann, L. Everlid, Proc. 64 th German Tribology Conference, September 25-27, 2023, Göttingen, Germany.