24th International Colloquium Tribology - January 2024 37 Lubricants Technology for Improving the Protection Performance of Reduction Gears in Transaxles for Electric Vehicles D. Takekawa 1* , H. Tatsumi 1 , K. Matsubara 1 , K. Narita 1 1 Lubricants Research Laboratory, Idemitsu Kosan Co., Ltd, Ichihara, Japan * Corresponding author: daisuke.takekawa.7430@idemitsu.com 1. Introduction To reduce carbon dioxide emissions, electrified vehicles, such as battery electric vehicles (BEVs), are expected to become increasingly popular in the automotive industry. Transaxles for electric vehicles (E AXEL) is used for drivetrains in electric vehicles. E AXEL is effective in reducing carbon dioxide emissions. Lubricants for E AXLE require a variety of performance. In particular, the important performance among them is the cooling of the motor and the protection of gears and bearings. In the past, it has been reported that it is important to reduce kinematic viscosity and increase heat transfer coefficient in order to improve motor cooling [1]. We have designed and built a new testing machine to evaluate the performance of lubricants in motor cooling [1]. Using this tester, we investigated the effect of the viscosity on the cooling of the motor. It was found that the lower the viscosity sample, the greater the temperature drop on the copper plate surface and the better the cooling. In addition, reducing the kinematic viscosity of the lubricating oil also has the effect of reducing stirring loss and improving efficiency. However, reducing the viscosity of the lubricating oil will reduce the thickness of the oil film on the contact part of the gear and the bearing, which will reduce the protective performance. Therefore, in this study, we report the results of a detailed examination of low viscosity and protection performance of gears and bearings. 2. Gear and bearing protection performance 2.1 Study on improving gear and bearing protection performance As above-mentioned, applying lubricant with lower viscosity to EVs would system potentially give an advantage for motor cooling to order to achieve a better motor efficiency. However, it is necessary for designing lower viscosity lubricants to consider a negative impact on the durability of gear and bearing, which are the components of the E AXLE. This implies that the role in lubricant additives would be more important for improving lubricity. In this section, we investigated the effects of extreme pressure agents (EP additives) on the seizure resistance and fatigue resistance of gears and the fatigue resistance of bearings. Table 1 shows the samples tested for evaluation. The kinematic viscosity of 100-°C. was adjusted to 3-mm 2 / s, and different phosphorus-based EP agents were blended. Table.1: Formulation of sample fluids and their kinematic viscosity at 100-°C. Oil-A Oil-B Oil-C BO    EP(S)    EP(P)-A  - - EP(P)-B -  - EP(P)-C - -  Others    Kinematic viscosity @100-°C, mm 2 / s 3.0 3.0 3.0 In particular, phosphorus-based additives had a significant impact on the fatigue life of gears and bearings. The results of the FZG gear fatigue resistance test are excerpted and shown in Figure 1. The Figure 2 shows the results of the angular bearing fatigue resistance test. Both FZG gear fatigue resistance test and angular bearing fatigue resistance test, compared with oils A and B, oil C showed better fatigue resistance. Figure 1: FZG gear fatigue resistance test results (C-PT/ 8.3/ 90). Figure 2: Angular bearing fatigue resistance test results (1800-rpm, 3.8-GPa, 100-°C). 38 24th International Colloquium Tribology - January 2024 Lubricants Technology for Improving the Protection Performance of Reduction Gears in Transaxles for Electric Vehicles 2.2 Analysis of factors that improve fatigue resistance Based on the above results, we investigated the reason why oil C exhibits excellent fatigue resistance. First, a friction test simulating the lubrication conditions of the FZG fatigue resistance test was performed using each test oil. The correlation between fatigue life and the amount of elements and surface shape of the tribofilm formed in the wear scar after the friction test was investigated. The correlation with each parameter is shown in Figure 3. Tribofilm surface roughness measured by AFM tended to correlate best with fatigue life of FZG. On the other hand, the correlation between the elemental content of tribofilm and fatigue life was low. Figure 4 shows the measurement results of the nanoscale wear mark surface shape after friction test by AFM. Oil C has a smoother surface shape than oils A and B, disperses contact stress, and is less likely to cause cracks that are the starting point of fatigue damage, so it is considered to exhibit a long fatigue life. This study suggests that additives that smoothly control the friction surface are useful for improving fatigue resistance. Figure 3: Correlation between fatigue life of FZG gear fatigue resistance test and each item. Figure 4: AFM measurement results of wear marks after friction test. 3. Copper corrosion protection performance The compatibility of the copper used in the motor and electronic components, which are components of the E AXLE, is also important. Since there are concerns that some types of EP additives may adversely affect copper compatibility, copper compatibility of each sample was confirmed in a copper plate immersion test. Table 2 shows the amount of copper elution from the copper catalyst after 150 hours at ISOT165-°C and the results of the copper plate corrosion test according to ASTM D130. It was confirmed that oil C, which has excellent fatigue resistance, is compatible with copper having the same performance as oils A and B. In general, sulfur-based additives have low copper compatibility, but the influence of phosphorus-based additives on copper compatibility is considered to be small. Table 2: Copper plate compatibility test results. Oil-A Oil-B Oil-C The amount of copper elution, ppm (ISOT165-°C, 150 h) 66 38 53 Appearance of copper plate (ASTM D130/ 150-°C, 3 h) 2a 2a 1b Conclusion In this study, we investigated the effect of phosphorus additives on the protective performance of gears and bearings, which are anti-performance when viscosity decreases. In addition, the compatibility of copper, which can be adversely affected by the use of extreme pressure additives, was also evaluated, and the following conclusions were obtained: 1. The type of phosphorus-based additive affected the fatigue resistance of gears and bearings. Oil C showed superior fatigue life to oil A and oil B. 2. As a result of investigating the factors that cause the good of specific phosphorus additives in gear fatigue tests and bearing fatigue resistance tests, it was found that the surface shape of the tribofilm measured by AFM had a great influence. 3. Compared to sulfur-based additives, it was found that phosphorus-based additives had a smaller effect on copper compatibility. In this study, we found that by properly selecting base oils and anti-wear agents, it is possible to design lubricants for E AXEL with excellent motor cooling and gear and bearing protection. References [1] Takekawa, D. Narita, K.: Lubricants Technology Applied to Transmissions in Hybrid Electric Vehicles and Electric Vehicles. SAE Technical paper 012338, 2019. [2] Onimaru, S. et.al.: Heat Analysis of the Hybrid Electric Vehicle (HEV) Motor Cooling Structure Using ATF. Denso Technical Review 17 (2008) 1, 19-25. [3] Seki, N. et.al.: Heat transfer engineering, published by Moriita syuppan, 1988.