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to such a property as lubricant film strength. However, as we know from practice, lubricants with similar pressure-viscosity coefficient may exhibit significant differences in lubricant film strength in the presence of extreme pressure additives.
Surface roughness plays a significant role in EHD lubrication – the local film thickness will obviously be smaller over peaks and larger over valleys. Besides significant impacts on friction and wear, roughness also affects the propensity for cavitation. To factor in roughness effects, flow-factors are used to correct the corresponding smooth film thickness. Homogenisation is another mathematical technique that can be used to statistically average surface roughness effects.
Different surface finishing methods produce different surface roughness profiles. As an example, in Figure 1, surface roughness profiles for conventionally ground and mechanochemically finished gears are shown. The latter demonstrate superior tribological performance.
surface finish quality. Besides common tribological problems, most of which engineers are familiar with from motor sports, some new problems related to electrification come into play. Thus, bearings in EV transmissions are prone to electrically induced bearing damage and may exhibit signs of pitting and fluting. Surface-initiated rolling contact fatigue is another common problem gaining increased attention lately. Proper lubrication is critical. In general, EV transmission fluids call for a somewhat different spectrum of properties compared to conventional ATF. The instant torque feature of an electric motor – which the majority of EV owners praise for rapid acceleration – puts a significant stress on transmission gears. On the contrary, under high-speed operation, excessive heat becomes a problem. To increase scuffing resistance, one can deploy anti-scuff additives, but doing so will increase risk of copper corrosion. Alternatively, one can increase oil viscosity, but doing so will undermine transmission efficiency and cooling efficiency at high speed, see Figure 2. Hence, to find the right balance is not a trivial task, and a trial-and-error approach is viewed by many as the preferred strategy despite its heavy cost burden.
Figure 1: Differences in surface roughness profiles and contact stress distribution for conventionally ground (left) and mechanochemically finished (right) gears.
Powertrain electrification has been a growing trend in the automotive industry. Electric motors used in Battery Electric vehicles (BEVs) operate at high speeds ranging from 3000 to 16,000 rpm, with high-performance motors reaching over 20,000 rpm. For instance, Tesla’s carbon-sleeved motor used in Tesla Model S Plaid may reach 24,000 rpm at the top speed of 330 km/h. There are experimental designs of interior permanent magnet synchronous motors (IPMSM) reaching 100,000 rpm. Most EV transmissions require a coupling between an oil-lubricated gearbox to an electrical motor that runs with minimal lubrication at very high rpm. High mechanical and thermal stresses transmission components – such as seals, bearings and gears – are exposed to may have a detrimental impact on their service life. This puts higher demands on materials and
14 LUBE MAGAZINE NO.181 JUNE 2024
Figure 2: Example of TEHD analysis of gear tribology in two extreme cases: low speed/high load (LSHL) and high speed/low load (HSLL).
CAE and mesh-less CFD simulation tools open new ways to the design and optimisation of lubrication and thermal management solutions for EV transmissions and e-axles. Additionally, mesh-less simulations can also provide valuable insights into the effects of different oil properties on cooling and lubrication efficiencies, thereby helping in matching the lubricant and hardware characteristics for optimal performance. The development process – both for hardware and lubricants – for EV transmissions can be radically accelerated and made more economical by skillfully deploying the modern CAE tools, see Figure 3. Basic tribological tests are still needed for evaluation of lubricant parameters such as pressure-viscosity coefficient or boundary friction coefficient that are not a part of official specifications. The data from these simple tests are verified and further refined by
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