Micropitting Performance of an e-Axle Fluid under High-Speed Forced Lubrication Using a Three-Roller Test

The growing demand for greater efficiency in electric vehicles requires e-axle gears to operate at higher rotational speeds while maintaining durability.However, high-speed operation promotes lubricant scattering and viscosity reduction, creating severe lubrication conditions that accelerate micropitting fatigue.Although numerous studies have investigated micropitting under low-speed, dip-feed lubrication conditions, only limited systematic information exists about micropitting behavior under high-speed operation with forced-feed lubrication.In this study, a three-roller-type micropitting tester was employed to clarify how the lubrication supply methods and sliding conditions affect micropitting damage under high-speed sliding conditions.First, dip-feed and forced-feed lubrication were compared at an entrainment speed of 3 m/s, Hertzian contact pressures of 3.0 and 3.5 GPa, and slide-roll ratios (SRR) of -0.5%, -2.0%, and -5.0%.Subsequently, the entrainment speed was increased to 6 m/s, and the effect of the lubricant flow rate under forced-feed lubrication was evaluated by varying the flow rate from 0.5 to 15 mL/s at the same SRR levels, with dip-feed lubrication included as a reference.Micropitting damage was quantified by the damaged area percentage.Cross-sections of the sliding surfaces were observed using scanning electron microscopy to evaluate the crack length and propagation angle.The results demonstrated that forced-feed lubrication significantly reduced micropitting compared with dip-feed lubrication.Higher lubricant flow rates initially decreased micropitting, but no further improvement occurred above 10 mL/s.These findings indicate that the lubrication stability under highspeed conditions, ensured by sufficient oil replenishment and effective heat dissipation, plays a crucial role in suppressing micropitting in e-axle gear applications.