Study on the Tribological Performance of Regenerated Gear Oil with Composite Additives

In this study, a comprehensive regeneration process was employed to enhance the recycling efficiency and performance of waste gear oil. The process began with the waste gear oil subjected to extraction flocculation, which was then followed by vacuum distillation for solvent removal. Then, catalytic hydrogenation was performed, and HiTEC 3339 additive was incorporated at concentrations that ranged from 0.25% to 1.5%, thus resulting in the regenerated gear oil. The tribological properties of the regenerated gear oil were investigated under various load conditions using a friction and wear testing apparatus. When a load of 10 N was applied, the filtered oil (Oil 2) exhibited an average friction coefficient of 0.092 and a volumetric wear rate of 8.25 × 10−8 mm3/Nm, which represented reductions of 8.23% and 42.7%, respectively, when compared to the unfiltered oil (Oil 1). As the load was increased to 50 N, Oil 2 demonstrated a wear rate of 23.4 × 10−8 mm3/Nm, indicating a 20.9% improvement in wear resistance. As the concentration of the additive increased, the following trends were observed: (i) Under a load of 10 N, the friction coefficients demonstrated a gradual decreasing trend, while at 50 N, the friction coefficients were remarkably similar and significantly lower than those at 10 N. (ii) The wear rates initially decreased and then increased. Among the tested lubricants, Oil 4 (containing 0.5% HiTEC 3339) exhibited the shallowest wear scar depth under various loads, which indicated superior anti-wear performance. When Oil 4 was thoroughly evaluated through bench tests, it indicated excellent extreme pressure and anti-wear properties, as well as superior rust and corrosion prevention capabilities and high–low temperature performance. The overall performance indicators of Oil 4 were discovered to be similar to those of fresh oil.