Torque loss reduction at differential gear unit is important to improve the fuel economy of automobiles. One effective way is to decrease the viscosity of lubricants as it results in less churning loss. However, this option creates a higher potential for thin oil films, which could damage the mechanical parts. At tapered roller bearings, in particular, wear at the large end face of rollers and its counterpart, known as bearing bottom wear is one of major failure modes. To understand the wear mechanism, wear at the rolling contact surface of rollers and its counterpart, known as bearing side wear, was also observed to confirm the wear impact on the tapered roller bearings. Because gear oils are also required to avoid seizure under extreme pressure, the combination of a phosphorus anti-wear agent and a sulfurous extreme pressure agent are formulated. Because the latter could cause an antagonistic impact on the former, we focused on control of active sulfur content as well as the treat ratios to reduce wear at the tapered roller bearing while maintaining anti-seizure. Additive screening bench tests were performed considering operational conditions in actual gear unit-stand tests on the basis of the above-mentioned concept and the best formulation was identified as a breakthrough for low-viscosity gear oil. This low-viscosity gear oil technology was validated in the unit-stand tests. The sample oil based on the technology demonstrated equivalent performance for anti-wear while maintaining good anti-seizure, to the high-viscosity gear oil.
