The priority for lubricant performance for metal V-belt-type CVT (B-CVTFs) should be the improvement of transmittable torque capacity between the belt and pulley plus excellent antishudder properties for lockup clutch used in B-CVTs. This study intends to investigate the effect of lubricant additives for improving these performances of B-CVTs. In addition, surface analysis techniques were utilized to gain a novel insight into the chemical composites and morphology of the tribofilms. As a result, it is vital for greater torque capacity to give higher boundary friction coefficient between the metal contacting interfaces, and the process of boundary lubricant film formation derived from antiwear additives used in B-CVTFs strongly impacts on the torque capacity. Moreover, it is found that a sort of lubricant formulation gave an excellent antishudder performance for wet clutch with keeping higher friction coefficient between the metals, which would result in improving the performance of B-CVTs. 1. Introduction There are two types of transmissions used in automobiles: automatic and manual. A market share of automatic transmissions referred to as ATs has been over 97% of all transmissions installed in automobiles in Japan [1]. In particular, there is an increasing number of passenger cars that feature continuously variable transmissions (CVTs) because CVTs achieve better fuel economy. Among CVTs, the number of cars with metal push belt-type CVTs (B-CVTs) has steadily increased each year and is currently applied to cars with an engine displacement of more than 3 liters. B-CVT consists of a steel belt made up of about 400 segments and laminated rings, as shown in Figure 1. Power is transmitted by the frictional force generated between the belt and pulley. In order to improve transmission efficiency and spread the application to CVTs to larger cars, belt CVT lubricant oils (B-CVTFs) must produce a higher transmission torque capacity between the belt and pulley. Therefore, the priority for the performance of CVT fluids should be focused on the improvement of torque capacity. Figure 1: Constitution of Belt CVT unit. There are some reports regarding the effects of lubricants on the performance of a CVT [2–4]. For example, the films generated by Zinc dithiophosphate (ZnDTP) additives in the contact regions are known to contribute to achieve a higher metal-metal friction coefficient [2]. The information gained regarding the tribofilms of ZnDTP is helpful when considering the additive reaction from lubricants. ZnDTP tribofilms do not develop in rolling contact or if
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