Novel Microsurface Machining Techniques for Improving the Traction Coefficient

This study examined methods of machining a microsurface texture on the surface of the rolling elements of a toroidal continuously variable transmission (CVT) for improving the traction coefficient. The microsurface texture of the toroidal surfaces consists of tiny circumferential grooves (referred to here as micro grooves) and a mirror-like surface finish similar to the rolling surface of bearings. Hard turning with a cubic boron nitride (cBN) cutting tool, grinding with a cBN wheel and micro forming were applied to machine the micro grooves. The results made clear the practical potential of each method. A micro forming device was also developed for use in actual production. A mirror-like surface finish and micro crowning of the convex portions of the microsurface texture were simultaneously executed by superfinishing them with a fine-grained elastic superfinishing stone. The results showed that a microsurface texture was successfully formed on the rolling elements of a toroidal CVT variator. The specifications of the microsurface texture were then optimized in tests conducted with a four-roller tester. The test results made it clear that the traction coefficient can be improved by the microsurface texture while assuring high durability at the same time. The newly developed machining techniques were then applied to the disks of an actual toroidal CVT variator. The results of a traction performance test revealed that the traction coefficient was improved without sacrificing durability.