Design principle of high power traction drive CVT

Better drivability and fuel consumption are continuous issues for driving system of automobiles. One of the answers is CVT (Continuously Variable Transmission). It was reported that push steel V-Belt CVT improved fuel consumption by more than 15%^[1]. More than 700,000 CVT units have been produced and they have obtained good results in the market. In 10 years, CVT has been spreading slowly throuout the world. But so far all the cars on the market have been small, that is less than 1300cc cars. Recently, some reports mentioned that a high power V-Belt was tested in 3.3L Van or Formula 1 racing cars, but CVT for large engines have not been put on the market yet. In recent years, requests for high power CVT for automobiles have come to light. Traction Drive CVT has the possibility of satisfying this requirement. In traction drive, power is transmitted by the shear force of the EHL (Elastohydrodynamic Lubrication)^[2] oil film existing between a pair of rolling elements. The performance of traction drive depends to a large extent on the rheological properties of the fluid at the EHL contact. This theory was made in 1960's ^[3]. As is well known, examples of traction drive CVT applied to automobiles in the 1960's were Toric transmission by Hewko^[4] and the vehicle test by Kraus^[5]. Basic research was performed by Lowenthal^[6] and Coy ^[7]. Traction fluid was developed in 1970's by Monsanto. But a traction drive CVT has not been put into practical use commercially. The reasons were as follows: (1)the materials of the traction drive rolling elements were not sufficiently reliable because of high pressure and high temperature at the traction contact point. (2)there was no traction oil which satisfied all the conditions of automobiles. (3)there were no bearings which could support high speed and large axial load. The authors started research and development of a Half Toroidal traction drive CVT in 1978. From 1982 to 1987, 8 prototypes were made and life tests performed. Some prototypes were installed in cars and road tests were performed. The result of this research was reported in 1990^[8]. Development of the speed ratio control mechanism and development of a double cavity CVT were reported by Nakano ^[9]. A New design for heavy truck was reported by Lohr and Dawe ^[10]. In the United Kingdom, Full Toroidal CVT has been developing earnestly ^[11]. For over 4 years we have been developing high power traction drive CVT by using better materials and heat treatment of rolling elements. Development of a better traction fluid for automobiles has helped this research progress tremendously. Now, the dream of a 5000cc CVT car is coming true. In this paper, the design principles behind high power traction drive CVT are presented.