Abstract
The authors developed the novel mechanism CVT (Shaft Drive CVT, S-CVT). The input and output shafts with conical disks are parallel and the idler shaft having conical rollers at both ends is placed perpendicular to the input/output shafts. This idler shaft transmits torque from the input shaft to the output shaft and its linear motion produces the speed variation by changing the contact point between the input/output disks and the idler roller. The design procedure and the evaluation of performance using the prototype were shown in this paper. The efficiency of traction drive transmission is expressed as the product of the speed efficiency and the torque efficiency. The theoretical efficiency of S-CVT was calculated by using the shear model of traction oil in the EHL contact for the speed efficiency and considering the loss torque of bearings for the torque efficiency. To improve the torque efficiency, the backup roller mechanism was devised. It is mounted to the opposite side of the idler roller, and contact with the input/output disks and the backup disks as well as the idler roller. It transmits torque between the input/output disks and the idler roller through the backup disks, and the amount of transmitted torque is enlarged twice. The result of theoretical calculation shows the improvement of the efficiency up to 88.6 to 91.8%. The effectiveness of the backup rollers was confirmed by the experiment, and the efficiency of 92.5% was obtained at uniform ratio. The efficiency of 2 to 5% was improved on each speed ratio comparing with the case without backup rollers. Moreover, a unique ratio changing mechanism was developed and the speed control performance was evaluated by the experiments. The construction of S-CVT is similar to the metal-belt CVT of which the ratio changing operation is performed by moving the input/output disks using two hydraulic pistons, respectively. The hydraulic power is required to clamp the belt not to slip and increased according to the high-power transmission. In the prototype S-CVT, only one motor is used to actuate disks. The mechanism using a ball screw and a gear train is developed without using any hydraulic systems. The power consumption of motor is so small because the contact force applying to the input/output disks are counterbalanced by the excellent arranged gear train. It is effective to improve the efficiency of total CVT system.
Keywords: Transmission, CVT, Traction Drive, Ratio Changing, Tribology