The torque-split gear transmission has been used in the transmission system of the rotorcraft, which undertaking high torque loads and requiring low weight. A universal mathematical design method of the torque-split gear transmission is proposed in this work. The teeth with the same phase of the two gears on the duplex idler are marked. And the meshing condition is defined by the whole pitch number between the two points along the pitch circle of the output gear. Then the relationship between the tooth number and gear positions is established by using this meshing condition. Unlike other existing design method, this method involves the idlers, input gear and output gear that engaged directly, and it is suitable for the design of multiple types of torque-split gear transmission. This method is validated through numerical examples of the torque-split transmission with symmetrical duplex idler. Moreover, practical applications of the torque-split transmission with planetary duplex idler, coplanar gear and concentric face gear are also studied with this method. A large number of discrete gear position solutions are observed under the same tooth number design. And different gear positions correspond to different dynamic and load sharing characteristics of the torque-split gear transmission.
