Compact pericyclic continuously variable speed transmission systems: Design features and high-reduction variable speed case studies

The Pericyclic Variable Speed Transmission (PVT) is a dual-input one output differential power train system which can be adapted to various vehicle systems as a main transmission component. The PVT system can also be operated as a fixed speed drivetrain when one of the input members is held stationary. Wide range of reduction ratios, (1.05:1 up to 50:1), can be achieved for a fixed speed PVT in one compact unit. To explore the range of potential PVT based variable speed transmission concepts, two bounding cases were considered for a 600HP light rotary-wing aircraft. The total speed reduction ratio of the helicopter transmission is 285.7:1 for the low rotor speed (140 RPM), and 133:1 for the high rotor speed (300 RPM). The transmission in the case studies is the combination of a 6.25:1 ratio PVT as the first reduction stage and 45:1 ratio PVT for the second reduction stage, in the first case study the speed increase is achieved by a 45:1 ratio PVT unit while in the second case study it is increased by the 6:25:1 ratio PVT. Critical design elements, such as kinematical features, power flow, power circulation, and component torque on the two conceptual PVT designs are analyzed to investigate the impact of different reduction ratio variable speed PVT units on the transmission system configuration. The PVT design procedure starts with the determination of the number of teeth on the gears to achieve the low speed reduction ratio. Then, the speed of the speed control member is calculated to achieve required output speed increase. Both systems are designed to minimize internal power circulation during all operating speeds. The analysis shows that both systems branch approximately 50% of the engine power to the Controllable VariableSpeed Unit (CVU) at increased rotor speed (300RPM). The kineto-static torques of the components are obtained by the system kinematics and the power flow. The torque carried by CVU of the 6.25:1 ratio variable speed unit is 2% of the 45:1 ratio variable speed system, in a more compact and light weight CVU units; hence, in general it is advantageous to couple the CVU unit with the low reduction ratio PVT.