Synthesis of seven-speed planetary gear trains for heavy-duty commercial vehicle

• Six-speed planetary gear trains are synthesized with the deducing synthesis method.
• Lever analogy was used to derive 2-set planetary gear trains with high reduction ratio.
• A design method of planetary gear train based on original scheme is developed.
• Six 7-speed planetary gear trains are obtained for commercial vehicles.

To reduce the design cost of planetary gear trains (PGTs) with torque converter, a systemic synthesis method of seven-speed PGTs is proposed based on six-speed PGTs for heavy-duty commercial vehicles. First, the six-speed PGTs with three degrees of freedom (DOFs) and three planetary gear sets (PGSs) are synthesized with the method of switched-input and variable fixed interconnection. Second, seventeen valid six-speed PGTs are selected according to the requirements, such as the ratio step, efficiency, mechanism planarity and single-transition shift. In addition, by analyzing the speed ratio expression based on the lever analogy, two-PGS PGTs with a high reduction speed ratio are derived. Finally, the seven-speed PGTs are synthesized by the matching design between the six-speed PGTs and the two-PGS PGTs with the high reduction speed ratio. Six seven-speed PGTs with seven clutches and four PGSs are obtained for the heavy-duty commercial vehicles and military vehicles. This method is efficiently used to the serial design of PGT products. And analysis of the current product schemes shows this design method is valid.