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Multi-gear planetary gear transmission has become a research and development hot spot of automobile enterprises, and excellent transmission topological structure determines the automobile transmission performance, Therefore, it is of great significance to research and develop new transmission topological structure. Firstly, this paper analyzes the structural characteristics of common seven-speed transmission, and puts forward a topological structure of three-row series-parallel seven-speed automatic transmission, which consists of an SS-CR double-row parallel structure and a single-row double-stage planetary row series-parallel connection, and can realize seven forward gears and one reverse gear with only six shift elements. On this basis, the equivalent lever diagram and the speed diagram are designed, and the transmission parameters of each gear are calculated in detail and optimized by using the triangle similarity principle. The results show that only two shift elements are required for each gear of the new transmission. And in addition to the conventional step-by-step lifting gear, spanning lifting gear can be realized between D1-D4 and D5, between D4 and D6, and between D5 and D7. Its transmission ratio range is 9.77, which is 64.02% higher than that of Mercedes-Benz 722.9. The transmission ratio of the lowest gear D1 is 3.76% higher and that of the highest gear D7 is 36.74% lower.
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