Important lines Tangency of the both base circles (基圆公切线) is the common norma K Line of action(力作用线) Axis of transmission(啮合线或传动线) Conclusion these three lines are the same lineImportant lines Conclusion: these three lines are the same line. Axis of transmission(啮合线或传动线) Line of action（力作用线） Tangency of the both base circles （基圆公切线）is the common normal . rbp rbg rp rg