Abstract: The coupling gear (30) has a first torque transfer shaft (32) and a second torque transfer shaft (33) embedded in a house (31) in a rotating way, the input end (32b) of the first torque transfer shaft (32) is connected to the intermediate connecting piece (13b) of the first motion transfer unit (13) of the first connecting part-unit (10), and its output end (32c) is connected to the intermediate connecting piece (23b) of the first motion transfer unit (23) of the second connecting part-unit (20) while the input end (33b) of the second torque transfer shaft (33) is connected to the motion piece (14) of the first motion transfer unit (13) of the first connecting part-unit (10), and its output end (32c) is connected to the motion piece (14) of the first motion transfer unit (23) of the second connecting part-unit (20).