Connecting rod assembly mainly is made up of  big-end bearing shell (upper bearing inserts), small-end bearing shell (copper sleeve, lower bearing inserts), connecting rod bolt and connecting rod body. according to the relative motion between components being plane motion or spatial motion, it can be classified into plane connecting rod mechanism and spatial connecting rod mechanism.According to the number of components in the mechanism, it can be classified into four-rod mechanism,five-rod mechanism etc.. Normally,the connecting rod mechanism of five rods and over five rods is named as multi-rod mechanism. When the degree of the connecting rod is 1, it is named as single freedom connecting mechanism; When the degree of the freedom of connecting rod is greater than 1,it is named as multiple freedom connecting rod mechanism. According to the kinematic chain of the connecting rod mechanism being open chain or closed chain , it can be classified into open chain connecting rod mechanism (manipulator is normally a spatial open chain connecting roe mechanism with its kinematic pair being revolute pair of sliding pair ) and closed chain connecting rod mechanism. As the components of plane connecting rod mechanism with single closed loop are at least  numbered 4, so the simplest plane closed chain connecting rod mechanism is four-rod mechanism. Other multiple rod closed chain mechanism is only to increase rods on that basis ;as the number of the  components of spate connecting rod mechanism with single closed loop is  at least 3,so it can be spatial three-rod mechanism composed of three components.