Abstract: Mobility is one of the most fundamental concepts in type synthesis, kinematics and dynamics of mechanisms. In the past several decades, strong interest of many scholars at home and abroad has been mainly shown in mobility analysis of spatial parallel mechanisms. A principle of mobility analysis is introduced for two-level and two-loop spatial mechanisms with coupling chains, which contains more complex coupling relations than spatial parallel mechanisms. Through deriving the twist equation of two-level and two-loop spatial mechanisms with coupling chains, the mathematic model of coupling relations caused by coupling chains is established. Based on the twist equation, the method for rigid substructure detection and the principle of mobility analysis of the class of mechanisms are proposed. Mobility property can be determined by the characteristics of the linear combinations of the limb twist system of the equivalent sub-parallel mechanisms of two-level and two-loop spatial mechanisms with coupled chains. Two typical examples are used to illustrate effectiveness of the proposed method.

Key words: mobility analysis, screw theory, spatial mechanisms, coupling chains