Abstract: The force analysis of parallel mechanism or complex mechanism by using the traditional method, especially, when it needs to calculate constraint reactions of all kinematic pairs, is quite inconvenient because a large number of simultaneous equations need to be solved. Using screw theory to analyze such mechanisms will simplify the problem. With this method, it can develop the force analysis of the mechanism including active forces under external force and constraint reactions of all kinematic pairs, and it can deal with the active forces and constraint reaction forces of every kinematic pair respectively. The advantage is that the method can ensure the number of equilibrium equations being no more than six each time. For understanding the characteristics of this force analysis method based on screw theory, a single-loop spatial mechanism is taken as an example, which consists of two revolute pairs (R), a spherical pair (S) and a universal pair (U), i.e. RSUR mechanism. It needs to solve its active forces and constraint reactions of all kinematics pairs with this method. Through the solution it can be seen that the method greatly simplifies the process of calculating the force analysis and the solution process is clear and easy to understand.

Key words: Active forces, Complex mechanism, Constraint reactions, Force analysis, Parallel mechanism, Screw theory