Abstract: As some kinematics problems are studied by the screw theory, the dependency of screw system is a key problem. By linear algebra theory, the dependency of screw system which is constituted by three line vectors and three couples, four line vectors and two couples or five line vectors and one couple, is discussed. A matrix is composed of screws. The dependency of screw system is determined by the order of the matrix. Criterions of the dependency are given. The dependency on different geometrical conditions are listed. All of the results are useful for studying parallel mechanism kinematics, such as degrees of freedom and input selecting.

Key words: Dependency, Parallel manipulator, Screws theory