In this work, two degree-of-freedom rotational mechanisms (2-DOF RMs) also called 2-DOF pointing mechanisms are discussed systematically about their motion characteristic and orientation description methods. Based on the graphical approach, 2-DOF RMs are divided into three types in term of the features of their instantaneous rotational freedom lines, and the models of the spinning motions for all the three types of 2-DOF RMs are built respectively by constraint conditions. According to the spinning motion models, the orientation description methods suitable for the three types of 2-DOF RMs are introduced in detail severally to simplify the kinematic equations in succedent work. Then the correctness of the spinning motion models is verified via the virtual prototype simulation with several examples. Finally, the methods suitable for describing the orientation of these mechanisms are proposed. Conclusions would facilitate the analysis of kinematics of 2-DOF RMs and exhibit the motion characteristics of 2-DOF RMs expressly.

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