Derivation of an equation of motion of closed loop mechanisms by the motor algebra is presented in this paper. The equation can be given by first determining the velocities and accelerations of the links, next deriving the equilibrium equations of the forces and moments on links, and then appling conditions of the virtual work of joint motions to the equilibrium equations. Simple description of the derivation method are shown by using the motor algebra. In order to reduce the computational cost for the dynamic analysis, it is essential to utilizing the geometrical specialities in a mechanism to the derivation of an equation of motion. It is shown, by two examples of the derivation for a translational table mechanism and a parallel mechanism, that the expression of derivation by the motor algebra makes their utilization easier.
