Abstract: Dynamics model of 4-SPS/CU parallel mechanism with spherical joint clearance is formulated based on augmented method in the equations of motion for mechanism systems. The driven chain in this parallel mechanism is regarded as a unit via the method of mass moment in order to simplify the dynamics modeling. The kinematic model of spherical joint with clearance is established on the ground of contact type between elements in the clearance joint. The contact deformation and relative contact velocity are presented on account of the above research. A new contact force model is presented, which replaces the constant stiffness coefficient of the Flores contact model with nonlinear stiffness coefficient considering to the coupling between the contact stiffness of contact elements and dynamics model of mechanism system. The normal contact force between the contact elements is evaluated based on the new contact model, and the tangential contact force is derived according to the Coulomb friction model with dynamic correction coefficient to ensure the stability of numerical computation. The contact forces between the socket and ball are converted to the mass center of driven chain and upper platform interconnected by this clearance joint, and the converted contact force is integrated to the generalized force in equations of motion of 4-SPS/CU parallel mechanism to introduce the effect of spherical joint clearance. The Baumgarte stabilization method is used for solving dynamics equations in order to stabilize the constraints violation. The influence of spherical joint with clearance on the dynamics performance of this parallel mechanism is predicted by numerical analysis

Key words: 4-SPS/CU parallel mechanism, Coulomb friction model, dynamics, Flores contact model, spherical joint with clearance