Joint clearance exists in mechanical multibody systems,which affects the constraint conditions,the degree of freedoms,and the topological structures of the mechanical system.Based on the Hertz's contact law and the Coulomb's friction law,a mechanical model of the planar revolute joint with clearance was built.A two-beam mechanical arm with one rigid beam and one flexible beam was studied,and the elastic distortion of the flexible beam was described by the geometrical deformation constraint method and the modal condensation method.By using the Kane's equation,a complete dynamic model of the mechanical arm,which considered two planer revolute joints with clearance,was built,and the model was more realistic and precise compared to the conventional one.