A computer-implemented method for designing a 3D modeled object representing a transmission mechanism with a target 3D motion behavior. The method including obtaining a 3D finite element mesh and data associated to the mesh, performing a topology optimization based on the mesh and on the associated data, therefore obtaining a density field representing distribution of material quantity of the 3D modeled object. The method further includes computing a signed field based on the density field and the associated data, identifying one or more patterns of convergence and divergence in the signed field, each pattern forming a region of the signed field, and for each identified pattern, identifying a joint representative of the identified pattern and replacing a part of the density field corresponding to the respective region formed by the identified pattern by a material distribution representing the identified joint.