This abstract describes an approach to the organization of qualitative design knowledge about mechanism’s functioning. This knowledge is being formed on the basis of analytical results obtained in quantitative simulation. Smooth response functions generated by simulation system are approximated by differences of first three orders. Attention is drawn on capabilities of sign combinations of the differences to provide qualitative reasoning when searching for synthesis solutions. The tasks arising are examined from this viewpoint: extraction of qualitative features of function, their classification, setting dependencies between parameters and features, derivation of tendencies of change of the features.