The gripping contact forces distributed between the interface of the tongs and the work-piece of the gripper of forging robots are considered as an equivalent resultant force whose contact model is friction point contact. So the gripping operation is similar to multi fingered robot hands, and the operation theory of N robot fingers to grasp an object can be used for the analysis of the force-closure and the optimization of contact forces of heavy gripping mechanisms. After the configuration of heavy duty gripping mechanisms is analyzed, the position distribution of the resultant contact forces is optimized, the iterative algorithm of linear constrained gradient flows is used to calculate and analyze force-closure conditions. The force closure features for different forging robots and in different operation conditions and mechanisms are analyzed. Simulation and experimental results demonstrate the effectiveness of the force-closure method.