Abstract: This paper discusses the calculation of plasticlimit of bending moment of a workpiece, the determination of motor power matching astake-flange machine and the choice of geometric shape and dimension ofworkpiece sections in the process of the machine design. Plasticity theoryis applied to solve these problems and a new simplified method basedon Tresca yielding's law is presented according to the features of theproblems in order to solve the problems mentioned above. Theplastic stress and strain distribution of the workpiece and the respective mathematical formulas are obtained. Acalculating example from a factory is given.