The diploma thesis deals with the desing of the reaction arm for the reaction arm for the tightening tool and the subsequent FEM analysis. The aim is to design the construction of the reaction arm attached to the assembly table. At the end of the raction arm, a tightening device from ATLAS COPCO will be screwed on and then modeled in the Creo parametric 4.0 program. In the next step, we will use a numerical simulation of stress, strain and bending to make sure that the structure is deminsioned for sufficient rigidity and funcrtionality of the arm.