Failure mechanism and optimization of cardan shaft for positive displacement motor in ultra-deep drilling

As the key equipment of deep drilling, the universal shaft of screw drill is easily damaged due to the increase of eccentric torque, which significantly affects the drilling efficiency, cost, and safety. However, there is a lack of research on the failure factor analysis and innovative design of the universal shaft of screw drill. In this study, the structure and mechanical characteristics of the existing universal shaft are analyzed, and it is concluded that the universal shaft has serious stress concentration and fatigue damage due to high torque. Then, the key failure area and failure mechanism of the universal shaft were determined by finite element analysis and fatigue life prediction. Furthermore, a new type of cycloidal universal shaft is designed and optimized. The simulation results show that the maximum stress value of the optimized universal shaft in the key stress parts is reduced by 55%, and the fatigue life is increased by about 82%. The effectiveness of the new structural design optimization scheme is verified, which provides relevant data support for the design and application of screw drilling tools.