Simulation analysis and prevention strategies for fatigue fracture of the swivel shaft in concrete pump trucks

During the pumping operation of concrete pump trucks, the swivel shaft primarily withstands a series of complex loads such as bidirectional bending moments and torque (stemming from the self-weight of the boom in the amplitude plane, the self-weight of the concrete; wind load and rotational inertia force in the rotation plane; torque from the delivery pipe and the concrete inside), as well as lateral wind load on the concrete and the impact vibration caused by the flow of concrete. These factors can easily lead to fractures due to the loads exceeding the material load limit of the swivel shaft. This study targets the swivel shaft fracture accidents during the construction process of concrete pump trucks, hypothesizing that the accidents occur due to improper installation angles of the swivel shaft (angle between the oil channel hole and the cylinder axis). Firstly, numerical simulation of the onsite conditions is conducted and compared with the results observed from the actual accidents. Secondly, by fixing the onsite conditions (keeping the boom posture unchanged), a 360 degrees rotational simulation calculation of the fractured swivel shaft is performed to identify the relationship between high stress intervals and the swivel shaft rotation angle and the angle between the second variable oil cylinder axis. Finally, a full-condition calculation is conducted for the boom and the swivel shaft. The results indicate that the high stress area of the swivel shaft does not change with the swivel shaft rotation angle (when the boom is fixed) but is related to the position of the second variable oil cylinder axis and is symmetrically distributed around it. Therefore, adjusting the swivel shaft rotation angle to avoid the high stress area near the symmetry axis of the second variable oil cylinder can prevent the fracture accidents of the swivel shaft.