Failure Analysis and Design Optimisation of Steering Linkage Pivot Shaft of Commercial Vehicle

Commercial vehicles have steering systems with one or more steering links connecting the steering gear box pitman arm and front axle steering arm. In case of twin steer vehicles, intermediate pivot arm is used to transfer the motion proportionately between the two front axles. Intermediate pivot arm is also used in some longer front over-hang vehicles to overcome their packaging constraints and to optimize the mechanical leverage. The pivot shaft is a mechanical part of the intermediate pivot arm assembly upon which pivot arm can swivel in one axis. Steering forces transferred through the drag links generates resultant forces and bending moments on the pivot shaft. In this work, study has been carried out on premature failure of the pivot shaft in city bus application model (Entry + 1 step). Metallurgical analysis of failed part indicated the failure to be due to fatigue. Pivot shaft was tested in rig with similar load conditions in order to replicate the failure. The failure mode and location of both, tested as well as field failure samples were identical. Material properties of failed samples were also studied. In order to understand the reason for premature failure, CAE-MBD analysis was performed followed by structural analysis of the component. It was found that the bending moment and Von-Mises stress in the component were higher compared to other existing proven vehicles. This increase in the loading was found to be mainly because of steering linkage layout on to the intermediate pivot arm. The steering linkage layout was reworked and optimized, which resulted in reduction in bending moment and stress in the pivot shaft.