Analysis of fracture failure of manual transmission gear shift arm based on dynamic synchronous shifting

The reliability of the gear shift control mechanism in transmissions is critical to driving safety. Aiming at the fracture issue of the gear shift arm in commercial vehicles equipped with the 8GS46B transmission, this study identified the root cause of the fracture and proposed engineering solutions to improve its reliability through the analysis of dynamic gear shift forces and failure mechanisms. A force model of the gear shift system was established based on gear shift synchronization performance; the dynamic loads borne by the gear shift arm were determined by integrating GSA (Gear Shift Analysis) tests, and the comparison between transient structural and static structural simulations revealed that transient structural simulation is more consistent with actual operating conditions. Under a load of 540 N, the directional change of the gear shift force caused a change in the stress state: the maximum stress at the welding center was 237 MPa without deflection, while after a 10° deflection, the stress at the welding center increased to 258 MPa and the stress at the bending position reached 222 MPa. Judged based on the strength design criteria and the fourth strength theory, these stress values exceeded the yield strength of Q235 steel (235 MPa), which is the fundamental cause of the fracture. This study proposed three improvement schemes: Scheme 1 involves increasing the width to 42 mm, which reduces the maximum equivalent stress to 171 MPa (before deflection) and 195 MPa (after deflection), respectively; Scheme 2 is replacing Q235 steel with Q355 steel, resulting in maximum equivalent stresses of 240 MPa and 280 MPa (before and after deflection, respectively); Scheme 3 adopts local optimization (fillet at the welding end and thickening at the bending position), which decreases the maximum equivalent stress to 111 MPa (before deflection) and 123 MPa (after deflection)—representing a reduction of 53.1 % and 52.3 % compared with the original structure. The safety factor of all three schemes reaches above 1.2, meeting the strength requirements; after a comprehensive evaluation, Scheme 1 is identified as the preferred scheme because it requires no process modification and balances economic efficiency. This study not only resolves the fracture problem of the gear shift arm, but its methodological framework also provides important reference for improving the reliability of similar structural components.