Fatigue life analysis of the ball screw in a steering assist electric cylinder for the second axle of commercial vehicles

Abstract

This study addresses the combined axial–radial loading of ball screws in the second-axle steering assist electric actuator within a dual-front-axle electric power steering system for commercial vehicles. A theoretical load distribution model incorporating radial load effects is established, and contact fatigue life is evaluated using Hertzian contact theory and the S–N curve. Results show that radial loads exacerbate non-uniform circumferential load distribution among balls, increasing local ball–raceway contact loads and stresses. As the radial load increases from 0 kN to 6 kN under a constant axial load of 38 kN, the theoretical fatigue life decreases from 536.1 h to 366.8 h. An explicit dynamic simulation model is developed using LS-DYNA, with MPP parallel computing employed to enhance solution efficiency for the complex contact model. Fatigue life simulations are subsequently performed via nCode, yielding trends consistent with theoretical predictions and a maximum relative error of 6.89%. Bench durability tests confirm that no apparent early-stage failures—such as pitting, spalling, or cracks—occur within 105 cycles. The findings provide a reference for fatigue life assessment of ball screws in steering assist electric actuators under combined loading conditions.