Frictional behavior of bolted joints during impact tightening

Impact tightening is widely used in industrial assembly of bolted joints. Friction plays a decisive role in the impact tightening process of bolted joints, yet the dynamic frictional behavior during this process remains insufficiently understood. While previous studies have focused on single bolt sizes, this work expands the experimental investigation to M10, M16, and M20 bolted joints, revealing scale-dependent frictional mechanisms. The influences of bolt diameter, impact wrench power, socket length and preload level on the distribution of coefficients of thread and bearing friction are analyzed. Results show that both bolt diameter and impact wrench power significantly influence the dynamic coefficients of friction, with smaller bolted joints exhibiting higher coefficients and greater scatter. These transitions indicate changes in real contact conditions and stick–slip behavior under impulsive torque input. These insights advance the understanding of frictional dynamics in impact tightening and provide quantitative input for improving predictive models of impact tightening and preload estimation, supporting the development of more reliable industrial assembly processes.