Effect of Tightening Speed on Thread and Under-Head Coefficient of Friction

Applying torque to a fastener creates friction as well as clamp-load. Friction and clamp-load are inversely proportional: as friction increases, the amount of clamp-load generated decreases. The speed at which a fastener is tightened has a pronounced affect on the magnitude of friction, and thereby clampload generated in a metal joint. This paper examines the relationship between the tightening speed with friction and clamp-load. The applied torque, clamp-load, and under-head torque were measured as the fastener was tightened within a torque/tension load cell. The corresponding thread torque, under-head and thread coefficients of friction values were calculated from equations given in both the DIN946 and ISO16047 standards. One bolt, one washer, and one nut (all of similar hardness) were used for all trials of this study. Two different anti-seize lubricants were used (one composed of molybdenum-disulfide and graphite and the other of mineral oil, nickel, and graphite). Lubricant was applied to both the thread and under-head regions of the bolt prior to torquing. Further trials were executed with the bolt, washer, and nut in the dry, or plain, condition (all of similar hardness). The results of this investigation indicated that the thread and under-head coefficients of friction decreased as speed increased. The programmed speeds ranged from 25 to 700 r/min, with a torque target of 28 N·m. Since the friction coefficients decreased, the amount of clamp-load generated at target torque increased with increasing speed. The average difference in clamp-load from minimum to maximum speed was determined to be 6.3 kN for both lubricants and 1.2 kN for the unlubricated condition.