The Effect of Coating and Tightening Speed on the Torque-Tension Relationship in Threaded Fasteners

This study investigates the effect of fastener coating, tightening speed, and repeated tightening on the torque-tension relationship, and on the coefficients of thread and underhead friction. Selected coatings provide three distinctly different levels of friction between threads and under the rotating fastener head; these coatings are assigned a low, medium, or high friction designation. Due to the fact that the torque-tension relationship is highly sensitive to normal variations in friction, the findings of this study would improve the reliability and safety of bolted assemblies, especially in critical applications.

For the three selected coatings, the experimental procedure determines the coefficients of thread and underhead bearing friction as well as the overall fasteners torque-tension relationship at two tightening speeds for three fastener sizes with fine and coarse threads. Additionally, the effect of repeated tightening is examined.