Dynamic Frictional Effect of Tightening Speed on Clamping Force in High Strength Bolts

While working with slip critical connections, one of the most critical factors for high strength bolts is to achieve to a specific value of pretension. This pretension varies depending on several factors such as condition of thread, nonlinear local behavior (head, thread, nut, and other components of the connection), lubrication, and dynamic frictional effect of tightening speed. The current study addresses one of these factors which is the dynamic effect of tightening speed of the bolt performed by automatic machines. In this respect, four samples were carefully examined in laboratory showing that the friction factor of bolt tightening with minimum speed is smaller than the same with average speed. The results indicate that lower tightening speed with smaller torques concludes to a higher pretention, while increasing the tightening speed results in a greater friction factor and nut factor, and consequently, making it difficult to reach to the needed pretention.