Study on the Load Conversion of Bolt-Nut Fasteners between Tightening and Service Conditions

At present, the common method to realize the pre-tightening effect of threaded fasteners is by applying an accurate tightening torque on them. However, when removing the tightening torque, the load characteristics will have a transient conversion, so that the pre-tightening loads will be significantly different from the tightening condition. For the aim of understanding the pre-tightening effect under the service condition, it is important to model the load conversion between tightening and service conditions. Based on the most basic mechanical equilibrium analysis, the load characteristics of bolt-nut fasteners under a round of the tightening and service conditions were studied. The torque equilibrium equations were built respectively, the load conversion between two conditions was analyzed and an approximate relationship between the applied tightening torque under the tightening condition and the pre-tightening torque under the service condition was obtained. In the meanwhile, a dynamic strain measuring system was proposed to accurately determine the pre-tightening loads in real time. Results verified that the thread torque and the bearing friction torque balance each other and act as pre-tightening torques under the service condition. The final pre-tightening loads can be roughly estimated to be larger than half of the difference in tightening torques by wrenches, but smaller than half of the sum of tightening torques, or can be accurately measured through dynamic strain measurements.