An experimental study on the relationship between torque and preload of threaded connections

In order to investigate the relationship between torque and preload during the tightening process by using torque-only control method, a new measuring device was designed in this study. Nickel-based superalloy bolts and nuts of three specifications M8 x 1, M10 x 1.25, and M10 x 1.5 were used in threaded connections to implement the pre-tightening test at room temperature. Pre-tightening tests under four working conditions for every bolt were conducted. Torque-preload curves were drawn according to the test results, and nut factors were obtained by curve fitting method. The effect of surface lubrication and surface roughness on nut factor was analyzed. The results show that the nut factor under lubrication condition is smaller than that without lubrication when the surface roughness remains the same and that under the same lubrication condition, the larger the surface roughness, the greater the nut factor. The results show that the device proposed can be used to measure the preload accurately during the pre-tightening process of threaded connections. Finally, the influence of the screwing times on the nut self-locking torque was studied.