An investigation of the thread and under-head coefficients of friction on clamp-load in the bolted joint

The coefficient of friction (CoF) has a dominant effect on the clamp load in a bolted joint. Friction occurs at threads as well as in the under-head region of the bolt-head or nut. Tests were conducted to determine the effect of lubrication, tightening speed, flange diameter, length of engagement, and the nut, bolt, and washer hardnesses on clamp load and thread and under-head CoFs. In most cases a tightening torque of 45 N-m was used. A finite element analysis model of an experimental bolted joint was developed to determine the clamp load on individual threads, flank pressure, and bolt elongation. The bolt in the model was tightened dynamically by applying a torque at the flats of the hexagonal head.; The results indicate that the flange size has a pronounced effect on the clamp load as compared to the length of thread engagement; as the flange diameter increased, the magnitude of the clamp-load also increased. A medium hardness bolt coupled with a high hardness washer produced higher clamp-load compared to any other material hardness combination. The thread and under-head coefficient of friction decrease with the tightening speed. The under-head CoF has a more pronounced effect on the clamp load than the thread CoF. The load distribution on different threads was also obtained from the FEA model and compared with the previous analytical and FEA (static) models. The 1 st thread was found to support the largest portion of the clamp load while the last thread the smallest portion. However, the percentage of the load carried by the 1st thread was different from those of the static models. The FEA results were also compared with the experimental results. There was a difference of about 5.2% between the FEA computed and experimentally determined clamp load. The experimental thread torque data was coupled with the FEA results to obtain the thread coefficient of friction values on each of the engaged threads. The coefficient of friction at the 1st thread was largest and decreased with the thread number.