Bolt nut connections are widely used to connect mechanical elements in the industry because they are easy to install, remove and maintain. When subjected to complex working environments, failure of bolt nut connections may happen and even cause accidents. For example, the transverse load may cause self-loosening of the bolt nut connections, and cyclic axial load may cause fatigue failure of the bolt nut connections. Therefore, bolt nut connections with high fatigue strength, anti-loosening performance, and low cost are always needed. In our teams’ previous researches, it has been found that both fatigue life and anti-loosening performance of the bolt nut connections can be improved by introducing a pitch difference between the nut and the bolt. For pitch difference bolt nut connections, the load distribution gradually decreases from the free side to the clamped side of the nut rather than decreases from the clamped side to the free side of the nut as common bolt nut connections, and this is the reason why the fatigue life of pitch difference nut can be improved. Besides, when the pitch difference is large enough, a prevailing torque, which is commonly used for anti-loosening, will occur and prevent the nut from self-loosening effectively. However, when the pitch difference is small, the fatigue life can be improved significantly, and the improvement of anti-loosening performance is limited. On the other hand, when the pitch difference is relatively large, the bolt nut connections show excellent anti-loosening performance and the improvement of fatigue life is much smaller than that when the pitch difference is small. In one word, significant improvement of the fatigue life and anti-loosening performance cannot be achieved at the same time only by introducing a pitch difference to a common bolt nut connection. In this study, the mechanism of anti-loosening performance of pitch difference bolt nut connections is investigated. Besides, the effect of some geometric aspects of pitch difference bolt nut connections, such as thread root radius and nut height, are investigated to obtain high fatigue life and excellent anti-loosening performance at the same time. This thesis is composed of a total of 5 chapters and organized as follows. In Chapter 1, the history of bolt nut connections and some typical failures of bolt nut connections are introduced. Besides, previous researchers’ efforts on improving fatigue life and anti-loosening performance of bolt nut connections are summarized. In addition, previous researches on pitch difference nut of our team are introduced. In Chapter 2, the mechanism of anti-loosening performance of the pitch difference nut is studied by Junker experiments and finite element simulation. Loosening experiments of M12 bolt nut connection with a pitch difference of α=0, 30, 40, 50μm are done by Junker’s type loosening test. According to DIN 25201, when the baseline fastener loosens after 300±100 cycles vibration, and the residual axial force of the test specimen is no less than 80% with 2000 vibration cycles, the anti-loosening performance of the test specimen can be regarded as excellent. It is found that α = 40, 50μm meet the standard. Although α=35μm does not meet the criteria, the clamping force F=4.0kN 22%% is maintained at vibration number n =1500, and the loosening resistance is substantially maintained. The relationship between clamping force and tightening torque （F−T relation） and the loosening process of pitch difference nuts are analyzed FEM. It is found that at the early stage of vibration, the elastic energy releases and the clamping force decreases fast. With the loosening of the nut, two sides of the nut contact the bolt thread. Owing to the elastic energy stored between two ends of the nut, a tightening process occurs and the tightening angle in every cycle increases with the decrease of clamping force. Thus, the decrease rate of the clamping force becomes slower with the increasing the vibration number, when the clamping force decreases to a certain value, the tightening angle almost equals to the loosening angle in one vibration cycle, i.e., a residual clamping force and a residual prevailing torque remains steady and loosening of the bolt nut connection will not continue. In Chapter 3, the effects of thread root radius on fatigue and anti-loosening performance of pitch difference nut are studied experimentally and analytically for M16 bolt nut connections. It is found that by enlarging the root radius from ρ to 2ρ0, the fatigue life of bolt nut connections can be improved by more than 30% because both stress amplitude and mean stress at the thread roots can be reduced. In addition, by introducing a suitable pitch difference between the root radii enlarged bolt-nut connections, the fatigue limit can be further improved by 25%. This is because when no pitch difference, the crack initiation always occurs at No.1 or No. 2 threads close to the bolt head, causing the final failure; however, under a suitable pitch difference, the crack initiation occurs at No.6 or No.7 threads far away from the bolt head. Good anti-loosening performance can be expected for the bolt-nut connections having enlarged root radius because the prevailing torque Tp = 19Nm and the residual prevailing torque Tup = 10Nm are not smaller compared to other special bolt-nut connections. Since those values are not smaller compared to other special nuts such as U-Nut and Super Slit Nut, good anti-loosening performance can be expected for the enlarged root radius of bolt-nut connections under the suitable pitch difference. In Chapter 4, the effect of nut height on anti-loosening performance and fatigue life of M12 bolt nut connections having a pitch difference between the nut and the bolt is investigated in this study. The relations between tightening torque and clamping force under different nut heights and pitch differences are obtained by experiments and finite element analysis. It has been found the anti-loosening property for pitch difference bolt nut connections depends on the produce of pitch difference and thread number n. Besides, the stresses at the thread roots of the bolt are analyzed using the axisymmetric model. When the pitch difference is less than 25μm, the fatigue life increment remains steady if increasing the nut height from 10.5mm to 14.0mm. It can be concluded that by increasing the nut height and decreasing the pitch difference at the same time, a better combination of anti-loosening performance and fatigue properties can be obtained. Chapter 5 provides the major conclusions, the most significant outcomes and contributions, and suggestions for future works.

九州工業大学博士学位論文 学位記番号：工博甲第528号 学位授与年月日：令和3年9月24日

令和3年度

九州工業大学博士学位論文（要旨）学位記番号：工博甲第528号 学位授与年月日：令和3年9月24日

1. Introduction||2. Mechanism of anti-loosening performance of pitch difference bolt nut connections under transverse vibration||3. Root radius effect on fatigue strength and anti-loosening performance of pitch difference bolt nut connections||4. Nut height effect on fatigue strength and anti-loosening performance of pitch difference bolt nut connections||5. Conclusions and suggestions for future works