13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- to the normal bolt. For the case of α=αmiddle, as Figure 12 shows, because of the large pitch difference, it is seen that the mean stress values for most threads is over the yield stress the material. Conclusions In this study, a slightly pitch difference is designed between bolt and nut. Four different levels of pitch difference α=0, α=αsmall, α=αmiddle and α=αlarge are experimentally considered. The relationship between prevailing torque and axial force for each case of α has been analyzed as the first step. Next, the loosening experiment and the fatigue experiment have been performed successively to study the anti-loosening performance and the fatigue strength of bolt. Finally, by using the finite element method, the effect of pitch difference on the stress status of the bolt thread bottom has been numerically analyzed. The conclusions can be summarized as follows: (1) It is found that α=αmiddle is the most desirable pitch difference to realize both the anti-loosening and clamping abilities. (2) It is found that α=αsmall is the most desirable pitch difference to extend the fatigue life of the bolt and nut. Compared with the normal bolt and nut, the fatigue life for α=αsmall can be extended by about 1.4 times and the fatigue life for α=αmiddle can be extend ed by about 1.2 times. (3) According to the FEM analysis, it is found that the stress amplitude at the No.1 thread bottom decreases significantly for α=αsmall, and due to the large plastic deformation at No.7 and No.8 thread bottoms, the stress amplitude here is almost the same as other thread bottoms. Acknowledgements This study is supported by Hidetoshi TAMASAKI, in KHI corporation, Moji-ku Kitakyushu, Japan. We also show great gratitude to Yang Yu, who provided much valuable work during the writing of this paper. References [1] Daiki Kogyo KK., 2004, Super Slit Nut, Japan Patent (in Japanese) 2004–218674. [2] Hard Lock Kogyo KK., 2002, Hard Lock Nut, Japan Patent (in Japanese) 2002–195236. [3] Minamida KK., 1999, Anti-loosening Nut, Japan Patent (in Japanese) H11–177902. [4] Kazuo, M., Stress Analysis of a Bolt-Nut Joint by the Finite Element Method and the Copper-Electroplating Method. The Japanese Society of Mechanical Engineers, 19(1976) 360–368 [5] Nishida, S., et al, 1980, Screw Connection Having Improved Fatigue Strength, United States Patent 4,189,975. [6] Noda, N.-A., Kuhara, M., Xiao, Y., Piao, Q C, Takase, Y. and Nishida, S.-I., Anti-loosening performance of bolts and nuts which have slightly different pitches. The Japan Society of Mechanical Engineers, 46 (2008) 71–72 [7] Xiao, Y., Noda, N.-A., Morita, M., Nishida, S.-I., Stress reduction effect of bolts and nuts which have slightly different pitches. The Japan Society of Mechanical Engineers, 62 (2009) 29–30
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