13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of Pitch Difference on Anti-loosening Performance and Fatigue Strength for High Strength Bolts and Nuts Xin Chen1,*, Nao-Aki NODA1, Yu-Ichiro AKAISHI1 Yoshikazu SANO1 and Yasushi TAKASE1 1 Department of Mechanical Engineering, Kyushu Institute of Technology, Fukuoka 804-8550, Japan * Corresponding author: xin96995@126.com Abstract A slight pitch difference is considered between the bolt and nut in this study. Here, the pitch of nut is α µm larger than the pitch of bolt. In the first place, the distance δ, of nut screwed onto bolt without any prevailing torque, is experimentally and analytically obtained for each level of α. Then, the relationship between axial force and prevailing torque during tightening process is tested. According to the obtained torque-axial force relationship, for four different levels of α, the loosening experiment based on NAS3350 (National Aerospace Standard) is performed, and the loosening and dropping status of the nuts are investigated. Considering both the anti-loosening ability and the clamping ability of the nut, the desirable range of α is discussed. After that, according to the loosening experiment results, three levels of α are selected to make a further study about the effect of the pitch difference on the fatigue life of bolt. The fatigue experiment is performed and the S-N curves are obtained. Finally, the finite element method is used to make a simulation of the fatigue experiment and the mean stress and stress amplitude at each thread bottom of bolt are analyzed. Keywords Bolted Joint, Pitch Difference, Anti-loosening Performance, Fatigue Life 1. Introduction The bolts and nuts are important joining elements disassembled easily for maintenance and widely used because of low cost. To ensure the structure safety, the anti-loosening performance and high fatigue strength have been always requested, and therefore, several special nuts were invented in order to prevent self-loosening [1-3]. The previous experimental results showed that the fatigue strength may be improved under suitable pitch error existing between bolt and nut [4]. The tapered bolt named CD bolt proposed by Nishida, indicated higher fatigue strength experimentally and being widely used in structures [5]. The authors previously analyzed the anti-loosening effect and stress reduction effect for the bolt and nut having slightly pitch difference by applying the finite element method [6-7]. However, the previous studies are limited to theoretical investigations. Therefore, the effect of pitch difference on anti-loosening performance has not been experimentally verified yet, and an experimental analysis for the effect of pitch difference on the fatigue life is also necessary. In this paper, the M16 (JIS) bolt and nuts are employed and designed with a slight pitch difference. The standard M16 bolt and nut usually have the same pitch of 2000µm. Here, we assume that the nut pitch is equal or slightly larger than the bolt pitch, and we will consider four types of pitch difference, that is, α=0, α=αsmall, α=αmiddle and α=αlarge. The clearance between bolt and nut is assumed as a common dimension of 125µm. At the first place, the position where the prevailing torque appears will be discussed towards four different levels of pitch difference. Then, the relationship between axial force and prevailing torque will be experimentally obtained. To investigate the effect of pitch difference on the anti-loosening performance, the loosening experiment will be performed by using the four groups of bolt joint specimens. Considering the clamping force and loosening status of bolts and nuts during the loosening experiment, the desirable pitch difference will be discussed. Next, according to the loosening experiment results, then the
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