13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- fatigue experiment will be performed to obtain the S-N curves. After that, the finite element method will be applied to simulate the fatigue experiment, and then the endurance limit diagrams will be discussed. 2. Relationship between prevailing torque and axial force For the normal bolt and nut, the nut can be screwed onto the bolt without torque until touching the clamped member. When a slightly pitch difference α is introduced on each nut pitch, the height of the nut increases accordingly, and therefore, during the screwing process, the contact status will also changes between the bolt threads and nut threads. It can be imagined that the nut threads may get stuck in the bolt threads if the pitch difference α is large enough, and in that case, a prevailing torque is necessary to continue screwing the nut to the requested position. Figure 1 shows a schematic illustration of a screwing status appeared until which the nut becomes immovable without any prevailing torque. Actually, as the nut is screwed onto the bolt, the pitch difference will be accumulated between the bolt threads. Finally, the two ends threads of nut contact with the bolt threads at the same time as shown in Figure 1. Theoretically, when a pitch difference is added on each nut pitch, the distance δ which nut be screwed onto bolt without any torque, can be obtained by formula (1). y nC δα = P 2tanθ , y x C C= tanθ (1) where δ is the distance where the prevailing torque appears, α is the pitch difference, n is the thread number of nut, θ is the thread angle, P is the pitch of bolt, Cx and Cy are the clearance between bolt and nut (see Figure 1). To clarify the mechanism, four sets specimens of bolts and nuts with different levels of α are investigated. The four levels of α has a relationship of α=0<αsmall<αmiddle<αlarge, where α=0 represents the normal bolt and nut. As seen in Figure 2 (a), the thread length of bolt is of 42mm and the height of nut is 16mm. The clamped plate, shown in Figure 2 (b), has a width of 34.5mm. It is known that without clamped plate, the normal nut (α=0) can be screwed onto the blot until δ equals 42mm. However, with increasing the pitch difference, the fitting distance δ without any prevailing torque will decrease. The relationship between δ and the pitch difference are investigated. Table 1 shows the measured value of fitting distance δ for four levels of α. Figure 1. Schematic illustration for bolt and nut which have slight pitch difference
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