13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Table 1. Fitting distance δ without any prevailing torque (a) Without clamped plate (b) With clamped plate Figure 2. Fitting status [mm] When α=αsmall, the nut can be screwed to the requested position for clamped plate (δ=25.5mm) without any prevailing torque. For the cases of α=αmiddle and α=αlarge, a prevailing torque is necessary to carry the nut touching the clamped plate. Next, we will focus on the relationship between axial force and prevailing torque of the bolt in the tightening process. Figure 3 shows the experimentally obtained torque-axial relationship of the bolt during the tightening process until the nut reaches the position in Figure 2(b). Whether the prevailing torque is needed or not, for all the nuts, the final tightening torque is set as the same value of 70 N·m. For the normal bolt and nut, the axial force produces just after the torque is applied on the nut. At the point of torque equals 70 N·m, the axial force, in other words, the clamping force of the bolt, reaches 24 kN. The case of α=αsmall has the same torque-axial force relationship with α=0. When α increases to αmiddle, a prevailing torque of 40 N·m is needed before the nut becomes contacting with the clamped plate. Under the same tightening torque of 70 N·m, the axial force of bolt is reduced by 4 kN compared with the normal case. When α=αlarge, the prevailing torque increases accordingly, on the other hand, the axial force decreased significantly to 8 kN. Figure 3. Relationship between torque and axial force Pitch difference α (µm) δ (mm) 0 42 αsmall 25.5<δ<42 αmiddle 16<δ<25.5 αlarge δ<16 α=0 α=αsmall α=αmiddle α=αlarge Axial force F (kN) Torque T ( N m⋅ ) α=0<αsmall<αmiddle<αlarge
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