8 in design of bolts sometimes special bolts are used

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(8) In design of bolts, sometimes special bolts are used to have the threaded length shorter than the standard threaded length such that the threaded portion of the bolt does not extend into the joint, explain the advantage of this practice (5pts). The threaded length has a smaller area therefore increasing the stress. Therefore, in some application custom-made bolts are used such that the threaded length does not go into the joint so that the nominal area will undergo the stress. (9) Find the wrench torque required to create the recommended preload for a permanent fastener if M 30 ISO 9.8 with coarse threads is used, assume the fastener is lubricated during the assembly (15pts). The wrench torque can be obtained from Eq 8-27 : 𝑇𝑇 = 𝑘𝑘𝐹𝐹 𝑖𝑖 𝑑𝑑 From Table 8-1 page 404, the tensile area, 𝐴𝐴 𝑡𝑡 = 561 mm 2 From Table 8-11 page 427, the proof strength, 𝑆𝑆 𝑃𝑃 = 650 MPa From Table 8-15 page 431, for a lubricated assembly, 𝐾𝐾 = 0.18 Permanent assembly, therefore, 𝐹𝐹 𝑖𝑖 = 0.9 𝐹𝐹 𝑝𝑝 = 0.9 𝑆𝑆 𝑃𝑃 𝐴𝐴 𝑡𝑡 = 328.185 kN 𝑇𝑇 = 𝑘𝑘𝐹𝐹 𝑖𝑖 𝑑𝑑 = (0.18)(328.185)(0.03) = 1.772 KN. m
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