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To determine how much preloading force is applied, there are three methods:
Torque method (±25% accurate): use a torque wrench to measure the amount of applied torque and isolate the preload force in the equation T = F⋅D⋅K.
Turn of nut method (± 15% accurate): tighten the suit enough to bring the two mating surfaces together and then turn it through a defined angle. The amount of preload force will depend on the thread pitch, bolt length, and the material’s Young modulus.
Bolt elongation method (± 1-5% accurate): measure the elongation of the bolt from before the point in which torque is applied and after. It is assumed that preload force will linearly increase as the bolt elongates. F = k⋅ΔL. 'ΔL’ is the change in length of the bolt due to tightening. ‘k' is the bolt's stiffness, which can be calculated using the bolt's material properties and geometry k = (E⋅A)/L. 'E' is the Young's modulus, 'A' is the cross-sectional area, and 'L' is the effective length of the bolt.
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