Relationship between bolt tightening method and preload

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There are three common methods for tightening large-diameter bolts: torque method, tension method, and heating method. The torque method works by applying a certain amount of torque to the nut, causing it to rotate; the nut then exerts force on the bolt through the inclined surfaces of its threads, resulting in axial stretching of the bolt. The bolt rebounds within its elastic range, and this rebounding force constitutes the preloading force, which in turn generates a clamping force on the flange. Commonly used torque application tools include: open wrenches, set-screw wrenches, electric torque wrenches, hydraulic wrenches, etc. Due to the high friction between the threaded surfaces, a large portion of the applied torque is required to overcome this friction. Using this method requires an understanding of the lubrication condition of the threaded friction surfaces; under different lubrication conditions, the preload obtained when the same torque is applied varies significantly. For tightening by stretching, a tool (bolt stretcher) is first used to pre-stretch the bolt to the desired degree of stretch (the degree of stretch corresponds to the preload force), after which the nut is tightened with a very slight force so that it fits against the flange surface. Once the pre-stretching tool is removed, the bolt is thus tightened. Since the effects of the thread inclined surfaces and the friction coefficients between the nut and the flange end face are minimal, the loading pre-tension is quite close to the final preload required; therefore, nuclear reactor bolts are generally tightened simultaneously using multiple tensioners in order to achieve a more precise preload. The heating method involves first stretching the bolt through heating, and then tightening the nut with a smaller force; once the bolt cools down, it is pre-tightened. The latter two methods can both avoid the influence of friction between the threaded inclined surface and the nut and the flange end face. The preload is quite accurate.