The hydraulic tension pre-tightening uses the hydraulic pressure to stretch the bolt to a predetermined elongation through a special tensioner, so that the nut can be tightened without load, and the pre-tightening force of the bolt can be accurately ensured. The pre-tightening method of the four bolts is actually analyzed from the stress condition of the bolts. There are only two kinds of them: one is to tighten the nut with a mechanical and hydraulic wrench, and the screw is inclined by the spiral sliding surface to make the bolt elongate; and the electric heating and hydraulic stretching pre- The tight method is to use the thermal expansion force and hydraulic pressure to extend the bolt, and then easily turn the nut. When the external factor is removed, the pre-tightening bolt is in an elastic deformation state. The difference between the two is that during the pre-tightening process, the thread is always subjected to tensile stress and torsional shear stress much higher than in the working state, while the latter is separated in the two stages of elongation and contraction. The threads are not subject to any additional stresses during operation.

The method of using a torque wrench or a fixed torque wrench to control the pre-tightening force is simple and convenient, but the accuracy is poor (the pre-tightening torque is greatly affected by the friction force during the pre-tightening process), and is not suitable for a large bolt connection. Therefore, in recent years, the method of measuring the elongation of the bolt is usually used to control the pre-tightening force of the bolt, and the elongation can be calculated according to the predetermined value of the pre-tightening force. For the large-diameter bolt, the heating or hydraulic stretching pre-tightening method is adopted. Perform so that the bolts reach the desired preload. Generally speaking, in the early bolt pre-tightening, the mechanical method was purely manpower and felt. With the step-by-step improvement of the pre-tightening force, the torque wrench, the labor-saving wrench, the hydraulic wrench and the electric heating method were gradually developed. , hydraulic stretching method.

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