The relationship between the angle of the nut and the elongation of the bolt is extended by heating. Since the elongation of the bolt is not easy to measure, it can be converted into the rotation angle of the nut. By rotating the nut to a suitable angle, the bolt can be pre-cooled after the bolt is cooled. tight. The pitch P of the nut and the bolt elongation Δl, the relationship of the rotation angle of the nut: Δl = P / 360 °.
Application examples show that the F2 reducer of the continuous casting sheet mill manufactured by the company is made of M125×6 stud bolts. The stud bolts are made of 42CrMo, and the quenching hardness is 255~269HB. The total length is 2210mm, and the calculated length l=1830mm. Since the pre-tightening torque required by the bolt is 8431kgm, and the wrench space is limited, the hydraulic wrench cannot be used to pre-tighten the bolt. Therefore, the method of pre-tightening by heating is adopted. First, drill a 25mm through hole in the center of the stud bolt. First tighten the nut to the surface of the box. Then use the flame or steam to heat the bolt through the through hole in the center of the stud. The stud is heated and the line expands. Produces elongation. According to the amount of elongation, the angle of rotation of the nut is calculated, and the nut is tightened and rotated to a predetermined angle, and the bolt is pre-tightened after the bolt is cooled.
Calculate the pre-tightening force from the pre-tightening torque. The pre-tightening torque T=8431kgm=82623800Nmm, take K=0.21[1], the nominal diameter of the thread d=125mm. Bring the above parameters into the formula (1) to obtain the pre-tightening force: F0 =T/(Kd)=3147573N.
Calculate the elongation of the bolt from the preload force. The calculated length of the bolt is l830mm, the elastic modulus of the material is E=206GN/m2[1], the diameter of the M125×6 bolt is d1=118.505mm, and the diameter of the bolt is drilled. D2=25mm, cross-sectional area of ​​bolt: A=π(d12-d2)/4=π(118.5052-252)/4=0.01053m2.
The elongation of the bolt according to formula (6): Δl=F0l/(EA)=2.655mm2.3 Determination of heating temperature Calculated by room temperature 20°C, the linear expansion coefficient of the material [1]: α=11.8×10-6 °C-1. Temperature rise according to formula (7): Δt = Δl / (αl) = 123 °C. From this, it can be seen that the heating temperature of the bolt is t=20+Δt=143°C.
The rotation angle of the nut is determined by the pitch P=6 mm of the M125×6 bolt, and the bolt elongation Δl=2.655 mm. According to the formula, the nut rotation angle is:=360Δl/P=159.3°. Through the above calculations, to achieve the pre-tightening of the bolt, the bolt can be heated to 143 ° C, and then the nut is rotated 159.3 °, after the bolt is cooled, the pre-tightening torque required for the bolt connection can be achieved.
The conclusion is too heavy. In the F2 reducer manufactured by the company, the bolts in the center of the bolt are used for pre-tightening the bolts on the upper and lower casings, which solves the problem that the hydraulic wrench cannot be used due to the space limitation of the wrench. . The equipment has been installed and operated for one and a half years, and no loose bolts have occurred. Therefore, for large-diameter bolts, especially when the wrench space is limited, the method of heating is used to pre-tighten the large-diameter bolts, which not only solves the narrow working space. The inconvenience is brought about, the strength of the fastening bolt is greatly reduced, and the work efficiency is greatly improved, which is a good pre-tightening method.
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