Everyone knows that in fasteners, the tightening method of threaded connections is selected, and it also understands the characteristics of various tightening methods. According to the design, the initial pre-tightening force dispersion is required, the size of the pre-tightening force, and the use conditions, etc., to be reasonable Select the tightening method. The tightening factor is an important condition for selecting the thread tightening method. The fastener thread tightening method mainly includes three commonly used typical tightening methods: torque method, angle method and torque slope method.
1. Torque method
The torque method is a method of tightening control in the elastic zone using the linear relationship between torque and preload. This method only controls a certain tightening torque when tightening. Therefore, because this method is easy to operate, it is a general and conventional tightening method. However, since about 90% of the tightening torque acts on the thread friction and the friction of the bearing surface, the actual effect on the axial preload is only about 10%, and the dispersion of the initial preload is caused by friction during the tightening process, etc. The degree of control of the factors varies, so the tightening method has a large dispersion, which is suitable for the fastening of a part, but is not suitable for the connection of important and key parts.
2. Rotation Angle Method (Guide: Two Methods of Bolt Production Stress Tightening)
Rotation angle method is to rotate the bolt relative to the nut by an angle when tightening, which is called the tightening angle. A certain tightening angle is used as an index to control the initial pre-tightening force. This tightening method can be used in the elastic zone and the plastic zone. According to the relationship between the tightening angle and the pre-tightening force, when the slope of the Q-F curve changes sharply, the dispersion of the pre-tightening force will increase with the setting error of the tightening angle. Therefore, when the rigidity of the connected part and the bolt is high, the fastening of the elastic zone is unfavorable; when the plastic zone is fastened, the dispersion of the initial pre-tightening force mainly depends on the yield point of the bolt, and the rotation angle The error has little effect on it, so this fastening method has the advantage of maximizing the use of bolt strength (that is, a higher pre-tightening force can be obtained).
It should be noted that this tightening method will plastically deform the shank and threaded shank of the bolt when tightening in the plastic zone. Therefore, its applicability should be considered when the bolt has poor plasticity and when the bolt is repeatedly used. In addition, when the pre-tightening force is too large, which will cause damage to the connected parts, the yield point of the bolt and the upper limit of the tensile strength must be specified.
3. Torque slope method
The torque slope method uses the change of the torque slope value in the Q-F curve as an indicator to control the initial preload. This tightening method usually takes the yield tightening axial force of the bolt as the target value for controlling the initial pre-tightening force. This tightening method is generally used when the initial pre-tightening force dispersion of the bolt is small and the bolt strength can be utilized to the maximum. However, since this tightening method controls the initial pre-tightening force basically the same as the angle method of the plastic zone, it is necessary to strictly control the yield point of the bolt. Compared with the corner method in the plastic zone, this tightening method has fewer problems in the plasticity of the bolt, that is, repeated use, and has certain advantages. However, the tightening tool is more complicated and more expensive.
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