Choosing the tightening method of the threaded connection should be based on the characteristics of the various tightening methods, and the tightening method should be reasonably selected according to the requirements of the initial preload force dispersion degree, the preloading force and the use conditions. The requirement for the degree of dispersion of the initial preload force is usually expressed by the tightening factor (Q), which is also commonly referred to as the initial preload force dispersion. Although the initial preload force dispersion is different due to the different tightening tools and the accuracy, the initial preload force dispersion is different when tightening due to the different tightening methods. Therefore, the tightening factor is different. It is an important condition for selecting the screw tightening method.
Torque method
The torque method is a method of tightening control in the elastic zone using a linear relationship between torque and preload. This method controls only a certain tightening torque when tightening, and therefore, because the method is easy to operate, it is a generally conventional tightening method. However, since about 90% of the tightening torque acts on the friction of the thread and the friction of the bearing surface, the true acting on the axial preload is only about 10%, and the dispersion of the initial preload is the friction during the tightening process. The degree of control of the factors varies, so the degree of dispersion of the tightening method is large, suitable for the fastening of general parts, and is not suitable for the connection of important and critical parts.
Corner method
The corner method is a method of controlling the initial preload force by using a certain tightening angle as an index to rotate the bolt relative to the nut at an angle when tightening. This tightening method can be used in the elastic zone and the plastic zone. It can also be seen from the standard Figure 3 "Relationship between the tightening angle and the preload" that when the slope of the QF curve changes abruptly, the dispersion of the preload force increases with the setting error of the tightening angle. Therefore, in the case where the rigidity of the connected member and the bolt is high, the fastening of the elastic region is disadvantageous; when the plastic region is fastened, the dispersion of the initial preload force mainly depends on the yield point of the bolt, and the corner angle The error has little effect on it, so the fastening method has the advantage of maximizing the strength of the bolt (ie, a higher preload).
Torque slope method
The torque slope method is a method of controlling the initial preload force by using the change in the torque slope value in the QF curve as an index. This tightening method generally uses the yielding and tightening axial force of the bolt as a target value for controlling the initial preload. This tightening method is generally used when the initial preload force dispersion of the bolt is small and the bolt strength can be utilized to the utmost. However, since the tightening method has basically the same control of the initial pre-tightening force as the cornering method of the plastic zone, strict control of the yield point of the bolt is required. Compared with the cornering method of the plastic zone, the tightening method has fewer problems in the plasticity of the bolt, such as repeated use, and has certain advantages. However, the fastening tool is complicated and expensive.
It should be noted that the tightening method causes plastic deformation of the bolt portion and the threaded rod portion when the plastic region is tightened. Therefore, the applicability should be considered in the case of poor plasticity of the bolt and repeated use of the bolt. In addition, when the pre-tightening force is too large and the connected member is damaged, the yield point of the bolt and the upper limit of the tensile strength must be specified.
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