Depth analysis of the role of fastener thread tightening length

The mechanical properties of fastener threads, whether according to American standards, general standards, national standards, and daily standards, have requirements for the thread length of various performance testing items. If you design a threaded hole, you may think about it: how many buckle threads are needed to ensure a strong and qualified bolt pair? Some people say that 7 teeth is enough. When the bolt is tightened, it will become a little longer. The axial force of the entire Bolt is borne by the thread buckle, but the load on each buckle is different. Because the bolt is elongated, the ratio of the first buckle thread to the bolt axial force is the highest, and then the thread buckle is reduced in sequence, and the thread with more than six buckles will no longer bear the bolt axial force, in a state that can basically be considered free, it is no longer helpful for the firmness of the bolt pair. There are Reference Data said that the proportion of the first buckle thread is as high as 47%, and there are also "iron brother" version. According to the results of the general Photoelastic experiment, the first tooth bearing is generally above 34%.