The special place is that there is a chute on the screw, and there is a part of the deep groove near the head, that is the place where the fastening screw (pin) on the matching "sleeve" (no pattern nut) is embedded, because of the embedding of the pin, installation When turning the wrench, the bolt can be driven into the bolt ball, and the pin slides in the chute. When the bolt is tightened in place, the fastening screw is exactly at the position of the deep groove. At this time, tighten the fastening screw to make it fall into the deep groove. The effect of stopping back and preventing loosening.