The teeth of tubing threads specified in GB9253.3-88 have the following characteristics:
The taper of tubing thread is 1:16, and the angle bisector of thread tooth is perpendicular to the thread axis.
The shape of the top and bottom of the teeth is circular arc, and the radius of the arc is the same as that of the flattened height of the top and bottom of the teeth, because the angle of the tooth is 60。.
The basic hand tightness gap between the top of external thread and the bottom of internal thread or between the top of internal thread and the bottom of external thread is 0.076mm. According to the regulation of GB9253.3-88 on the thread size of tubing connection and non-thickened tubing and Extra-thick tubing, the number of tightly spaced teeth A of tubing thread connection is 2 teeth, while when machine tightening, the end face of coupling or internal thread can be tightened to the plane of vanishing point, that is, the apparent number of remaining teeth is zero after machine tightening.
It can be inferred that, due to the taper of 1:16, the tooth size of the external thread increases by 1:16/2*6.35=0.1984357mm in the radius direction in any axial section of the coupling or the internal thread. In this way, the theoretical interference between the top of the external thread and the bottom of the internal thread or between the top of the internal thread and the bottom of the external thread is 0.198-0.076=0.122 mm after the tubing thread connection is machine tightened. This is the reason why tubing threads are tightly connected and well sealed.