The national standard stipulates that the tolerance of the external thread of tubing thread is within the length of L4-g and the maximum allowable error of 25.4mm for any section. The tolerance value of every 25.4 mm is ±0.076 mm.
When assembling tubing threads, under the condition of the same profile parameters, when the pitch error is large, the hand-tight connection contacts will produce three states:
1. The taper of the internal and external threads basically coincides, resulting in an increase in the hand-tightness gap between the top of the external threads and the bottom of the internal threads and between the top of the internal threads and the bottom of the external threads.
2. The taper of internal and external threads does not coincide. Several threads with larger pitch error are located in the joint sealing position, which also causes the increase of hand tightness gap.
3. The taper of internal and external threads does not coincide, but several threads with larger pitch error are not located in the connecting sealing part, while the hand tightness gap between the top of the external threads and the bottom of the internal threads connecting the sealing part does not change much.
According to the relationship between pitch deviation and diameter, when the profile angle of thread is 60°, the diameter equivalent of pitch deviation is 1.732 △P (△P is the pitch deviation between any two teeth). Whether the pitch deviation is positive or negative, the diameter equivalent always enlarges the pitch of external thread and decreases the pitch of internal thread, which is reflected in the connection of tubing threads. The hand tightness gap between the threaded teeth bottom increases the diameter equivalent value.
According to the pitch tolerance stipulated in the national standard, assuming that the accumulative pitch error of the two interconnected inner and outer tubing threads is 0.152 mm, the diameter equivalent is 1.732*0.152=0.2632 mm, and the radius direction is 0.1316 mm. That is to say, when the tubing threads are connected, the hand tightness gap between the top and bottom of the external threads will be increased by 0.1316 mm due to the pitch error.