In the production of steel pipe, the single length is usually less than 14m, and some service life of steel pipe is often much longer than this length. For example, in geological drilling and oil drilling, the drilling depth is usually more than 1000 meters, up to 7000 meters; the transportation line for oil, natural gas, water, alkali, minerals and other transportation lines is several kilometers or even hundreds of kilometers. Therefore, it is necessary to use the corresponding coupling (nozzle) to connect the two ends of the steel pipe thread. Thread connection requires high connection strength and good air tightness to prevent steel fracture and leakage of transport media. In order to meet the needs of various uses, various forms of threads have been developed in the world, but they mainly use the following three types:


(1) dome triangle thread. It is characterized by simple threading. But the thread is not uniform and deformed, so the connection strength is low, only the thread side seal, sealing performance is poor.
(2) Spiral trapezoidal (British) threads. It has the characteristics of high thread accuracy, high processing requirements and high connection strength. Relying on the compression seal of the side and top of the thread, the sealing performance is relatively high.
(3) Straight connection threads. Its characteristic is not to take over, but the side of the pipe should be thicker, one end of the automobile thread, the other end of the automobile thread, processing is more difficult. Thread precision is high, connection strength is high; relying on top and side seals, sealing performance is good.