Thread Content
Selection factors: Thread profile, thread series, and thread fit grade are the three factors to consider when selecting a thread. Thread profile: China adopts the metric standard for ordinary threads, and the design thread profile of its internal threads is identical to the basic thread profile ； As for the profile design of external threads, a minimum root radius is required only when the mechanical property is greater than or equal to 8. For imperial series threads, there are many different thread profiles, but the basic profiles commonly used in pressure vessels are UN and UNR. The UNR thread profile requires that the bottom contour of the external thread be in the form of a specified arc, whereas the UN thread profile, as originally designed, can have a flat bottom at the base of the external thread. In reality, due to modern thread machining methods and tools, many UN threads are manufactured as UNR threads 100%. When threading by turning, if a round-bottomed thread is used, it must be specified as an UNR thread; otherwise, an UN thread might be provided. For pressure vessel designers, when choosing metric threads, in the vast majority of cases it is not necessary to consider the thread profile; only the mechanical property grade of the fastener needs to be taken into account. However, when choosing imperial series threads, it is important to note that high performance requirements are imposed on fasteners; especially in situations where fatigue loads are likely to occur, it is necessary to shape the root profile of the external thread into a specified arc. The root radius of external threads can slightly increase the static tensile strength of fasteners, but the main purpose of the root radius is to improve the fatigue resistance of fasteners, which is an important criterion for deciding whether to use UNR threads. In the standards for metric ordinary threads, threads are classified as coarse or fine based on their pitch; for example, the pitch of the coarse thread for M48 is 6, while for the corresponding fine thread, the pitch is at most 4. The imperial series of threads is a set that is distinguished within a diameter series by the diameter per tooth count, expressed as diameter/tooth count. Among them, there are 3 commonly used and important types of pressure vessel threads: coarse thread (UNC), fine thread (UNF), and 8-thread (8-UN). Grade of fit: For ASME imperial threads, Grade 1 threads (1A, 1B) are used in applications where free tolerances and allowances are required, so that assembly can be facilitated even with relatively shallow threads. This grade enables fast and easy assembly and disassembly, which is an excellent design concept for mechanical fasteners. Only for sizes of 1/4 inch and above. It is less commonly used in pressure vessels. Grade 2 threads (2A, 2B) are the most widely used; among them, 2A external threads have a margin, while grade 2B internal threads do not have any margin. This type of thread is widely used in the pressure vessel industry. Grade 3 threads (3A, 3B) are used for high-strength fasteners that require high levels of tightness and precision in assembly, and they are less commonly used in the pressure vessel industry. Although British thread standards can, in principle, be used in combination – for example, a 2A external thread can be paired with 1B, 2B, and 3B internal threads – their combined use is avoided as much as possible in the pressure vessel industry. In our country, the fits for threads are generally defined by directly using tolerance grade symbols. Whether it is imperial or metric threads, the key to the thread fit grade lies in using thread tolerances to determine the degree of tightness of the thread fit. There is a misconception regarding thread fits: many people believe that the tighter the tolerances, the better the fit will be, and thus the better the quality and performance of the assembly. But it turns out that this understanding is usually incorrect.