Thread feature code and marking example
Thread characteristic codes and markings are standardized identifiers that indicate parameters such as thread type, size, and precision grade. They serve as a crucial basis for thread design, manufacturing, inspection, and interchangeability. Correctly understanding and using thread characteristic codes and markings ensures the reliability and interchangeability of threaded connections, avoiding assembly errors and performance issues caused by unclear markings. Thread characteristic codes, consisting of letters and numbers, represent the thread type (e.g., common thread, trapezoidal thread, pipe thread, etc.). Markings, consisting of the characteristic code, dimensional parameters, tolerance zone code, engagement length code, and direction of rotation, fully describe all thread parameters. Both international and domestic standards clearly define thread characteristic codes and marking methods, such as the national standards GB/T 192-2003, “Basic Profiles of Common Threads,” and GB/T 5796-2005, “Trapezoidal Threads.”
Common threads (also known as metric threads) are the most widely used thread type. Their characteristic code is “M,” and they are suitable for general mechanical connections. Common thread designations consist of the characteristic code, nominal diameter × pitch, tolerance zone code, engagement length code, and direction of rotation. The nominal diameter is the major diameter of the thread, and the pitch is the axial distance between adjacent threads (coarse threads may not have a pitch designation). For example, “M20×2-6g-LH” indicates a left-hand, fine-pitch common thread with a nominal diameter of 20mm, a 2mm pitch, a 6g tolerance zone for the pitch and top diameters; “M16-7H” indicates a right-hand, coarse thread with a nominal diameter of 16mm, a 7H tolerance zone for the pitch and top diameters (right-hand is the default direction of rotation and may be omitted). The tolerance zone designation consists of the tolerance grade and basic deviation. The tolerance grade indicates accuracy (smaller numbers indicate higher accuracy), and the basic deviation indicates the position of the tolerance zone (uppercase letters indicate internal threads, lowercase letters indicate external threads). The thread length designations are short (S), medium (N), and long (L). Medium thread length designations may be omitted. Ordinary threads are simple and straightforward in their characteristic designations and markings, making them the most commonly used in mechanical engineering.
Trapezoidal threads are a type of thread used to transmit power and motion. Their characteristic code is “Tr,” and they are suitable for use in transmission mechanisms such as screws and nuts. Trapezoidal threads are designated by their characteristic code, nominal diameter × pitch, tolerance zone code, engagement length code, and direction of rotation. The nominal diameter is the major diameter of the thread. For single-start threads, the pitch is indicated directly; for multi-start threads, the lead and pitch are indicated (lead = pitch × number of starts). For example, “Tr40×10-7e” indicates a single-start trapezoidal thread with a nominal diameter of 40mm, a pitch of 10mm, a pitch diameter tolerance zone of 7e, and a right-handed thread. “Tr32×12 (P6)-8H-L” indicates a right-handed trapezoidal thread with a nominal diameter of 32mm, a lead of 12mm, a pitch of 6mm (double start), a pitch diameter tolerance zone of 8H, a long engagement length, and a right-handed thread. The tolerance zone designation for trapezoidal threads only indicates the pitch diameter tolerance zone, as the tolerances for the top and bottom diameters are already controlled by the pitch diameter tolerance. Trapezoidal threads have a 30° flank angle and, compared to conventional threads, offer greater load capacity and higher transmission efficiency. Their characteristic designations and marking methods are similar to those for conventional threads, but attention should be paid to the marking method for multi-thread threads.
Pipe threads are types of threads used for pipe connections. They are categorized as non-threaded and threaded, respectively. Their characteristic codes are “G” and “R” (including Rc, Rp, R1, and R2). Non-threaded (G) pipe threads rely on gaskets for sealing. Their designation consists of a characteristic code, size code, and tolerance grade. The size code is in inches (e.g., 1/2, 3/4, etc.) and does not directly indicate the major diameter of the thread. The specific dimensions must be consulted with the standard. For example, “G1/2 A” indicates a 1/2 size, A-grade precision, non-threaded pipe thread; “G3/4 B-LH” indicates a 3/4 size, B-grade precision, left-hand, non-threaded pipe thread. Threaded (R) pipe threads rely on the inherent sealing properties of the thread. Rc designates a tapered internal thread, Rp designates a cylindrical internal thread, R1 designates a tapered external thread mated with Rp, and R2 designates a tapered external thread mated with Rc. For example, “Rc1” indicates a tapered internal thread with a size code of 1; “Rp3/4” indicates a cylindrical internal thread with a size code of 3/4; and “R2 2” indicates a tapered external thread with a size code of 2 that mates with a tapered internal thread. Pipe threads do not include pitch and tolerance zone codes in their markings; their parameters are uniquely determined by the size code, a key difference from conventional and trapezoidal threads.
Corresponding designations and markings apply to other thread types, such as rectangular threads, buttress threads, and metric tapered threads. Rectangular threads, due to their rectangular profile and difficulty in machining, have been gradually replaced by trapezoidal threads. Their designation is “B,” and their marking method is similar, such as “B40×10-7e.” Buttress threads, with their asymmetrical triangular profile, are suitable for transmission mechanisms subject to unidirectional forces. Their designation is “B” (in the old standard) or “Tr” (in the new standard, shared with trapezoidal threads and distinguished by their flank angle). The flank angle must be specified during marking , such as “Tr40×10 (3°/30°)-8H” for a buttress thread with a flank angle of 3°/30°. Metric tapered threads, used in applications requiring sealing, are designated “ZM,” and their marking, such as “ZM10×1,” indicates a metric tapered thread with a nominal diameter of 10 mm and a pitch of 1 mm. In practical applications, it’s important to select the appropriate characteristic code and marking method based on the thread’s intended use and standard to ensure correct thread identification and use. For imported equipment or international projects, it’s also important to be familiar with thread marking methods based on international standards (such as ISO) to avoid misunderstandings due to discrepancies.
