Diameter and pitch series of common threads
The diameter and pitch series for common threads are fundamental specifications for thread design and processing. They define the basic dimensional parameters of threads and ensure the interchangeability and versatility of threaded connections. Common threads can be divided into connecting threads and transmission threads based on their application, with connecting threads being the most widely used. Their diameter and pitch series adhere to the national standards GB/T 193-2003 and GB/T 196-2003. Thread diameters include major, minor, and pitch diameters. The major diameter is the nominal diameter and maximum diameter of the thread, used to indicate the thread specification. For example, M10 represents a common thread with a major diameter of 10mm. The pitch diameter is a key parameter that determines the thread’s fit and lies between the major and minor diameters. The minor diameter is the minimum diameter of the thread and is often used as a basis for strength calculations. M16 common bolts produced by a standard parts manufacturer strictly adhere to the standards for major, minor, and pitch diameters to ensure that the fit with the nut meets the 6g/6H tolerance requirements.
Common threads are available in two pitch series: coarse and fine, providing flexibility in thread selection. Coarse threads have a larger pitch, greater strength, and improved assembly efficiency, making them suitable for general connections, such as fixed connections in mechanical equipment. Fine threads have a smaller pitch and better self-locking properties, making them suitable for thin-walled parts, precision connections, or applications requiring frequent disassembly, such as instrumentation. Common threads with the same nominal diameter can have a variety of pitches. For example, the coarse pitch of an M10 thread is 1.5mm, while fine pitches are available in 1.25mm, 1mm, and 0.75mm. When selecting the pitch, consider factors such as connection strength, sealing, and assembly space. For example, for connections in vibrating environments, fine threads are recommended for their improved self-locking properties; for heavily loaded connections, coarse threads are preferred for their enhanced strength. In the connection of engine cylinder blocks, a certain automobile manufacturer uses M12×1.75 coarse threads for parts subject to greater stress, and M12×1.25 fine threads for thin-walled parts, taking into account both strength and sealing.
Common threads are categorized by nominal diameter, ranging from tiny to larger diameters to meet the needs of various applications. National standards specify a nominal diameter range for common threads of 1mm to 600mm. Commonly used diameters include 1mm, 1.2mm, 1.4mm, 1.6mm, 2mm, 2.5mm, 3mm, 4mm, 5mm, 6mm, 8mm, 10mm, and 12mm. Larger diameters such as 16mm, 20mm, 24mm, and 30mm are also widely used in machinery manufacturing. The choice of nominal diameter depends on the size and load conditions of the connected parts. For example, M3-M6 threads are commonly used in small electrical equipment, while M16-M30 threads are commonly used for connections in large machinery. When designing a crane connection structure, a heavy machinery manufacturer selected an M24 coarse thread based on calculations to ensure it could withstand the maximum working load without breaking.
The diameter and pitch combinations of common threads must adhere to the preferred series specified in the standard to enhance interchangeability and standardization. The national standard categorizes thread diameter and pitch combinations into three levels: preferred, secondary, and avoided as much as possible. Threads in the preferred series offer greater versatility and cost-effectiveness. For example, M6×1, M8×1.25, and M10×1.5 are all in the preferred series. During design, threads in the preferred series should be prioritized to reduce the number of tools and measuring tools required, thereby lowering production costs. When special applications require non-standard diameter and pitch combinations, these must be clearly indicated on the drawings, and the corresponding tools and measuring tools must be custom-made. Statistics from a standardization research institute indicate that products using preferred series threads have production efficiency over 20% higher than those using non-standard series threads, and procurement costs are approximately 15% lower.
International standards exist for the diameter and pitch of common threads. For example, ISO standards are essentially consistent with my country’s national standards, ensuring thread interchangeability in international trade and technological exchange. When repairing exported products or imported equipment, it’s important to note the differences between these standards. For example, American standard threads use inch-based diameters and pitches, which are not interchangeable with metric standard threads, requiring the use of separate tools and measuring tools. With the advancement of globalization, thread standardization is becoming increasingly international, and many companies have adopted international standards for thread design and processing to meet the demands of the international market. An export-oriented company manufactures all its machinery using ISO standard common threads, with diameters and pitches conforming to ISO 724, ensuring their versatility and competitiveness in the international market. In short, common thread diameters and pitches are fundamental technical specifications in machinery manufacturing. The correct selection and implementation of these standards is crucial for ensuring product quality, improving production efficiency, and reducing costs.
