Types and size calculation of triangular threads
Triangular threads are the most widely used thread type in mechanical engineering. Their triangular profile offers advantages such as easy processing, reliable connections, and excellent interchangeability. They are primarily used for fastening connections and in some transmission applications. There are numerous types of triangular threads. According to national standards, they can be divided into common threads (metric triangular threads), imperial triangular threads, and pipe threads (some with triangular profiles). Based on the thread angle, they can be categorized as 60° triangular threads and 55° triangular threads. Different types of triangular threads have different dimensional parameters and applications. Dimension calculations must be performed according to the corresponding national standards to ensure interchangeability and connection quality. Understanding the types of triangular threads and dimensional calculation methods is crucial for thread design, processing, and inspection.
Common threads (also known as metric triangular threads) are the most widely used triangular threads in my country, with a 60° pitch angle. According to the national standard GB/T 192-2003, “Basic Profiles of Common Threads,” the basic profile is formed by truncating the top and bottom of an original triangle. These threads feature identical basic dimensions for both internal and external threads, and tolerance-controlled clearances. Common threads can be categorized by pitch as coarse or fine. Coarse threads have a larger pitch and higher strength, making them suitable for general fastening connections, such as connecting housings in mechanical equipment. Fine threads have a smaller pitch and better self-locking properties, making them suitable for thin-walled parts, precision adjustment mechanisms, and applications subject to impact loads, such as pressure vessel end cap connections and machine tool feed screws. The nominal diameter of a common thread is the major diameter of the thread (external or internal). The commonly used nominal diameter range is 1-100mm. The pitch increases with the nominal diameter. For example, a coarse thread with a nominal diameter of 10mm has a pitch of 1.5mm, while fine threads have pitches of 1.25mm, 1mm, and 0.75mm. Common threads are marked as “M × nominal diameter × pitch.” For example, “M16 × 2” indicates a fine common thread with a nominal diameter of 16mm and a pitch of 2mm, while “M20” indicates a coarse common thread with a nominal diameter of 20mm (pitch of 2.5mm).
The British triangular thread is a common thread type used in British countries. It has a flare angle of 55° and its dimensions are expressed in inches. The nominal diameter is the major diameter of the thread (in inches), and the pitch is expressed in threads per inch (threads/in). British triangular threads can be divided into Whitworth and British Standard Whitworth (BSW) threads based on their application. Whitworth threads are the most commonly used British thread type, suitable for general fastening. They have a flare angle of 55°, with rounded crests and roots, offering excellent strength and interchangeability. The nominal diameter of British threads ranges from 1/4 to 6 inches, with the number of threads per inch decreasing as the nominal diameter increases. For example, a 1/2-inch British thread has 12 threads per inch (pitch 2.117mm), while a 1-inch British thread has 8 threads per inch (pitch 3.175mm). Imperial threads are designated “nominal diameter (inch) – threads per inch.” For example, “1/2-12” indicates an imperial thread with a nominal diameter of 1/2 inch and 12 threads per inch. Because imperial threads differ from conventional threads in their thread angles and unit of measurement, they are not interchangeable. Therefore, special care must be taken to distinguish between the two when repairing imported equipment and conducting international trade.
The non-threaded pipe thread and the threaded pipe thread part of the pipe thread are triangular tooth type with a tooth angle of 55°. They are mainly used for pipe connection. The nominal diameter is the nominal diameter of the pipe (inch).
