Fastener knowledge: six elements of thread

The thread profile is used to determine the geometry of the thread. Its thread elements mainly include five elements: profile, large, (medium and small) diameter, number of threads, lead, direction of rotation and working height, and only these six Only the internal and external threads with the same elements can be screwed together.

1. Tooth shape: On the section passing through the thread axis, the profile shape of the thread is called the tooth shape. The angle between two adjacent flanks is called the profile angle. Commonly used common thread has a triangular profile with a profile angle of 60°.

2. Major diameter, minor diameter and intermediate diameter: Major diameter refers to the diameter of an imaginary cylindrical or tapered surface that coincides with the top of the external thread and the bottom of the internal thread. The major diameter of the external thread is represented by d. The major diameter of the internal thread is represented by D. The minor diameter refers to the diameter of an imaginary cylindrical or tapered surface that coincides with the root of the external thread and the crest of the internal thread. The minor diameter of the external thread is represented by d1, and the minor diameter of the internal thread is represented by D1. Between the major diameter and the minor diameter, imagine a cylindrical surface (or conical surface).In its axial section, the width of the tooth and the groove width on the plain line are equal, then the diameter of the imaginary cylindrical surface is called the pitch diameter, and d2( Or D2) said.

3. Line number: The number of spiral lines forming a thread is called the line number. There are single-thread and multi-thread threads. The multi-thread threads are evenly distributed in the section perpendicular to the axis. (Guide: Knowledge about the material of stainless steel fasteners)

4. Lead: The axial distance between two adjacent teeth on the pitch line corresponding to two points is called the pitch. On the same spiral line, the axial distance between two adjacent teeth corresponding to two points on the median diameter line is called the lead. The relationship between the number of lines n, the pitch P, and the lead S is: Su003dnP

5. Direction of rotation: Seen along the axis, the thread that rotates clockwise becomes a right-hand thread, and the thread that rotates counterclockwise is called a left-hand thread.

6. Working height: the distance between the overlapping part of the two-phase matching thread profile in the direction perpendicular to the thread axis, etc. The nominal diameter of the thread is the nominal diameter of the pipe thread, except for the pipe inner diameter, and the outside diameter is the nominal diameter. The thread has been standardized, and there are metric (metric) and imperial systems. The international standard adopts the metric system, and China also adopts the metric system.

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