The thread is divided into a triangular thread, a rectangular thread, a trapezoidal thread, and a zigzag thread according to its sectional shape (tooth shape).  Helical Spring,Cylindrical Spring,Compression Spring Compression Spring,Extension Spring,Torsion Spring Co., Ltd. , http://www.nssprings.com
The triangular thread is mainly used for coupling (see threaded connection), and the rectangular, trapezoidal and zigzag threads are mainly used for transmission.
The external thread of the thread distributed on the outer surface of the mother body is called the internal thread on the surface of the mother body. The thread formed on the cylindrical body is called a cylindrical thread, and the thread formed on the cone body is called a tapered thread. The thread is divided into two types of left-handed and right-handed in the direction of the spiral, and generally uses a right-handed thread.
Threads can be divided into single-line and multi-wire, and the connection is mostly single-line; when used for transmission, it requires fast or high efficiency, and adopts double or multi-line, but generally does not exceed 4 lines.
The main parameters of the cylindrical thread are outer diameter (d), inner diameter (d1), median diameter (d2), pitch (t), number of lines (n), lead (s = nt), angle of elevation (λ) and tooth shape. Angle (α) and so on. Except for the pipe thread with the inner diameter of the pipe as the nominal diameter, the other threads have the nominal diameter of the outer diameter.
The thread pair whose thread angle is smaller than the friction angle does not loosen under the action of the axial force, and is called self-locking, and its transmission efficiency is low.
Among the cylindrical threads, the triangular thread has good self-locking performance. It is divided into coarse and fine teeth, and the general connection is mostly with coarse thread. Fine teeth have small pitch, small angle of lift and better self-locking performance. They are often used in thin-walled tubes of small parts, with vibration or variable load connections, and fine-tuning devices. Pipe threads are used for tight fitting of the pipe. Rectangular threads have high efficiency, but they are often difficult to grind, and the inner and outer threads are difficult to screw and center, so they are often replaced by trapezoidal threads. The working edge of the zigzag thread is close to the rectangular straight edge and is used to withstand the unidirectional axial force.
The shape of the conical thread is triangular, mainly relying on the deformation of the tooth to ensure the tightness of the thread pair, and is mostly used for the pipe fitting.
Thread type classification: according to the sectional shape