The shape of the thread

Fastening connection is one of the important roles of thread, and the reliability of its connection directly affects the normal operation of mechanical equipment. With the development of industrial technology, threaded connection is widely used in various industries, according to its different uses can be divided into fastening thread, transmission thread, special thread and pipe thread. The fastening thread is widely used in the machinery manufacturing industry and is also the most widely used thread Type.

Classification and use of fastening threads

A screw thread is a continuous raised part with a specific cross-section produced on the surface of a cylinder or cone. According to the use can be divided into transmission thread, fastening thread, special thread, pipe thread. The use of fastening threads is more extensive, mostly used in the machinery and equipment industry, its role is to tighten and connect, requiring threaded connections with screwability and reliability. Fastening threads are divided into gap screw thread, large gap screw thread, tight screw thread, small screw thread and ultra-fine screw thread. Transmission threads are mainly used to transfer power and movement, such as screw and nut in machine tools. The screw drive is required to be accurate and reliable. The contact between threads should be good and wear-resistant. Special thread according to the use is divided into oil thread, aviation thread, lamp head thread, gas cylinder thread, etc. Pipe thread is a thread located on the pipe wall for connection, there are 55 degree non-sealed pipe thread and 55 degree sealed pipe thread, mainly used for pipe connection.

Classification and use of fastening threads

Clearance fit thread

The common thread is generally more common with the gap fit thread, which is a metric thread and a non-sealed thread for the connection function. The basic tooth type of ordinary thread is triangle, the tooth type Angle is 60°, and the thread feature code is M.

Large clearance fit thread

This kind of thread generally has hot dip galvanized thread and high temperature thread. In order to apply to the working environment, the thread of the component is generally coated with a thicker surface treatment, so that the thread matching gap is relatively large, in order to facilitate the rotation between the threads.

Tight fit thread

This mainly includes transition fit threads and interference fit threads. The transition fit thread refers to the possibility of gap fit or interference fit in the thread fit, and the overlap of internal and external thread tolerances. It is suitable for the double stud fixed at one end of the body, and can prevent the stud from slipping out of the body when loosening the nut at the other end. The interference fit thread relies on the interference value of the internal and external threads to generate elastic pressure between the surfaces of the parts after assembly, so as to obtain a tight connection. The connection structure is simple, the centering accuracy is good, the bearing capacity is high, and the work can be more reliable under the impact vibration load, but the threaded parts are not easy to disassemble.

Minor thread

This kind of thread has small size and poor strength. Compared with the ordinary thread, the cutting height at the small path is 0.321H to improve the tensile strength of the external thread. Small thread is mainly used for clocks, cameras, instruments and so on.

Ultra-fine thread

According to the size of the pitch, the thread can be divided into coarse thread and fine thread. Superfine thread is a kind of fine thread. This kind of thread self-locking performance is good, anti-vibration and anti-loosening ability is strong, but because of the shallow depth of the thread teeth, the ability to withstand greater tension is lower than the coarse thread. Fine thread is suitable for thin-walled cylindrical parts, such as fine thread is widely used in various optical instruments.

Basic type of common thread

Through the section of the thread axis, the thread shape formed after cutting the original triangle and the bottom at the specified height.

Geometric parameters of common thread:

Diameter and pitch

Major diameter

Is the diameter of an imaginary cylinder or cone that is tangent to the apex of the external thread or the base of the internal thread. The internal thread is represented by the letter D, and the external thread is represented by the letter d.

footpath

The diameter of an imaginary cylinder or cone tangent to the base of an external thread or the apex of an internal thread. The internal thread is represented by the letter D1, and the external thread is represented by the letter d1.

Pitch diameter

The diameter of an imaginary cylinder at the equal width of the groove and the bulge in the tooth profile through the axial section of the thread. The internal thread is represented by the letter D2, and the external thread is represented by the letter d2.

Screw pitch

The axial distance between two adjacent teeth on the thread corresponding to two points on the middle diameter line. Pitch is denoted by the letter P.

Profile Angle

Profile Angle refers to the Angle between two sides of the thread profile in the axial interface of the thread. One half of it is the tooth half Angle. The Angle of the common thread is 60°, and the half Angle of the thread is 30°.

Flank Angle

The Angle between the vertical line of the thread axis and the side of the thread. The theoretical side Angle on the common thread is 30°.

Screw length and contact height

The screw length refers to the length of the screw threads that fit together along the axis of the thread. The contact height refers to the distance of the threads that fit each other, and the distance of the joint part in the direction of the vertical thread axis.

Basic dimensions of common threads

The basic dimensions of ordinary threads are mainly the large diameter, the middle diameter and the small diameter of the thread. Picture Calculation of the thread and the diameter of the thread are inseparable from the pitch of the thread and the diameter of the thread can be calculated according to the following formula:

D2 = D - 2 x 3/8 h = D - 0.6495 P

D2 = d - 2 x 3/8 h = d - 0.6495 P

D1 = D - 2 x 5/8 h = D - 1.0825 P

D1 = d - 2 x 5/8 h = d - 1.0825 P

The above can be calculated according to the thread large diameter D/d and pitch P, the diameter and diameter of the thread can be calculated. The thread profile diagram is as follows: