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Knowledge of Hexagon Bolts

Aug. 24, 2020

Hexagon bolts are one of the most commonly used bolts in standard parts.

Thread elements:

The thread type, major diameter, pitch, number of threads and direction of rotation are called the five elements of Hex Bolts.

Tooth type

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

Major path, trail and middle 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 of stainless steel bolts is represented by d, and the major diameter of the internal thread is represented by d. D said. 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 tooth width and 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) means.

Hex Bolts

Hex Bolts

Number of lines

The number of spirals forming a thread is called the number of threads. There are single-thread and multi-thread threads. Multi-thread threads are evenly distributed in a section perpendicular to the axis.


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: S=n·P


Viewed along the axis, the clockwise rotation of the hexagonal bolt becomes a right-hand thread, and the counterclockwise rotation is called a left-hand thread.

Anti-loose techniques

1. Anti-loose friction

This is the most widely used anti-loosening method. This method produces a positive pressure between the thread pairs that does not follow the external force to produce a friction that can restrain the relative rotation of the thread pairs. This positive pressure can be achieved by pressing the thread pair axially or simultaneously in both directions. For example, elastic washers, double nuts, self-locking nuts and nylon insert lock nuts are adopted. This kind of anti-loosening method is more convenient for the disassembly of the nut, but in the environment of impact, vibration and variable load, the pre-tightening force of the bolt will be reduced due to slack at the beginning. As the number of vibrations increases, the lost pre-tightening force is dull. The increase will eventually cause the nut to loosen and the threaded connection to fail.

2. Mechanical anti-loose

Common mechanical anti-loosening methods: use split pins, stop gaskets, and string wire ropes. The mechanical anti-loosening method is more reliable, and the mechanical anti-loosing method should be used to deal with important connections.

3. Permanent anti-loose

Commonly used permanent anti-loose are: spot welding, riveting, bonding and so on. In this method, most of the threaded fasteners are crushed during disassembly and cannot be reused.

4. Riveting and anti-loosening

After tightening, methods such as punching, welding, and bonding are used to make the thread pair lose the movable pair characteristics and the connection becomes impossible to disconnect. The disadvantage of this method is that the bolt can only be used once, and the disassembly is very difficult, and the bolt pair must be crushed before disassembly.

5. Anti-loose structure

The anti-loose structure does not rely on manpower outside the circle, only its own structure. The structural anti-loosening method, namely the Down's thread anti-loosening method, is also the best anti-loosening method that has begun to be advanced and the results are not known to most people.

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