Overview and classification of bolts

A bolt is a threaded fastener with a cylindrical rod at the head. A cylindrical part with a thread at one end is called a stud and is used to mate with the nut. The other end is called the bolt head, which is called the bolt head. Bolts are usually made of metal. Bolts used in special occasions, such as electrical insulation or corrosion protection materials, also have a variety of non-metallic materials.
Bolts are used to connect or fasten in mechanical structures or building components, which are usually classified as fasteners in mechanical parts. In most cases, bolts are manufactured according to relevant national or international standards. Therefore, during operation and cycling, bolts are also classified as standard parts along with other parts produced according to mandatory standards. Bolts, screws, and nuts (also known as nuts) are often referred to in many fields as screws or non professional narratives.
The difference between the bolt and the screw lies in two aspects: one is the shape. The stud part of the bolt is strictly required to be cylindrical, which is used to install the nut, but sometimes the stud part of the bolt is conical or even central. The other is to use the function, the object of the screw is the target material, not the nut. In many cases, the bolt can work alone and can be screwed into a pre drilled hole without a nut and bolt fit. At this point, the bolts are classified into screws according to their functions. There are many kinds of bolts, but most of them are different from bolts and bolt heads.

1. Cylindrical thread can be divided into coarse thread, fine thread and inch thread, so it is called fine thread bolt and inch bolt;
2. The shape and use of bolt head are different, which can be divided into hexagon head bolt, square head bolt, semicircular head bolt, countersunk head bolt, hole bolt, T-head bolt, hook head (anchor bolt), etc.
3. Expansion bolts are derived from bolts according to the additional expansion parts.
4. According to the material and manufacturing accuracy of bolts, high strength bolts, high precision bolts and locking bolts can be obtained.
5. Bolts can be divided into more than ten grades according to performance grade, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9.  
Bolts of grade 8.8 and above are made of low carbon alloy steel or medium carbon steel and heat treated (quenched and tempered). They are usually referred to as structural steel bolts and the rest are commonly referred to as ordinary bolts. The marking of bolt performance grade consists of two parts, which represent the nominal tensile strength and yield strength ratio of bolt material respectively
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