The difference between a bolt and a screw lies in two aspects: one is the shape aspect, the stud part of the bolt is strictly controlled as a cylindrical shape, and is used to install the nut, but the stud part of the screw is sometimes tapered or even better; On the one hand, it is the application function, and the other half of the screw tightening is the target material instead of the nut. In many applications, the bolts also work independently, and are directly screwed into the pre-drilled holes without nuts so much as to cooperate. At this time, the bolts are functionally classified as screws. There are various types of bolts, but most of them are determined by their studs and bolt heads. 1. The thread of the column can be divided into coarse thread, fine thread and thread nominal diameter thread, so it is called fine thread bolt and thread nominal diameter bolt. 2. Bolt heads are divided into hexagonal head bolts, square head bolts, semicircular head bolts, countersunk head bolts, hole bolts, T-shaped head bolts, hook head (foot) bolts, etc. depending on the shape and main purpose of the bolt head. 3. According to the additional expansion parts, maijin bolts are sent to expand expansion bolts and so on. 4. Bolts can be made into high-strength bolts, high-precision bolts and fastening bolts according to the commonly used materials and manufacturing precision. 5. Bolts are divided into more than 10 grades such as 220.127.116.11.18.104.22.168.22.214.171.124.126.96.36.199.9.12.9 according to performance standards, among which bolts of grade 8.8 and above are made of low-carbon environmental protection alloy steel or low-carbon steel and After the heat treatment process (heat treatment. Quenching), they are collectively referred to as high-toughness bolts, and others are collectively referred to as general bolts. The bolt performance level model consists of two parts, each indicating the nominal compressive strength and yield ratio of the bolt material.