Fasteners are a type of mechanical parts that are used for fastening connections and are extremely widely used. It is widely used in various industries, such as machinery, equipment, vehicles, ships, railways, bridges, buildings, structures, tools, instruments, chemicals, meters and supplies, etc. It is the most widely used mechanical basic part.
It is characterized by a wide variety of specifications, different performance and uses, and a high degree of standardization, serialization, and generalization. Therefore, some people call a type of fasteners that have national standards as standard fasteners, or simply standard parts.
　1. Screw torque requirements: the torque of the hexagon socket screw is relatively larger, the torque of the hexagon socket screw is smaller, and the torque of the cross recess is smaller (so this type of screw is generally a normal grade screw).
　　2. Assemble the hexagonal bolts generally with adjustable wrenches, box wrenches and open-end wrenches. The assembly efficiency of the adjustable wrench is low, but it is very versatile, and it is suitable for various head specifications of the hexagonal screws; the torx wrench is the most efficient, but it is not suitable for some occasions. A torx wrench has only 2 heads, so it can only be used Two types of hexagon head screws; open-end wrenches are similar to torx wrenches, but it can be used with a longer sleeve.
　　The smaller the size of the outer hexagonal screw, the higher the corners and corners of the outer hexagonal bolt, otherwise its head will easily slip when subjected to (wrench) force.
　　Heat treatment is mainly for carbon steel screws, mainly quenching and tempering heat treatment and carburizing heat treatment, to meet the requirements of different environments for screw strength.
　　1. Quenched and tempered heat treatment: products with strength grades of 8.8 and above are all quenched and tempered heat treated products. The characteristic of this heat treatment is that the hardness is relatively uniform inside and outside.
　　When the same material is heat treated, the higher the hardness, the worse the toughness. Therefore, it is necessary to have a safe match, while meeting the hardness, the toughness is also guaranteed.
　　2. Carburizing heat treatment: self-tapping screws basically need this treatment, which is characterized by a very hard surface and a soft core; because it needs to tap into a hard iron plate