Screw is just a general term for fasteners, which includes bolts, screws, and nuts. As long as there is a threaded fastener, a bolt is a type of fastener composed of a head and a screw (cylindrical body with external thread), which needs to be matched with the nut. A type of fastener composed of a head and a screw (cylindrical body with external thread), which needs to be matched with the nut, is used to tighten and connect two parts with through holes. This type of connection is called bolt connection. If the nut is unscrewed from the bolt, it can separate these two parts, so bolt connection is a detachable connection.

A type of fastener where the bolt has no head and only has threads on both ends. When connecting, one end must be screwed into the part with an internal threaded hole, and the other end must pass through the part with a through hole, and then the nut must be screwed on, even if these two parts are tightly connected as a whole. This type of connection is called a bolt connection and also belongs to a detachable connection. Mainly used in situations where one of the connected parts has a large thickness, requires a compact structure, or is not suitable for bolt connection due to frequent disassembly.

Screws are also a type of fastener composed of a head and a screw, which can be divided into three categories according to their purpose: machine screws, set screws, and special purpose screws. Machine screws are mainly used for fastening a part with a fixed threaded hole to a part with a through-hole, without the need for a nut to fit (this type of connection is called a screw connection, which is also a detachable connection; it can also be used with a nut to fit between two parts with through-holes for fastening connection.) Set screws are mainly used to fix the relative position between two parts. Special purpose screws, such as lifting ring screws, are used for lifting parts.

Nuts have internal threaded holes and are generally in the shape of flat hexagonal columns, as well as flat square columns or flat cylindrical shapes. They are used in conjunction with bolts, screws, or machine screws to tighten and connect two parts, making them a whole.

Speaking of hardness, let's first popularize the knowledge of hardness. Hardness is an indicator of bolt performance that can be measured using a hardness tester. There are three commonly used methods for expressing hardness:

1. Vickers hardness (HV) - The Vickers hardness value (HV) is obtained by dividing the load value by the surface area of the material indentation pit, when a diamond square cone indenter with a top angle of 136 ° and a load of up to 120kg is pressed into the surface of the material.

2. Brinell hardness - The measurement principle is to use a certain size of test force F (N) (the unit of force is usually kilogram force kgf, note: 1kgf=9.8N, kgf means one kilogram of force), press a quenched steel ball or hard alloy ball with a diameter of D (mm) into the surface of the tested metal, maintain the specified time, remove the test force, measure the average diameter of the indentation d (mm) using a reading microscope, and then calculate the Brinell hardness HB value according to the formula, or check the HB value from the prepared Brinell hardness table based on d.

3. Rockwell hardness - an indicator used to determine the hardness value based on the depth of indentation plastic deformation, with a hardness unit of 0.002 millimeters. Using different indenters and testing forces in the Rockwell hardness test will result in different combinations, corresponding to different scales of Rockwell hardness (HRA, HRB, HRC).