Bolts are widely used in detachable connections in mechanical manufacturing, and are generally used in conjunction with nuts (usually apply with one or two  washers).

a) General purpose of bolts: There are many kinds, including hexagon head and square head. Hexagon head bolts are the most common one. They are classified into A, B, C and other product grades according to their manufacturing accuracy and product quality. The A and B grades are mostly used. They are mainly used for important, high-precision assembly and subject to greater impact, vibration or where the load changes. Hexagon head bolts can be divided into two types: hexagon head and large hexagon head according to the size of the head supporting area and the size of the installation position. The head or screw with a hole for the type to be used when locking. The square head of the square head bolt has a larger size and a force-bearing surface, which is convenient for the wrench mouth to be stuck or against other parts to prevent rotation. It is commonly used on rough structures, and sometimes also used in T-slots to facilitate bolts in loose adjustment position in the groove. See GB8, GB5780～5790, etc.

b) Special purpose bolts: Including T-slot bolts, joint bolts and anchor bolts. T-slot bolts are mostly used in places that need to be disconnected frequently. Anchor bolts are used to fix the frame or motor base in the cement foundation. See GB798, GB799, etc.

c) High-strength bolt connection pair for steel structure: Generally used for friction-type connection of steel structures such as buildings, bridges, towers, pipeline supports and hoisting machinery, see GB3632, etc.