Screws are usually used alone (sometimes with washers). They are generally used for tightening and should be screwed into the internal threads of the body.

a) Machine screws: There are many varieties due to different head and groove shapes. There are several types of heads: cheese head, pan head, countersunk head and semi-countersunk head. The head grooves have slotted, phillips and hexagon socket. The phillips screw has good neutrality when screwing, and the head strength is greater than that of the slotted type, and it is not easy to be bald. It is generally used in mass production. Hexagon socket screws can apply a large tightening torque, have a high connection strength, and the head can be embedded in the body. They are used for connections that require a compact structure and a smooth appearance. See GB65, GB67～69 and GB818～820, etc.;

b) Set screws: Set screws are used to fix the relative position of the parts. The head has slot and hexagon socket. Slotted and hexagonal sockets are easy to sink into parts. The end of the set screw varies according to the requirements. Generally, the most commonly used three types are cone end, flat end and dog end. The tapered end is suitable for parts with low hardness, when using a non-pointed tapered screw, a hole should be punched on the top tight surface of the part, and the tapered surface is pressed against the edge of the hole. The end is a flat-end screw, which has a large contact area and does not damage the surface of the part after being tightened. It is used to tighten a flat surface with a large hardness or where the position is frequently adjusted. The screw with the dog end at the end does not damage the surface of the part. It is mostly used to fix the parts mounted on the pipe shaft (thin-walled part). The dog end is pushed into the hole on the shaft. The anti-shear effect of the round end can transmit larger load. . See GB71, GB73～75, GB77～78, etc.;

c) Hexagon socket screws: Hexagon socket screws are suitable for occasions where the installation space is small or the head of the screw needs to be buried, see GB70, GB6190～6191 and GB2672～2674, etc.;

d) Screws for special purposes: Such as positioning screws, non-stripping screws and eye bolts, see GB72, GB828~829, GB837~839, GB948~949 and GB825.