As a standardized fastener that pre-assembles screws, nuts, and washers,  are widely used in machinery, construction and other fields with the advantages of efficient installation and reliable connection. The classification of their types is based on differentiated design of structure and function.

External hexagonal combination screws are the mainstream in the market, accounting for more than 60%. The screw head is an external hexagon, with flat washers and nuts, and the specifications range from M3 to M16. The 8.8-grade high-strength version is suitable for automobile chassis and engineering machinery, while the 10.9-grade version is used in heavy-duty scenarios such as wind power equipment. Its wrench operation space is small and suitable for compact structure installation.

The internal hexagonal combination screw has its own internal hexagonal groove, and is often equipped with spring washers and flat washers. The tightening torque is 15% higher than that of the external hexagon. It is mostly used in precision machinery, such as machine tools and molds, with M4-M10 specifications as the main ones. The stainless steel version is favored in food machinery and medical equipment because of its rust resistance.

The pan head combination screw has a round head with a cross slot or a slotted slot, and is matched with a flat washer and a nut. It is suitable for low-load scenarios such as furniture and electronic equipment. Most of them are small specifications of M3-M8. The surface is galvanized to prevent oxidation. No professional tools are required for installation, and it is suitable for manual operation.

The flange surface combination screw has a flange on the screw head, eliminating the need for additional washers. The flange surface tooth pattern enhances anti-slip properties, and the anti-loosening performance is 40% higher than that of ordinary models. It is often used in vibration environments, such as motors and pump equipment. The M6-M20 specifications are suitable for a variety of working conditions, and the sealing type with a rubber ring can prevent liquid leakage.

Different types of  have different fastening requirements. When selecting, it is necessary to combine the load, environment, and installation space for comprehensive judgment. Its pre-assembly characteristics can reduce assembly steps and improve connection efficiency and stability.