Hexagonal nuts are used in conjunction with screws and bolts to connect and fasten machine parts. The most widely used type is the Class 1 hex nut, with Class C nuts used for surfaces with rough finishes and lower precision requirements, while Class A and B nuts are used for more polished surfaces with higher precision requirements.

Type 2 hexagonal nuts have greater thickness and are often used in applications requiring frequent assembly and disassembly. Thin hexagonal nuts have reduced thickness and are used where space is limited on connected machine parts.

Hexagonal nuts are classified by nominal thickness into Type I, Type II, and thin varieties. Above grade 8, they are divided into Type I and Type II. The design of hexagonal nuts provides excellent torque transmission capacity and can withstand significant axial and radial loads.

• Construction: Used in steel structures, frames, and bridges to ensure stability and safety between components.
• Mechanical Engineering: Fixes parts in automobiles, ships, aircraft, and machine tools to ensure proper operation and stability.
• Electronics: Special hex bolts ensure tightness and stability of electronic components.
• Home Maintenance: Used in furniture assembly, bicycle repair, and other household applications.

Hex nuts have various classifications and specifications to meet different engineering needs. Type 1 has single-sided chamfers while Type 3 has double-sided chamfers. Pocket nuts with non-through screw holes are used decoratively, especially where exposed bolt ends might be hazardous. Butterfly nuts are used in space-constrained applications like automobile wheels.

Carbon steel classification:

• By Use: Carbon structural steel, carbon tool steel, and free-cutting structural steel
• By Smelting Method: Open hearth steel, converter steel
• By Deoxidation: Boiling steel (F), killed steel (Z), semi-killed steel (b), special killed steel (TZ)
• By Carbon Content: Low carbon (≤0.25%), medium carbon (0.25%-0.6%), high carbon (≥0.6%)
• By Quality: Ordinary (higher P,S), quality (lower P,S), high quality (lowest P,S)

Bolt and nut strength grades typically match (e.g., bolt 8.8 with nut 8), but sometimes nuts have slightly lower strength than bolts for safety and cost considerations, as bolt failure is harder to detect.