The following is a detailed introduction to bolts:
1. Material
The choice of bolt material directly determines its strength, corrosion resistance, and applicable environment. Common materials include:
Carbon steel: The most common material. Strength grades vary depending on the carbon content. The most typical are 4.8 and 8.8 (medium carbon steel, commonly used structural steel) and 10.9 and 12.9 (alloy steel, high strength). The surface is usually galvanized (for rust prevention), blackened, or nickel-plated.
Alloy steel: Carbon steel with added elements such as chromium, molybdenum, and vanadium, resulting in higher yield strength and fatigue resistance. Commonly used in high-strength bolts (e.g., grade 10.9, 12.9) in automobiles, wind power generation, and heavy machinery.
Stainless steel: Excellent corrosion resistance and aesthetics. Common materials include SUS304 (general purpose, corrosion resistant) and SUS316 (marine grade, resistant to chloride ion corrosion). Strength typically ranges from A2-70 to A4-80. While slightly less hard than high-strength alloy steel, it is the preferred choice for humid and acidic/alkaline environments.
Non-ferrous metals: Such as brass, copper, and aluminum alloys. Commonly used in electrical equipment (good conductivity), precision instruments, or decorative applications; non-magnetic and rust-resistant.
Special alloys: Such as titanium alloys (aerospace, high-end bicycles, low density and high strength), and heat-resistant steel (engines, high-temperature environments).
2. Main Types
Bolts come in many types, typically classified by head shape, thread type, and performance/application:
By Head Shape:
Hexagonal Head Bolts: The most standard type, capable of withstanding high force, easy to tighten with a wrench, suitable for most applications requiring high-strength connections.
Internal Hex Socket Head Bolts: The head fits into the tool hole, aesthetically pleasing, and space-saving, suitable for mechanical assemblies with limited space or requiring countersunk installation.
Square Head Bolts: Square head, capable of withstanding high torque, commonly used for heavy-duty or simple tooling.
Countersunk/Semi-countersunk Head Bolts: Conical head, the surface is flush with the connected parts after installation, used for equipment or furniture requiring a smooth surface.
By Performance and Structure:
High-Strength Bolts: Typically refer to grade 8.8 and above, used in bridges, building steel structures, heavy machinery, and other applications subject to huge dynamic loads.
Expansion Bolts: Used in concrete or brick walls. The tapered design expands the sleeve, generating friction to secure objects (e.g., air conditioner brackets, guardrails).
Anchor bolts: Pre-embedded in concrete foundations, used to secure large equipment, towers, or factory columns.
Eye bolts/Lever bolts: Ring-shaped heads, used for hoisting equipment or as traction connection points.
By thread type:
Fully threaded: The entire bolt is threaded, suitable for withstanding axial tensile forces.
Partially threaded: The diameter of the smooth portion of the bolt is slightly larger than the thread root diameter, providing better shear resistance, often used for positioning connections.
3. Application Areas
Bolts are used in every corner of modern industry and life:
Construction Engineering: Connections of steel structure frames, bridges, tower cranes, and high-speed railway tracks. Here, high-strength large hexagonal head bolts and torque-shear bolts are crucial for connecting lifelines.
Transportation: Fixing automobile engines, chassis, and wheel hubs; mating aircraft wings to fuselages; tightening railway sleepers. These applications require bolts with extremely high fatigue resistance and anti-loosening properties.
Electronics and Precision Instruments: Fixing laptop casings, mobile phone internal structures, watch straps, and hard disk drives. Miniature hexagonal or precision slotted/Phillips head bolts are primarily used, requiring stringent dimensional accuracy and surface treatment.
Energy infrastructure: Wind turbine tower connections (using ultra-long, high-strength bolts), oil pipeline flange connections (high-pressure and corrosion-resistant bolts), and photovoltaic bracket fixing (rust-resistant self-tapping bolts).
Everyday life: Furniture assembly (such as the hexagonal wood screws commonly used by IKEA), bathroom accessories (expansion bolts), and adjustment parts for bicycles and sports equipment.
