What is a Hex Head Bolt

What is a Hex Head Bolt?

Hex bolt, also known as a hex head bolt or hex cap screw, is a standard fastener with a head in the shape of a regular hexagonal prism and a partially or fully threaded shank. It can be categorized into hex head and socket head types, and further divided into partially threaded and fully threaded hex bolts based on thread length.

It is typically used in conjunction with one or more nuts and washers. Tightening is achieved by applying torque to the nut or by screwing the bolt into a pre-tapped internal thread hole, thereby fastening two or more components with through-holes together.

The iconic hexagonal head design provides an efficient interface for applying force using tools like wrenches and sockets, making it one of the most fundamental and widely used connecting elements in numerous fields such as machinery, construction, and equipment assembly.

Threaded Fasteners

Threaded fasteners rely on the mechanical self-locking action of threads and clamping force to function. They represent the most core and extensive category of fasteners, of which the hex bolt is one type.

Main Product Categories Include:

Bolt: Typically refers to an externally threaded cylindrical fastener used with a nut for through-hole connections.

Screw: Typically used without a nut, it is directly screwed into an internal threaded hole in the component being joined.

Stud: Has threads on both ends. One end is screwed into a base material, and the other end is secured with a nut.

Nut: Used in conjunction with bolts or studs, featuring an internal thread in the center.

Self-tapping Screw: Has a cutting or forming tip at the front, allowing it to tap its own matching thread in a pre-drilled hole or thin sheet metal, eliminating the need for pre-tapping.

Wood Screw: Features deep threads with a large pitch, specifically designed for direct screwing into wood.

Characteristics and Main Uses of Hex Bolts

Characteristics of Hex Head Bolt

Excellent Fastening and Loosening Resistance: When used with a nut, it can generate significant clamping force, ensuring the connection remains stable under vibration and impact loads. Loosening resistance can be further enhanced by adding spring washers, lock nuts, etc.

Convenient Installation and Removal: The standard hexagonal head design allows for efficient installation and removal using a variety of tools, from simple wrenches to professional power tools, facilitating maintenance.

Reliable Connection, High Load-Bearing Capacity: This type of connection is rigid and can withstand shear forces, tensile forces, and combined loads, making it particularly suitable for critical structural connections requiring high strength and reliability.

Standardized Specifications, High Interchangeability: There are unified or corresponding standard systems globally, with clear dimensions and property classes, simplifying procurement, inventory management, and replacement.

Uses of  Hex Bolt

Hex bolts are almost ubiquitous in our production and daily life. Their main applications cover:

Structural Connections: Steel structure buildings (factories, bridges), frame assembly of heavy equipment.

Machinery and Equipment Assembly: Fixing components of various machine tools, engines, pumps, valves, conveyor equipment, etc.

Automotive Manufacturing and Repair: Fastening for automobile chassis, engines, body parts, etc.

Infrastructure Construction: Connections in railways, highways, and energy facilities (wind power, hydropower).

Daily Maintenance & Repair: A common spare fastener for tasks ranging from furniture assembly to household appliance repair.

Relevant Production Standards for Hex Bolts

Production standards specify the dimensions, tolerances, mechanical properties, testing methods, and technical requirements for bolts, forming the foundation for ensuring their interchangeability and functionality.

Main International Standards:

ISO 4014 (Class A/B) / ISO 4017 (Class C): International Organization for Standardization standards, defining product grade (precision) and dimension series. They serve as a common language for global trade.

DIN 931 (partially threaded)/ DIN 933 (fully threaded): German Industrial Standards, which have widespread influence in Europe and globally.

ANSI/ASME B18.2.1: American Society of Mechanical Engineers standard, the primary reference for imperial (US) bolts.

JIS B 1180: Japanese Industrial Standard.

Chinese National Standards (GB):

GB/T 5780 (Class C) / GB/T 5781 (fully threaded Class C): Correspond to ISO 4017 for commercial-grade bolts.

GB/T 5782 (Class A/B) / GB/T 5783 (fully threaded Class A/B): Correspond to ISO 4014 for precision bolts, the most widely used.

GB/T 5785 (Class B, thin shank) / GB/T 5786 (fully threaded Class B, thin shank): Used for special application requirements.

Common Material Standards

The material of a bolt directly determines its strength, toughness, and suitability for specific environments.

Carbon Steel and Alloy Steel:

The most widely used, differentiated by property class (e.g., 4.8, 8.8, 10.9, 12.9).

International/Chinese (GB): Classes like 4.8, 8.8, 10.9, 12.9. The number before the decimal point indicates the nominal tensile strength (×100 MPa), and the number after indicates the yield strength ratio. For example, Class 8.8 means tensile strength ≥800 MPa and yield strength ≥640 MPa.

US System (SAE/ASTM): Such as Grade 2, Grade 5, Grade 8, or ASTM A325, A490, A193, etc.

Stainless Steel:

Main Grades: A2-70, A4-80 (Austenitic stainless steel, e.g., 304, 316), C1-110 (Martensitic stainless steel, e.g., 410). The numbers indicate the property class, e.g., A2-70 means tensile strength ≥700 MPa.

Standards: GB/T 3098.6, ISO 3506-1. Offers good corrosion resistance, suitable for damp, chemical, and other environments.

Non-Ferrous Metals:

Brass, Aluminum Alloy: Often used for electrical conductivity, non-magnetic requirements, or decorative appearances. Strength is relatively lower.

Standards: Relevant non-ferrous metal material standards.

Surface Treatment Methods for Hex Bolts

Surface treatments aim to enhance corrosion resistance, improve appearance, or meet special functional needs.

Electro-galvanizing: The most common and economical anti-corrosion treatment, with a bright white or blue-white appearance (after passivation). Offers relatively shorter salt spray test resistance. Complies with standards like GB/T 5267.1.

Hot-dip Galvanizing: Features a thick zinc layer, strong corrosion resistance, long service life, but a relatively rough surface with a spangled pattern. Commonly used in harsh outdoor environments like structures and power towers.

Dacromet (Zinc Flake Coating): No risk of hydrogen embrittlement, thin and uniform coating, strong corrosion resistance (salt spray test can reach several hundred hours or more), but limited color options (silver-gray) and is environmentally friendly.

Mechanical Galvanizing: Suitable for high-strength bolts (≥Class 10.9), avoids hydrogen embrittlement issues, provides even coating, but average appearance and corrosion resistance.

Oxidation (Blackening/Bluing): Forms a dense oxide film on the surface, offers limited corrosion protection, mainly used for indoor environments with rust-preventive oil or for aesthetic decoration.

Stainless Steel (Natural Finish): Relies on the material's own corrosion resistance, requires no additional treatment, and maintains the metal's original color.

DIN 931 Hex Bolt Standard Dimensions

DIN 931 is an authoritative standard established by the German Institute for Standardization (DIN), specifying the dimensions, tolerances, and technical requirements for partially threaded, hexagon head bolts. This standard has been equivalently adopted by the international standard ISO 4014 and the European standard EN 24014. In China, its corresponding standard is GB/T 5782. The core content of these standards is highly consistent.

Related Standard: DIN 933 for fully threaded bolts, where the shank is threaded from just below the head to the end.

DIN 931 bolts are among the most fundamental and commonly used fasteners in almost all industrial sectors, including general machinery, equipment manufacturing, steel structures (non-slip-critical connections), automotive, rail transportation, electrical cabinets, and more.

A complete DIN 931 bolt designation typically looks like this:

DIN 931 - M12 x 60 - 10.9

DIN 931: Standard number

M12: Nominal thread diameter of 12 mm

x 60: Total bolt length of 60 mm

10.9: Property class (indicating a tensile strength of 1000 MPa and a yield ratio of 0.9)