Bolts are widely used fasteners in the fields of machinery and construction. Their main function is to connect and secure different parts or materials through threaded connections. Bolts offer high strength, ease of installation, and removal, playing a crucial role in modern industry. This article provides a detailed introduction to the definition, classification, composition, performance, selection principles, and application fields of bolts.

I. Definition of Bolts

A bolt is a fastener with external threads, typically consisting of a head and a shank. Bolts form secure connections by mating with a nut or threading directly into a material's threaded hole. Bolts can be used to fasten two or more pieces of material together, maintaining their relative position or bearing certain loads and forces.

II. Classification of Bolts

1. Classification by Head Shape

  • Hex Head Bolts: The most common type, with a hexagonal head, suitable for most engineering and mechanical connections.
  • Round Head Bolts: With a rounded head, usually used in applications with higher aesthetic requirements.
  • Countersunk Head Bolts: With a conical head, flush with the surface after installation, suitable for connections needing a smooth surface.
  • Square Head Bolts: With a square head, mainly used in wood structures and connections requiring anti-rotation.

2. Classification by Shank Type

  • Fully Threaded Bolts: Threads cover the entire length of the shank, suitable for applications needing a wide range of adjustment for tightening force.
  • Partially Threaded Bolts: Threads cover only part of the shank, suitable for connections needing specific positioning for tightening.

3. Classification by Material

  • Carbon Steel Bolts: High strength, low cost, suitable for most conventional applications.
  • Stainless Steel Bolts: Strong corrosion resistance, suitable for damp or corrosive environments.
  • Alloy Steel Bolts: High strength and hardness, suitable for heavy loads and high-temperature applications.
  • Plastic Bolts: Lightweight, corrosion-resistant, suitable for chemical environments or light-load applications.

4. Classification by Function

  • General Purpose Bolts: Used for standard mechanical and structural connections.
  • High-Strength Bolts: Used for connections requiring high strength, such as steel structures.
  • Expansion Bolts: Used for anchoring in concrete or brick walls.
  • Anchor Bolts: Used for securing equipment to foundations.

III. Composition of Bolts

1. Head

The head of the bolt comes in various shapes, commonly hexagonal, round, or countersunk. Its primary function is to provide a force surface for tightening tools, allowing the bolt to be tightened or loosened.

2. Shank

The shank is the main part of the bolt, featuring external threads. The length and diameter of the shank determine the bolt's application range and connection strength. The threads are usually triangular, providing good self-locking performance and tightening force.

3. Tip

The tip of the bolt is typically flat or slightly chamfered to facilitate insertion into the nut or threaded hole. Some special bolts may have sharp tips for self-tapping into specific materials.

IV. Performance of Bolts

1. Tightening Force

Tightening force is a crucial performance indicator of bolts, determining the strength of the connection. It depends on the thread design, shank diameter, and bolt material. High-strength bolts usually provide greater tightening force, suitable for load-bearing connections.

2. Tensile Strength

Tensile strength refers to the bolt's ability to withstand breakage under tensile force. It is related to the bolt's material and diameter, with high-strength bolts offering higher tensile strength.

3. Shear Strength

Shear strength refers to the bolt's ability to withstand breakage under shear force. Like tensile strength, it depends on the material and diameter, suitable for connections subjected to shear forces.

4. Corrosion Resistance

Corrosion resistance determines the bolt's lifespan in different environments. Stainless steel and galvanized bolts offer high corrosion resistance, suitable for damp or corrosive environments.

5. Thermal Stability

Thermal stability is the bolt's ability to maintain performance in high-temperature environments. Alloy steel bolts usually offer high thermal stability, suitable for high-temperature applications.

V. Principles for Selecting Bolts

1. Selection Based on Material

Different materials have different requirements for bolts. For metal materials, high-strength carbon steel or alloy steel bolts should be chosen, while for plastic or lightweight materials, plastic bolts may be suitable.

2. Selection Based on Application Environment

The application environment significantly impacts bolt selection. Stainless steel or galvanized bolts should be chosen for damp or corrosive environments, and high-temperature alloy bolts for high-temperature environments.

3. Selection Based on Load Requirements

Load requirements determine the bolt's specifications and type. High-strength bolts should be chosen for heavy load connections, while general-purpose bolts can be used for light load connections.

4. Selection Based on Connection Type

Different connection types require different bolts. Self-locking bolts are suitable for frequently disassembled connections, while general-purpose or expansion bolts are suitable for permanent connections.

5. Selection Based on Installation Conditions

Installation conditions also affect bolt selection. Hand-tightened bolts or those with special head shapes should be chosen for confined spaces or where tools cannot be used.

VI. Application Fields of Bolts

1. Construction Engineering

Bolts are widely used in construction engineering for connecting steel structures, bridges, and building frames, providing high-strength and reliable fastening solutions.

2. Machinery Manufacturing

In machinery manufacturing, bolts are used for assembling and securing equipment and mechanical components, ensuring structural stability and reliability.

3. Automotive Industry

In the automotive industry, bolts are used for assembling car bodies, chassis, and engine components, providing high-strength connections and ensuring the safety and durability of vehicles.

4. Aerospace

The aerospace industry has stringent quality requirements for bolts, which are used for assembling high-precision components of aircraft, spacecraft, and missiles, providing reliable fastening performance.

5. Furniture Manufacturing

Bolts are used in furniture manufacturing for connecting wooden and metal components, providing stable and aesthetically pleasing assembly results.

6. Power Industry

In the power industry, bolts are used for connecting and securing electrical equipment and transmission lines, ensuring the safety and stability of power systems.

7. Household Appliances

Bolts are used in household appliances for assembling and securing various components, ensuring the stability and safety of the appliances.


As an important fastener, bolts come in various types and have wide-ranging applications. Understanding the definition, classification, composition, performance, selection principles, and application fields of bolts is essential for improving assembly efficiency and connection quality. With technological advancements, the design and materials of bolts continue to innovate, providing better solutions for the fastening needs of various industries.

Bolts main content: