Bolts, as widely used fasteners in mechanical engineering, play a crucial role in connecting, fixing, and supporting structures. Although bolts may seem insignificant, the numbers and letters on them carry rich information that is essential for their correct selection and use. This article delves into the meanings of the numbers and letters on bolts and explores their practical value in engineering.

I. Basic Composition and Classification of Bolts

Bolts primarily consist of a shank and a head. The shank, equipped with threads, is screwed into the threaded holes of the connected parts. The head, on the other hand, mates with nuts or other fasteners to achieve a secure fastening. Depending on their usage and structural characteristics, bolts can be classified into various types, such as ordinary bolts, high-strength bolts, and bolts for reamed holes.

II. Meanings of Numbers and Letters on Bolts

The numbers and letters on bolts typically represent specifications, materials, performance grades, and other information, which are crucial for their correct selection and use. Here’s a detailed interpretation of their meanings:

  1. Specification and Size

The specification and size of bolts typically include the nominal diameter, pitch, length, and other parameters. The nominal diameter, denoted by the letter “d,” is an approximate value of the shank diameter and serves as the primary basis for selecting bolts and nuts. The pitch, represented by the letter “P,” is the distance between adjacent threads and determines the tightening force of the bolt. The length, indicating the distance from the head to the bottom of the bolt, is crucial for ensuring the proper installation depth.

  1. Material Markings

Material markings on bolts indicate the materials used in their manufacture, such as carbon steel or stainless steel. These markings help to understand the corrosion resistance, strength, and other performance characteristics of the bolts. For instance, stainless steel bolts are often marked with “SS” or specific types like “304” or “316,” indicating their stainless steel composition.

  1. Performance Grade

The performance grade of a bolt refers to its tensile strength, yield strength, and other performance indicators under specific conditions. The performance grade is typically represented by a combination of numbers and letters, such as “8.8” or “10.9.” The numbers represent the ratio of tensile strength to yield strength, while the letters indicate the manufacturing process and material type. Understanding the performance grade helps to select suitable bolts for specific applications, ensuring reliable and safe connections.

  1. Surface Treatment

Information about the surface treatment of bolts is also indicated by numbers and letters. These markings indicate whether the bolts have undergone treatments such as galvanizing or coating to enhance their corrosion resistance. For example, “Z” may represent zinc coating, while “C” may indicate a coating treatment. These markings aid in understanding the corrosion resistance of bolts and selecting suitable ones for different environments.

  1. Manufacturer Identification and Standard Number

Bolts may also bear manufacturer identification and standard numbers, indicating the manufacturer and the standards they comply with. These markings assist in tracing the source and quality of bolts, ensuring the purchase of qualified products that meet regulations.

III. Application of Numbers and Letters on Bolts in Engineering Practice

Understanding the meanings of numbers and letters on bolts is crucial in engineering practice. Firstly, correctly interpreting these markings helps to select bolts that meet requirements, ensuring reliable and safe connections. For instance, in structures subject to high tensile forces, high-strength bolts with higher tensile strength are preferred. In corrosive environments, stainless steel bolts with excellent corrosion resistance are chosen.

Secondly, knowledge of bolt specifications and sizes facilitates accurate and convenient installation. When selecting bolts, it is necessary to determine the appropriate bolt diameter, length, and pitch based on the hole diameter, thickness, and other parameters of the connected parts. Additionally, correctly identifying the material and performance grade of bolts helps to avoid potential safety hazards resulting from improper use.

Lastly, decoding the manufacturer’s identification and standard numbers on bolts can trace their origin and quality, providing a basis for engineering quality management. This ensures the purchase of bolts that comply with regulations and improves the overall quality of the project.

The numbers and letters on bolts serve as keys to decoding their meanings and applications. By understanding information such as specifications, materials, performance grades, surface treatment, and manufacturer identifications, we can select suitable bolts to ensure reliable, safe, and convenient connections. In engineering practice, correctly interpreting and applying this information is crucial for improving project quality and reducing safety risks. Therefore, we should attach great importance to the meanings of numbers and letters on bolts, continuously learn and master relevant knowledge, and better apply it to practical work.