Bolts, screws and fasteners are common terms used in the industry. Among them, bolts belong to a general category of fasteners. Based on the physical and mechanical principles of inclined plane rotation and friction, bolts fasten and position components, serving as the most fundamental and essential parts for mechanical assembly.
Widely applied in daily life and various industrial fields, bolts are known as "industrial rice". Their application scenarios cover electronic products, digital devices, electromechanical equipment, machinery, ships, vehicles, hydraulic engineering and chemical facilities. Different scenarios adopt bolts with corresponding specifications and precision levels. Precision micro bolts are used for delicate devices such as digital products, cameras, glasses and watches. Standard ordinary bolts are applied in household appliances, musical instruments and furniture. Large high-strength bolts are widely used in construction projects, steel structures and bridges. Transportation equipment including aircraft, rail transit and automobiles adopts bolt sets of various sizes. Bolts play an irreplaceable core role in all industrial assembly scenarios.
1. Classification of Bolt Performance Grades
Common performance grades for structural and industrial bolts include 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. Bolts of grade 8.8 and above are defined as high-strength bolts, which are made of high-quality carbon steel or alloy steel and undergo quenching and tempering heat treatment to enhance mechanical properties. Bolts below grade 8.8 are ordinary bolts that generally require no complex heat treatment.
Bolt performance grades follow the international standard "X.Y" marking system. The two sets of figures represent the nominal tensile strength and yield ratio respectively, enabling accurate calculation of mechanical parameters. Bolts of the same grade maintain consistent mechanical properties regardless of material sources or origins, ensuring universal design adaptability.
Grade Calculation Formula
1. Nominal tensile strength = X × 100 (MPa)
2. Yield ratio = Y ÷ 10
3. Nominal yield strength = X × 100 × (Y ÷ 10) (MPa)
2. Detailed Parameters of Common Bolt Grades
Nominal tensile strength: 400 MPa
Yield ratio: 0.8
Nominal yield strength: 320 MPa
Nominal tensile strength: 800 MPa
Yield ratio: 0.8
Nominal yield strength: 640 MPa
Grade 10.9 High-Strength Bolt
Nominal tensile strength: 1000 MPa
Yield ratio: 0.9
Nominal yield strength: 900 MPa
The shear resistance of bolts increases synchronously with performance grades. Grades 8.8 and 10.9 are the most widely used high-strength grades for steel structures and mechanical equipment. The strength grade of a bolt cannot be determined merely by its material and appearance specifications. Judgment must be based on grade marking, heat treatment process and factory test data, with Grade 5.8 as the minimum reliable general-purpose grade.
