When we purchase bolts, we need to pay attention to their tensile strength. Tensile strength is determined by the bolt's material and cold-hot processing, heat treatment methods, and is used to indicate the maximum ultimate strength that the bolt can bear when subjected to tension. The performance grades of bolts are generally divided into 10 grades, ranging from 3.6 to 12.9. Among them, the number before the decimal point represents 1/100 of the material's ultimate tensile strength, and the number after the decimal point represents 10 times the ratio of the material's yield limit to the ultimate tensile strength.
Similarly, the performance grades of nuts are divided into seven different grades, ranging from grade 4 to grade 12. The grade number represents 1/100 of the minimum stress that the nut is guaranteed to bear.
For imperial threads, the external threads have three tolerance grades: 1A, 2A, and 3A; the internal threads include 1B, 2B, and 3B grades. The higher the grade number, the tighter and more precise the thread fit.
For metric threads, the external threads have three tolerance grades: 4h, 6h, and 6g; the internal threads have three tolerance grades: 5H, 6H, and 7H. The thread fit preferably adopts the combination of H/g, H/h or G/h. For the threads of precision fasteners such as bolts and nuts, the standard recommends the 6H/6g fit method.
The calculation of bolt tensile strength is generally based on the nominal diameter. The calculation formula is F=σ×A, where F represents the ultimate tensile force, σ represents the material's tensile strength, and A is the effective stress-bearing area of the thread. For specific mechanical principles, you can refer to the knowledge of related disciplines such as "Mechanics of Materials".
In short, when selecting bolts, it is necessary to pay attention to their tensile strength grade, and select the appropriate bolt grade according to specific usage requirements and load conditions to ensure the safety and reliability of use.
