Performance grade of bolts

Thread Content

Performance grades of bolts: The performance grades of bolts used for steel structure connections include over 10 grades such as 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 made of low-carbon alloy steel or medium-carbon steel and have undergone heat treatment (quenching and tempering); they are commonly referred to as high-strength bolts, while the rest are known as ordinary bolts. The bolt performance grade designation consists of two digits, which represent the nominal tensile strength value of the bolt material and the yield strength ratio, respectively. For example: A bolt with a performance grade of 4.6 means that 1) the nominal tensile strength of the bolt material is at the 400 MPa level ； 2. The yield-to-tensile strength ratio of the bolt material is 0.6 ； 3. The nominal yield strength of the bolt material is 400×0.6=240 MPa; it is a high-strength bolt of grade 10.9. After heat treatment, the material achieves: 1. A nominal tensile strength of 1000 MPa for the bolt material ； 2. The yield strength ratio of the bolt material is 0.9 ； 3. The nominal yield strength of the bolt material is 1000×0.9=900 MPa. The performance grade of bolts represents a standard used internationally; bolts with the same performance grade have identical properties, regardless of their material or place of origin. In design, only the performance grade needs to be considered. The strength grades of 8.8 grade and 10.9 grade refer to the shear stress resistance levels of bolts being 8.8 GPa and 10.9 GPa respectively. The nominal tensile strength for 8.8 grade is 800 N/mm2, while the nominal yield strength is 640 N/mm2. Generally, bolts have their strength indicated as “X.Y”; where X*100 represents the tensile strength of the bolt, and X*100*(Y/10) represents its yield strength (as per the standard specification, yield strength/tensile strength = Y/10). For example, in the case of a 4.8 grade bolt, its tensile strength is 400 MPa ； The yield strength is: 400*8/10=320MPa. Additionally: Stainless steel bolts are usually designated as A4-70, A2-70, etc.; the meaning of these designations is explained separately. Units of measurement: There are mainly two systems for measuring length in the world today. One is the metric system, whose units include meters (m), centimeters (cm), millimeters (mm), etc.; this system is widely used in Europe, China, Japan, and other Southeast Asian countries. The other is the imperial system, whose main unit is the inch, which is equivalent to the old Chinese unit of chi; this system is more commonly used in the United States, the United Kingdom, and other European and American countries. 1. Metric measurement: (decimal system) 1 m = 100 cm = 1000 mm. 2. Imperial measurement: (octal system) 1 inch = 8 inches; 1 inch = 25.4 mm. 3/8″ × 25.4 = 9.52. 3. For products with a diameter of 1/4″ or less, their diameter is indicated by numbers such as 4#, 5#, 6#, 7#, 8#, 10#, 12#. Threads: A thread is a pattern of uniform spiral protrusions on the outer or inner surface of a solid. Based on their structural characteristics and uses, they can be divided into three main categories: (1) Ordinary threads: These have a triangular tooth shape and are used to connect or secure components. Plain threads are divided into coarse-thread and fine-thread types based on pitch, with fine-thread threads having a higher connection strength. (II) Transmission threads: The tooth profiles include trapezoidal, rectangular, serrated, and triangular shapes, among others. (III) Sealing threads: Used for sealed connections, mainly pipe threads, tapered threads, and tapered pipe threads. II. Thread fit grade: Thread fit refers to the degree of looseness or tightness between mating threads, and the fit grade is a specified combination of tolerances and deviations applied to the internal and external threads. (1) For unified British thread standards, there are three grades for external threads: 1A, 2A, and 3A; there are three grades for internal threads: 1B, 2B, and 3B. All of them feature a clearance fit. The higher the rank number, the tighter the coordination. In imperial threads, tolerances are specified only for grades 1A and 2A; the tolerance for grade 3A is zero, and the grade tolerances for 1A and 2A are equal. The larger the number of grades, the smaller the tolerance. 1, 1A, and 1B grades are very loose tolerance levels, suitable for the toleranced fits of internal and external threads. Grades 2, 2A, and 2B are the most commonly specified thread tolerance grades in the British series of mechanical fasteners. Levels 3, 3A, and 3B: they achieve the tightest fit through threading, and are suitable for fasteners with strict tolerances, used in critical designs where safety is important. 4. For external threads, grades 1A and 2A have a fit tolerance, while grade 3A does not. The tolerance of grade 1A is 50% greater than that of grade 2A, and 75% greater than that of grade 3A. For internal threads, the tolerance of grade 2B is 30% greater than that of grade 2A. Grade 1B is 50% larger than Grade 2B, and 75% larger than Grade 3B. (II) Metric threads: there are three thread grades for external threads, namely 4h, 6h, and 6g; there are three thread grades for internal threads, namely 5H, 6H, and 7H. (The thread precision grades for Japanese standards are divided into I, II, and III levels, with level II being the most common.) In metric threads, the basic deviations for H and h are zero. The basic deviation of G is positive, while the basic deviations of e, f, and g are negative. As shown in the figure: 1. H is the common tolerance zone for internal threads; it is generally not used for surface plating, or only a very thin phosphating layer is applied. The basic deviation for position G is used in special cases, such as thicker coatings, and is generally rarely employed. 2. g is commonly used to apply thin coatings of 6–9 um thickness; for example, if the product specifications require bolts with a coating thickness of 6 h, the thread tolerance range before coating is set at 6g. 3. The best thread fits are H/g, H/h, or G/h. For the threads of precision fasteners such as bolts and nuts, the standard recommends a fit of 6H/6g. (III) Thread markings. IV. Main geometric parameters of self-tapping and self-drilling threads: (I) Major diameter/outer diameter of the thread tooth (d1), which is the diameter of the imaginary cylinder on which the thread teeth coincide. The major diameter of the thread essentially represents the nominal diameter of the thread size. (II) Minor diameter/root diameter (d2): The diameter of the imaginary cylinder along which the roots of the threads coincide. (III) Tooth pitch (p): It is the axial distance between the corresponding two points of adjacent teeth on the midline. In the imperial system, the pitch is indicated by the number of teeth per inch (25.4 mm). The table below lists the common specifications for pitch (metric) and number of teeth (imperial). 1. Metric self-tapping screws: Specifications: ST 1.5, ST1.9, ST2.2, ST2.6, ST2.9, ST3.3, ST3.5, ST3.9, ST4.2, ST4.8, ST5.5, ST6.3, ST8.0, ST9.5. Pitch: 0.5, 0.6, 0.8, 0.9, 1.1, 1.3, 1.3, 1.3, 1.4, 1.6, 1.8, 1.8, 2.1, 2.1. 2. Imperial self-tapping screws: Specifications: 4#, 5#, 6#, 7#, 8#, 10#, 12#, 14#. Number of teeth: AB type – 24, 20, 20, 19, 18, 16, 14, 14; A type – 24, 20, 18, 16, 15, 12, 11, 10