The significance of valve fastener markings and thread fit grades

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This post was last edited by “Thankfully, you’re here” on July 7, 2016, at 11:16. The significance of markings on valve fasteners and the grades of thread fits: There are over 10 grades for bolts used in steel structure connections, namely 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 from low-carbon alloy steel or medium-carbon steel and undergo heat treatment (quenching and tempering); they are commonly referred to as high-strength bolts. All other bolts are known as ordinary bolts. The performance grade designation of bolts consists of two numbers, which represent the nominal tensile strength value and the yield ratio of the bolt material, 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 ratio of the bolt material is 0.6 ； 　　3. The nominal yield strength of the bolt material reaches 400 × 0.6 = 240 MPa. For high-strength bolts with a performance grade of 10.9, after heat treatment, the material can achieve: 1. The nominal tensile strength of the bolt material reaches 1000 MPa. file:///C:\Users\ADMINI~1\AppData\Local\Temp\ksohtml\wpsC757.tmp.png ； 　　2. The yield-to-tensile 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, it is sufficient to specify the performance grade alone. 　　The so-called strength grades of 8.8 and 10.9 refer to the shear stress resistance levels of bolts, which are 8.8 GPa and 10.9 GPa respectively. For grade 8.8, the nominal tensile strength is 800 N/mm², while the nominal yield strength is 640 N/mm². Generally, the strength of bolts is indicated as “X.Y”; here, X*100 represents the tensile strength of the bolt, and X*100*(Y/10) represents its yield strength (as per labeling regulations: yield strength/tensile strength = Y/10). For example, for a grade 4.8 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 elsewhere. file:///C:\Users\ADMINI~1\AppData\Local\Temp\ksohtml\wpsC758.tmp.png Units of measurement: There are mainly two units of length used in the world today. One is the metric system, with units such as meters (m), centimeters (cm), and millimeters (mm); 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: (Base-10) 1 m = 100 cm = 1000 mm.
2. Imperial measurement: (Base-8) 1 inch = 8 eighth-inches; 1 inch = 25.4 mm. 3/8￠￠ × 25.4 = 9.52.
3. For products with a diameter of 1/4￠￠ or less, their nominal diameters are indicated by numbers, such as: 4#, 5#, 6#, 7#, 8#, 10#, 12# threads.

I. A thread is a shape characterized by uniformly spaced helical protrusions on the outer or inner surface of a solid body. 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 classified 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. file:///C:\Users\ADMINI~1\AppData\Local\Temp\ksohtml\wpsC759.tmp.png II. Thread fit grade: The 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 classes 1A and 2A; the tolerance for class 3A is zero. Moreover, the tolerance values for classes 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 tolerance fits of internal and external threads. 　　Classes 2, 2A, and 2B are the most commonly specified thread tolerance grades in the British series of mechanical fasteners. 　　Classes 3, 3A, and 3B: they achieve the tightest fit when screwed together, and are suitable for fasteners with tight tolerances, used in critical designs where safety is important. 　　4. For external threads, classes 1A and 2A have a fit tolerance, while class 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. Class 1B is 50% larger than Class 2B and 75% larger than Class 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 grade 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 those of e, f, and g are negative. As shown in the figure: 1. H is the commonly used tolerance zone position for internal threads; it is generally not used for surface coating, or only a very thin phosphating layer is applied. The basic deviation for position G is used in special cases, such as thicker plating layers, 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 combinations are H/g, H/h, or G/h. For the threads of precision fasteners such as bolts and nuts, the standard recommendation is to use a 6H/6g combination. (III) Thread markings: file:///C:\Users\ADMINI~1\AppData\Local\Temp\ksohtml\wpsC769.tmp.png 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/tooth root diameter (d2): The diameter of the imaginary cylinder along which the tooth roots of the thread 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 thread pitches (metric) and number of threads per inch (imperial) for commonly used specifications.
1. Metric self-tapping screws:
Specifications: ST 1.5, ST 1.9, ST 2.2, ST 2.6, ST 2.9, ST 3.3, ST 3.5, ST 3.9, ST 4.2, ST 4.8, ST 5.5, ST 6.3, ST 8.0, ST 9.5
Thread pitches: 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 threads per inch: AB thread type – 24, 20, 20, 19, 18, 16, 14, 14; A thread type – 24, 20, 18, 16, 15, 12, 11, 10.
The above information pertains to the meaning of markings on valve fasteners as well as the relevant thread fit classes. It is compiled and published by Zhejiang Sandi Valve Co., Ltd., a company certified under ISO9001/TS/CE/API/EAC standards