Contact us

The performance grade of bolts for steel structure connection is divided into more than 10 grades, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc., among which the bolts of grade 8.8 and above are made of low carbon alloy steel or medium carbon steel and are heat treated (quenched and tempered), which are generally called high-strength bolts, and the rest are generally called ordinary bolts. The bolt performance grade mark consists of two parts of numbers, which respectively represent the nominal tensile strength value and yield strength ratio of bolt material.
For example: bolt with performance grade of 4.6 means:
The nominal tensile strength of bolt material is 400MPa; the yield strength ratio of bolt material is 0.6; the nominal yield strength of bolt material is 400 × 0.6 = 240mpa. The performance grade is 10.9 high-strength bolt. After heat treatment, the material can reach: the nominal tensile strength of bolt material reaches 1000MPa; the yield strength ratio of bolt material is 0.9; the nominal yield strength of bolt material reaches 1000 × 0.9 = 900MPa. The meaning of bolt performance grade is the international general standard. For bolts with the same performance grade, regardless of the difference in material and place of origin, their performance is the same, and only performance grade can be selected in design.
The strength grades 8.8 and 10.9 refer to the shear stress grades 8.8gpa and 10.9gpa.
8.8 nominal tensile strength 800N / mm2; nominal yield strength 640n / mm2.
Generally, the strength of bolt is indicated by "x.y", X * 100 = the tensile strength of bolt, X * 100 * (Y / 10) = the yield strength of bolt (because according to the identification, yield strength / tensile strength = Y / 10)
For example: grade 4.8, the tensile strength of the bolt is 400MPa; the yield strength is 400 * 8 / 10 = 320MPa.
In addition: stainless steel bolts are usually marked as a4-70 and A2-70, and the meaning is explained otherwise.
measure
There are two main types of length measurement units in the world, one is metric system, the other is metric system, which is widely used in Europe, China, Japan and other Southeast Asian regions, the other is British system, and the measurement unit is inch, which is equivalent to the old market size of our country, and more used in the United States, Britain and other European and American countries.
Metric measurement: (decimal) 1m = 100 cm = 1000 mm
English Measurement: (octal) 1 inch = 8 inch, 1 inch = 25.4mm, 3 / 8 × 25.4 = 9.52
1 / 4 of the following products are designated by serial number, such as 4 × 4, 5 × 6 × 7 × 8 × 10 × 12#
Thread
Screw thread is a kind of section with uniform helix protrusion on the outer or inner surface of solid. According to its structural characteristics and application, it can be divided into three categories: common thread: the tooth shape is triangle, which is used to connect or fasten parts. The common thread can be divided into coarse thread and fine thread according to the pitch. The connection strength of fine thread is higher. Transmission thread: the tooth shape has trapezoid, rectangle, saw shape and triangle, etc. Sealing thread: used for sealing connection, mainly pipe thread, taper thread and taper pipe thread.
Thread fit class
Thread fit is the loose or tight size between screw threads, and the grade of fit is the specified combination of deviation and tolerance on internal and external threads.
1. For unified inch screw thread, there are three grades of external screw thread: 1a, 2a and 3a, and three grades of internal screw thread: 1b, 2b and 3b, all of which are clearance fit. The higher the level number, the tighter the fit. In inch screw threads, the deviation is only specified for grades 1a and 2a, the deviation for grades 3a is zero, and the grade deviation for grades 1a and 2a are equal. The larger the number of grades, the smaller the tolerance.
L class 1a and 1b, very loose tolerance class, which is suitable for tolerance fit of internal and external threads.
L2a and 2b are the most common thread tolerance grades specified for British series mechanical fasteners.
L3a and 3b, screwed together to form the tightest fit, suitable for tight tolerance fasteners, used for safety critical design.
For external threads, there is a fit tolerance for classes 1a and 2a, but not for class 3a. Class 1A tolerance is 50% larger than class 2A tolerance, 75% larger than class 3A tolerance, and class 2B tolerance is 30% larger than class 2A tolerance for internal thread. Class 1b is 50% larger than class 2B and 75% larger than class 3B.
2. Metric thread, external thread has three thread grades: 4h, 6h and 6G, internal thread has three thread grades: 5h, 6h and 7h. (the accuracy level of Japanese standard thread is divided into three levels, I, II and III, generally II). In metric thread, the basic deviation of H and H is zero. The basic deviation of G is positive, and the basic deviation of E, F and G is negative.
H is the common position of tolerance zone of internal thread, which is not used as surface coating or extremely thin phosphating layer. The basic deviation of G position is used in special occasions, such as thick coating, which is rarely used.
G is often used for plating 6-9um thin coating. If the product drawing requires 6h bolt, 6G tolerance zone shall be adopted for the thread before plating.
The best combination of thread fit is H / g, H / h or g / h. for the threads of refined fasteners such as bolts and nuts, 6h / 6G fit is recommended in the standard.
3. Thread marking
Main geometric parameters of self tapping and self drilling thread
1. Major diameter / outer diameter of tooth (D1): it is the hypothetical cylinder diameter of thread crown coincidence. The major diameter of the thread basically represents the nominal diameter of the thread size.
2. Minor diameter / base diameter (D2): it is the hypothetical cylinder diameter of thread base coincidence.
3. Pitch (P): refers to the axial distance between two corresponding points of adjacent teeth on the meridian. In the British system, the pitch is indicated by the number of teeth per inch (25.4mm).
The following is a list of tooth pitch (metric system) teeth of common specifications (English system)
(1) Metric self tapping teeth:
Specification: s t 1.5, s t 1.9, s t 2.2, s t 2.6, s t 2.9, s t 3.3, s t 3.5, s t 3.9, s t 4.2, s t 4.8, s t 5.5, s t 6.3, s t 8.0, s t 9.5
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) British self tapping teeth:
Specification: 4 × 5 × 6 × 7 × 8 × 10 × 12 × 14#
Number of teeth: ab teeth 24, 20, 20, 19, 18, 16, 14, 14
A teeth 24, 20, 18, 16, 15, 12, 11, 10