Do You Know What The 8.8 on The Bolt Means?

Detailed explanation of bolt performance and threaded connection: from standards to applications

Bolts play a vital role in steel structure connections, and their performance level directly determines the strength and safety of the connection. The following will be elaborated in detail from three aspects: bolt performance level, measurement unit and thread-related knowledge, to help you gain a deeper understanding of this important field.

Overview of bolt performance level

The performance level of bolts used for steel structure connections is divided into multiple levels, from 3.6 to 12.9, with a total of more than 10 levels. Among them, bolts of level 8.8 and above are usually made of low-carbon alloy steel or medium-carbon steel, and are heat treated by quenching and tempering, and are called high-strength bolts; the rest are called ordinary bolts.

The significance of performance level:

The bolt performance level number consists of two parts of numbers

· The first half of the number indicates the nominal tensile strength of the bolt material (unit: MPa).

· The second half of the number indicates the ratio of the bolt's yield strength to the tensile strength (yield strength ratio).

For example:

·4.6 grade bolts:

• Nominal tensile strength is 400 MPa;

• Yield strength ratio is 0.6;

• Nominal yield strength is 400×0.6=240MPa

·10.9 grade high strength bolts:

• Nominal tensile strength is 1000 MPa;

• Yield strength ratio is 0.9;

• Nominal yield strength is 1000×0.9=900MPa

This type of designation has become an internationally accepted standard. Bolts of the same performance grade do not need to consider differences in materials and origins when designing.

Strength grades 8.8 and 10.9 refer to bolts with shear stress levels of 8.8GPa and 10.9GPa.

8.8 Nominal tensile strength 800N/MM2; Nominal yield strength 640N/MM2.

The strength of general bolts is expressed by "X.Y", X*100=tensile strength of this bolt, X*100*(Y/10)=yield strength of this bolt (because according to the marking regulations: yield strength/tensile strength=Y/10)

For example: 4.8 grade, the tensile strength of this bolt is: 400MPa; yield strength is: 400*8/10=320MPa.

In addition: stainless steel bolts are usually marked as A4-70, A2-70, and the meaning is explained separately.

Two systems of measurement units

In modern engineering design, length measurement units are mainly divided into metric and imperial.

1. Metric: in meters (m), centimeters (cm), millimeters (mm), commonly used in Europe, China, Japan and other places.

• (decimal) 1m = 100 cm = 1000mm

2. Imperial: in inches (inch), common in the United States and the United Kingdom.

• (octal system) 1 inch = 8 cents 1 inch = 25.4mm 3/8¢¢×25.4 =9.52

In small-sized imperial products, numbers are often used to indicate the diameter, such as 4#, 6#, 8#, etc.

Classification and matching of threads

Threads are an important connection medium between bolts and nuts. According to structural characteristics and uses, they can be divided into the following categories:

Ordinary threads: The tooth shape is triangular and widely used for connection or tightening.

• Coarse thread: Large pitch, suitable for general use;

• Fine thread: Small pitch, high connection strength, suitable for precision machinery.

Transmission thread: Mostly used for power transmission, tooth shapes include trapezoidal, rectangular, saw-shaped, etc.

Sealing thread: Mainly used for pipe connection, providing good sealing performance.

Thread matching grade:

Thread matching is an important indicator to measure the tightness of the thread.

1. Unified imperial threads: There are three thread grades for external threads, namely 1A, 2A and 3A, and three grades for internal threads, namely 1B, 2B and 3B, and all are clearance fits. In imperial threads, the deviation is only specified for 1A and 2A grades, the deviation for 3A is zero, and the grade deviations for 1A and 2A grades are equal. Generally speaking, the higher the grade number, the tighter the fit, and the larger the grade number, the smaller the tolerance. Among them, grades 1A and 1B are very loose tolerance grades, suitable for tolerance fit of internal and external threads; grades 2A and 2B are the most common thread tolerance grades specified for imperial series mechanical fasteners; grades 3A and 3B are screwed together to form the tightest fit, suitable for fasteners with tight tolerances, and are often used in safety-critical designs. For external threads, 1A and 2A grades have a fit tolerance, while 3A grade does not, and the 1A grade tolerance is 50% greater than the 2A grade tolerance and 75% greater than the 3A grade; for internal threads, the 2B grade tolerance is 30% greater than the 2A tolerance, and the 1B grade is 50% greater than the 2B grade and 75% greater than the 3B grade.

2. Metric threads: There are three thread grades for external threads, namely 4h, 6h and 6g, and there are also three thread grades for internal threads, namely 5H, 6H and 7H. In metric threads, the basic deviations of H and h are zero, the basic deviations of G are positive, and the basic deviations of e, f and g are negative. Among them, H is the common tolerance zone position of internal thread, which is generally not used for surface coating, or only uses a very thin phosphating layer; G position basic deviation is used for special occasions, such as thicker coating, which is rarely used; g is often used to plate a thin coating of 6-9um. For example, when the product drawing requires a 6h bolt, the thread before plating usually uses a 6g tolerance zone. In practical applications, the thread fit is best combined into H/g, H/h or G/h. For refined fastener threads such as bolts and nuts, the standard recommends the use of 6H/6g fit.

Thread marking

Main geometric parameters of self-drilling and self-tapping threads

1. Major diameter/thread outer diameter (d1): The diameter of the imaginary cylinder where the thread crests coincide. The major diameter of the thread basically represents the nominal diameter of the thread size.

2. Minor diameter/thread root diameter (d2): The diameter of the imaginary cylinder where the thread roots coincide.

3. Tooth pitch (p): It is the axial distance between two corresponding points on the midline of adjacent teeth. In the imperial system, the tooth pitch is expressed as the number of teeth per inch (25.4mm).

The following are the common specifications of pitch (metric) and number of teeth (imperial)

(1) Metric self-tapping:

Specifications: S T 1.5, S T1.9, S T2.2, S T2.6, S T2.9, S T3.3, S T3.5, S T3.9, S T4.2, S T4.8, S T5.5, S T6.3, S T8.0, S T9.5

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:

Specifications: 4#, 5#, 6#, 7#, 8#, 10#, 12#, 14#

Number of teeth: AB 24, 20, 20, 19, 18, 16, 14, 14

A-tooth 24, 20, 18, 16, 15, 12, 11, 10

In addition, the main geometric parameters of the thread include major diameter, minor diameter, pitch, etc., which directly affect the function and application of the thread.

Application and Summary

Bolt performance grade and thread parameters are basic knowledge in engineering design, especially in the fields of steel structure and machinery manufacturing. Its standardization and international application have promoted the global development of engineering technology. Whether choosing ordinary bolts or high-strength bolts, understanding their performance grades and matching requirements is the key to ensuring design safety and efficiency.