The machine has been working for a lifetime, what does the 4.4 and 8.8 on the bolt mean?

After working as a machine for so many years, you must not know the meaning of the labels on the screws, right?

The performance grades of bolts for steel structure connection are divided into more than 10 grades such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. Among them, the bolts of grade 8.8 and above are made of low-carbon alloy steel or medium-carbon steel and are heat-treated (quenching, tempering), commonly known as high-strength bolts, the rest are commonly known as ordinary bolts. The bolt performance grade label consists of two parts of numbers, which represent the nominal tensile strength value and yield ratio of the bolt material respectively. E.g:
Bolts of property class 4.6 mean:
The nominal tensile strength of the bolt material reaches 400MPa;
The yield ratio of the bolt material is 0.6;
The nominal yield strength of the bolt material is 400×0.6=240MPa.
Performance level 10.9 high-strength bolts, after heat treatment, can achieve:
The nominal tensile strength of the bolt material reaches 1000MPa;
The yield ratio of the bolt material is 0.9;
The nominal yield strength of the bolt material is 1000×0.9=900MPa.

The meaning of bolt performance grade is an international standard. Bolts of the same performance grade, regardless of the difference in material and origin, have the same performance, and only the performance grade can be selected in the design.
The so-called 8.8 and 10.9 strength grades mean that the shear stress resistance of the bolt is 8.8GPa and 10.9GPa
8.8 Nominal tensile strength 800N/MM2 Nominal yield strength 640N/MM2
General bolts use "X.Y" to indicate the strength, X*100=tensile strength of this bolt, X*100*(Y/10)=yield strength of this bolt (because according to the regulations: yield strength/tensile strength =Y/10)
For example, grade 4.8, the tensile strength of this bolt is: 400MPa; the yield strength is: 400*8/10=320MPa.
Another: stainless steel bolts are usually marked as A4-70, A2-70, the meaning is explained otherwise.
measure
There are two main units of length measurement in the world today, one is the metric system, and the unit of measurement is meters (m), centimeters (cm), millimeters (mm), etc. The species is the imperial system, and the unit of measurement is mainly inches, which is equivalent to the market inch of the old system in my country, and is widely used in the United States, Britain and other European and American countries.
Metric measurement: (decimal) 1m = 100 cm = 1000 mm
Imperial measurement: (8 system) 1 inch = 8 cents 1 inch = 25.4 mm 3/8 × 25.4 = 9.52
Products below 1/4 use the serial number to indicate their calling diameter, such as: 4#, 5#, 6#, 7#, 8#, 10#, 12#

thread
A thread is a shape with uniform helical protrusions on the cross-section of the solid outer or inner surface. According to its structural characteristics and uses, it can be divided into three categories:
Ordinary thread: The tooth shape is triangular and is used to connect or fasten parts. Ordinary threads are divided into two types: coarse thread and fine thread according to the pitch, and the connection strength of fine thread is higher.
Transmission thread: There are trapezoidal, rectangular, saw-shaped and triangular tooth shapes.
Sealing thread: used for sealing connection, mainly pipe thread, tapered thread and tapered pipe thread.
Sort by shape:

Thread fit class

The thread fit is the loose or tight size between the screwed threads, and the level of fit is the specified combination of deviations and tolerances acting on the internal and external threads.
1. For unified inch threads, there are three thread grades for external threads: 1A, 2A and 3A, and three grades for internal threads: 1B, 2B and 3B, all of which are clearance fits. The higher the rating number, the tighter the fit. In inch threads, the deviation is only specified for grades 1A and 2A, the deviation of grade 3A is zero, and the grade deviation of grades 1A and 2A is equal. The larger the number of levels, the smaller the tolerance.
Classes 1A and 1B, very loose tolerance classes, which are suitable for tolerance fits of internal and external threads.
Grades 2A and 2B are the most common thread tolerance grades specified for inch series mechanical fasteners.
Grades 3A and 3B, screwed together to form the tightest fit, are suitable for tight tolerance fasteners and are used in safety-critical designs.
For external threads, grades 1A and 2A have a fit tolerance, grade 3A does not. The 1A tolerance is 50% larger than the 2A tolerance and 75% larger than the 3A tolerance. For the internal thread, the 2B tolerance is 30% larger than the 2A tolerance. Class 1B is 50% larger than class 2B and 75% larger than class 3B.
2. Metric threads, there are three thread grades for external threads: 4h, 6h and 6g, and three thread grades for internal threads: 5H, 6H, 7H. (The Japanese standard thread accuracy grade is divided into three grades: I, II, and III, and is usually grade II.) In the 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 a common tolerance zone position for internal threads, and is generally not used as a surface coating, or a very thin phosphating layer is used. The basic deviation of G position is used for special occasions, such as thicker coating, which is rarely used.
g is often used to coat a thin coating of 6-9um. For example, the product drawing requires a 6h bolt, and the thread before plating uses a 6g tolerance zone.
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 a fit of 6H/6g.
3. Thread marking
Main geometric parameters of self-tapping and self-drilling threads
1. Major diameter/tooth outer diameter (d1): It is the imaginary cylindrical diameter of the coincidence of the thread crests. The major diameter of the thread basically represents the nominal diameter of the thread size.
2. Minor diameter/root diameter (d2): It is the diameter of the imaginary cylinder where the thread root coincides.
3. Tooth distance (p): the axial distance between adjacent teeth corresponding to two points on the middle meridian. In the imperial system, the pitch is indicated by the number of teeth per inch (25.4mm).
The following lists the common specifications of the pitch (metric) and the number of teeth (imperial)
1) Metric self-tapping:
Specifications: S T1.5, 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) Inch self-tapping:
Specifications: 4#, 5#, 6#, 7#, 8#, 10#, 12#, 14#
Number of teeth: AB teeth 24, 20, 20, 19, 18, 16, 14, 14
A tooth 24, 20, 18, 16, 15, 12, 11, 10