Having worked in the machinery industry for so many years, do you know the meaning of the markings on screws?
Steel structure connection bolts are classified into more than 10 performance grades, including 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 undergo heat treatment (quenching and tempering), commonly known as high-strength bolts. The rest are commonly known as ordinary bolts. The bolt performance grade designation consists of two parts: the nominal tensile strength and the yield strength ratio of the bolt material. For example:
A bolt with a performance grade of 4.6 means:
The nominal tensile strength of the bolt material reaches 400 MPa;
The yield strength ratio of the bolt material is 0.6;
The nominal yield strength of the bolt material reaches 400 × 0.6 = 240 MPa.
High-strength bolts with a performance grade of 10.9, after heat treatment, achieve the following:
Nominal tensile strength of the bolt material reaches 1000 MPa;
Yield ratio of the bolt material is 0.9;
Nominal yield strength of the bolt material reaches 1000 × 0.9 = 900 MPa.
The meaning of bolt performance grade is an internationally recognized standard. Bolts with the same performance grade have identical performance regardless of their material or origin; only the performance grade needs to be selected in design.
Strength grades 8.8 and 10.9 refer to the bolt's shear stress resistance grades of 8.8 GPa and 10.9 GPa, respectively.
The nominal tensile strength of 8.8 is 800 N/mm², and the nominal yield strength is 640 N/mm². Bolts typically use "X" and "Y" to indicate their strength. X * 100 = tensile strength of the bolt, X * 100 * (Y/10) = yield strength of the bolt (because according to the marking convention: yield strength / tensile strength = Y/10).
For example, a 4.8 grade bolt has a tensile strength of 400 MPa and a yield strength of 400 * 8/10 = 320 MPa.
Note: Stainless steel bolts are usually marked as A4-70 or A2-70, with different meanings.
1. Measurement
There are two main units of length measurement in the world today: the metric system, with units such as meters (m), centimeters (cm), and millimeters (mm), widely used in Europe, my country, and Southeast Asia (including Japan); and the imperial system, with units mainly in inches (inch), equivalent to the old Chinese cun (市寸), widely used in the United States, the United Kingdom, and other European and American countries.
Metric measurement: (decimal) 1 m = 100 cm = 1000 mm. Imperial units (octal): 1 inch = 8 cents, 1 inch = 25.4 mm, 3/8 × 25.4 = 9.52.
Products smaller than 1/4 inch are designated by serial numbers, such as: 4#, 5#, 6#, 7#, 8#, 10#, and 12#.
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Threads
A thread is a shape with uniform helical ridges on the cross-section of the outer or inner surface of a solid. Based on their structural characteristics and applications, they can be divided into three main categories:
Ordinary threads: Triangular tooth profile, used for connecting or fastening parts. Ordinary threads are further divided into coarse and fine threads according to pitch; fine threads offer higher connection strength.
Transmission threads: Tooth profiles include trapezoidal, rectangular, sawtooth, and triangular.
Sealing threads: Used for sealing connections, mainly pipe threads, tapered threads, and tapered pipe threads.
Classification by Shape:
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Thread Fit Grades
Thread fit refers to the degree of looseness or tightness between engaged threads. The fit grade is a specified combination of deviations and tolerances acting on the internal and external threads.
1. For Unified Imperial Threads, external threads have three thread grades:
1A, 2A, and 3A; internal threads have three grades: 1B, 2B, and 3B. All are clearance fits. The higher the grade number, the tighter the fit. In Imperial Threads, deviations are only specified for grades 1A and 2A; grade 3A has zero deviation, and the deviations for grades 1A and 2A are equal. The higher the grade number, the smaller the tolerance.
Grades 1A and 1B are very loose tolerance grades, suitable for tolerance-tolerant fits between 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 form the tightest fit, suitable for fasteners with tight tolerances, used in safety-critical designs.
For external threads, grades 1A and 2A have a fit tolerance, while grade 3A does not. The tolerance for grade 1A is 50% larger than that for grade 2A and 75% larger than that for grade 3A. For internal threads, grade 2B's tolerance is 30% larger than that for 2A. Grade 1B is 50% larger than that for grade 2B and 75% larger than that for grade 3B.
2. Metric threads: External threads have three thread grades:
4h, 6h, and 6g; internal threads have three thread grades: 5H, 6H, and 7H. (Japanese standard thread accuracy grades are divided into three levels: I, II, and III; level II is commonly used.) In metric threads, the basic deviations of H and h are zero. The basic deviation of G is positive, while the basic deviations of e, f, and g are negative.
H is a commonly used tolerance zone position for internal threads and is generally not used for surface coatings, or only for very thin phosphate layers. The basic deviation of the G position is used in special cases, such as thicker coatings, and is rarely used.
"g" is commonly used for plating thin layers of 6-9 μm. For example, if a bolt's drawing specifies a tolerance of 6h, the thread before plating should have a tolerance zone of 6g.
Thread fits are best combined as H/g, H/h, or G/h. For precision fasteners such as bolts and nuts, the standard recommends a 6H/6g fit.
3. Thread Marking
Thread marking is a standardized symbol system used to fully express the various technical parameters of a thread. It typically consists of thread characteristic symbols, size specifications, tolerance zone symbols, and other necessary information. Correct thread marking accurately conveys key information such as thread type, size, precision grade, and direction of rotation, and is an important technical language for ensuring thread interchangeability in mechanical design and manufacturing.
Examples of Unified Imperial Thread Markings:
1/4-20 UNC-2A: Indicates a nominal diameter of 1/4 inch, 20 threads per inch, coarse thread series, 2A grade external thread;
3/8-16UNC-2B LH: Indicates a nominal diameter of 3/8 inch, 16 threads per inch, coarse thread series, 2B grade internal thread, left-hand.
Examples of Metric Thread Markings:
M10×1-6g: Indicates a nominal diameter of 10mm, pitch of 1mm (fine thread), 6g grade external thread;
M12-6H: Indicates a nominal diameter of 12mm, standard pitch (coarse thread), 6H grade internal thread;
M20×2-6H/6g: Indicates a nominal diameter of 20mm, pitch of 2mm, 6H grade internal thread and 6g grade external thread mating.
Key Marking Rules:
Metric threads are indicated by "M", while imperial threads begin with the nominal diameter value.
Right-hand threads do not indicate the direction of rotation, while left-hand threads must be marked with "LH".
Coarse threads do not indicate the pitch, while fine threads must indicate the pitch.
In tolerance zone designations, uppercase letters indicate internal threads, and lowercase letters indicate external threads.
When marking mating parts, the numerator is the tolerance zone for internal threads, and the denominator is the tolerance zone for external threads.
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Main Geometric Parameters of Self-Tapping and Self-Drilling Threads
1. Major Diameter/Outer Diameter (d1): The diameter of the imaginary cylinder where the thread crests coincide. The major diameter basically represents the nominal diameter of the thread.
2. Minor Diameter/Root Diameter (d2): The diameter of the imaginary cylinder where the thread roots coincide.
3. Pitch (p): The axial distance between two corresponding points on the pitch diameter of adjacent threads. In imperial systems, the pitch is indicated by the number of threads per inch (25.4 mm).
The following lists the commonly used thread pitch (metric) and number of threads (imperial).
(1) Metric Self-Tapping Threads
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 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 Threads
Specifications: 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.
