Bolt performance grades
The performance grades of bolts used in steel structure connections include over 10 grades such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Among them, 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), and are generally referred to as high-strength bolts.
The rest are generally called ordinary bolts. The performance grade marking of bolts consists of two parts of numbers, representing the nominal tensile strength value of the bolt material and the ratio of yield strength to tensile strength.
For example, for a bolt with a performance grade of 4.6, it means:
1. The nominal tensile strength of the bolt material is 400 MPa;
2. The ratio of yield strength to tensile strength of the bolt material is 0.6;
3. The nominal yield strength of the bolt material is 400 × 0.6 = 240 MPa.
Another example, for a high-strength bolt with a performance grade of 10.9, after heat treatment, it can achieve:
1. The nominal tensile strength of the bolt material is 1000 MPa;
2. The ratio of yield strength to tensile strength of the bolt material is 0.9;
3. The nominal yield strength of the bolt material is 1000 × 0.9 = 900 MPa.
The meaning of bolt performance grades is an internationally recognized standard. Bolts of the same performance grade, regardless of their material and place of origin, have the same performance. In design, only the performance grade needs to be selected.
The so-called grade 8.8 and 10.9 for strength grades refer to the shear stress grades of the bolts being 8.8 GPa and 10.9 GPa. Generally, the strength of bolts is indicated by “*.Y”, where **100 = the tensile strength of this bolt, and **100 * (Y/10) = the yield strength of this bolt (because according to the marking rule: yield strength / tensile strength = Y/10).
For example, for a grade 4.8 bolt, its tensile strength is 400 MPa, and its yield strength is 400 * 8/10 = 320 MPa. For example, the bolts of Jergens lifting rings are all grade 12.9, with a tensile strength of 1200 MPa and a yield strength of 1200 × 0.9 = 1080 MPa, ensuring production safety.
Thread fit grades
Thread fit refers to the degree of tightness or looseness between mating threads. The grade of fit is the specified combination of deviations and tolerances on the internal and external threads.
(1) For unified inch threads, there are three grades of external threads: 1A, 2A, and 3A, and three grades of internal threads: 1B, 2B, and 3B, all of which are clearance fits. The higher the grade number, the tighter the fit. In inch threads, deviations are only specified for grades 1A and 2A, and the deviation for grade 3A is zero. Moreover, the grade deviations for 1A and 2A are equal. The larger the grade number, the smaller the tolerance.
1. 1A and 1B grades, very loose tolerance grades, suitable for the allowance fit of internal and external threads.
2. 2A and 2B grades, the most commonly used thread tolerance grades in the inch series of mechanical fasteners.
3. 3A and 3B grades, the tightest fit formed by mating, suitable for fasteners with tight tolerances, used in critical safety designs.
4. For external threads, grades 1A and 2A have a fit tolerance, while grade 3A does not. The tolerance of grade 1A is 50% larger than that of grade 2A and 75% larger than that of grade 3A. For internal threads, the tolerance of grade 2B is 30% larger than that of grade 2A. The tolerance of grade 1B is 50% larger than that of grade 2B and 75% larger than that of grade 3B.
(2) For metric threads, there are three grades of external threads: 4h, 6h, and 6g, and three grades of internal threads: 5H, 6H, and 7H. (The precision grades of Japanese standard threads are divided into I, II, and III, usually grade II is used.) In metric threads, the basic deviations of H and h are zero. The basic deviation of G is positive, and the basic deviations of e, f, and g are negative.
As shown in the following figure: 1. H is a commonly used tolerance zone position for internal threads and is generally not used for surface plating or with very thin phosphating layers. The fundamental deviation of the G position is used in special circumstances, such as for thicker plating layers, and is rarely used.
2. g is often used for thin plating layers of 6-9um. For example, if the product drawing specifies a 6h bolt, the pre-plating thread tolerance zone is 6g.
3. The best thread fit combinations are H/g, H/h, or G/h. For precision fastener threads such as bolts and nuts, the standard recommends a 6H/6g fit. For example, in Jergens’ “Metric Insert Threaded Inserts – Thin Wall” series of products, the internal thread grade is 6H and the external thread grade is 6g. This product creates a stable and durable thread in most materials and can be easily installed using standard tapping and drilling tools.
Identification of Unified Inch Screw Threads
Unified inch screw threads are widely used in English-speaking countries. This type of thread is divided into three series: coarse thread series UNC, fine thread series UNF, extra fine thread series UNFF, and a fixed pitch series UN.
Marking method: Thread diameter – number of threads per inch – series code – precision grade
For example: Coarse thread series 3/8 – 16 UNC – 2A
Fine thread series 3/8 – 24 UNF – 2A
Extra fine thread series 3/8 – 32 UNFF – 2A
Fixed pitch series 3/8 – 20 UN – 2A
The first digit 3/8 represents the thread outer diameter, with the unit being inches. To convert to the metric unit mm, multiply by 25.4, that is, 3/8 × 25.4 = 9.525mm; the second and third digits 16, 24, 32, 20 represent the number of threads per inch (the number of threads in a 25.4mm length); the letters after the third digit UNC, UNF, UNFF, UN represent the series code; the last two digits 2A represent the precision grade.
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