Bolt level, and the meaning of bolt levelClass 8.8 hexagon head bolt
GB 819-85: Cross Slot Countersunk Head Screw
GB/T 819.1-2000: “Cross recessed countersunk head screws Part 1: Steel Grade 4.8”
GB/T 819.2-2000: “Cross recessed countersunk head screws – Part 2: Steel 8.8, stainless steel A2-50, non-ferrous metal CU2 or CU3”
GB/T18230.1-2000: “Screw length of large hexagon head bolt for bolted structure according to GB/T3106 C class 8.8 and 10.9”
8.8 and 4.8 are the strength grades of bolts, generally 8.8 bolts are made of 45 steel, 4.8 bolts are made of Q235-A, but they are not limited to the above two materials
8.8 means:
The first 8 represents the tensile strength Rm
Rm=8×100 N/mm2 =800 N/mm2=800MPa
Second,.8 is a yield ratio of 0.8
Re800 * 0.8 = 640 mpa
4.8 means
Tensile strength Rm=4 x 100 N/mm2=400 N/mm2=400 mpa
Yield strength Re=400×0.8=320MPa
Bolt performance grades are 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9 and more than 10 grades, of which 8.8 and above bolts are made of low carbon alloy steel or medium carbon steel and heat treatment (quenching, tempering), collectively referred to as high strength bolts, and the rest are collectively referred to as ordinary bolts. The bolt performance grade label consists of two parts, which respectively represent the nominal tensile strength value and the flexure ratio of the bolt material.
Nuts and their grades
The nut is a nut, a part that is screwed together with a bolt or screw to play a fastening role, and a component that must be used in all production and manufacturing machinery. There are many kinds of nuts, according to the different materials, divided into carbon steel, high strength, stainless steel, plastic steel and other types. According to the product attributes corresponding to different national standards are divided into ordinary, non-standard, (old) national standard, new national standard, American system, British system, German standard and so on.
Hexagon nuts are divided into three types according to nominal thickness: type I, type II and thin type. Nuts above grade 8 are divided into two types: Type I and type II.
Type I hexagonal nuts are the most widely used, and type I nuts are divided into A, B, and C three levels, of which class A and class B nuts are suitable for machines, equipment and structures with small surface roughness and high precision requirements, while class C nuts are used for machines, equipment or structures with rough surface and low precision requirements; The thickness of type II hexagon nuts is relatively thick, and it is used in occasions where installation and disassembly are often required.
Nut commonly used national standard:
GB41 Hexagon nut Type I – Class C
GB6170 Type I hexagon nuts – Class A and B
GB6171 Type I hexagon nuts – Fine teeth – Class A, B
GB6172 Hexagon thin nuts – Class A, B – chamfer
GB6173 Hexagon thin nuts – Fine teeth – Class A, B
GB6174 Hexagon thin nuts – Class B – no chamfer
Gb6175ⅱhexagon nut – A, B class
GB6176 Type II hexagon nuts – Fine teeth – A, B class
GB6177 Hexagon flange nuts – Class A
GB55 thick hexagon nut
GB56 Hexagon super thick nut
GB1229 Large hexagon nut (High strength for steel structure)
Metric bolt grades are :4.6; 4.8; 5.8; 6.8; 8.8; 9.8; 10.9; 12.9.
The grades of metric nuts are :4; 5; 6; 8; 10; 4 of 12; 5 is the strength level of thin nuts.
Nut A; B; The C-class is equivalent to the American-made G2; G5; The performance level of G8 is basically the same as that of J995.
Type I nut thread specification is M1.6~M64 performance grade is 6; 8; 10; A2-50; A2-70; A4-50; A4-70; CU2; CU3 and AL4 grades, product grades A and B, A for specifications less than or equal to M16 specifications; Class B for specifications larger than M16! (A here; B refers to accuracy level);
Type II nut thread specifications are M5~M36, performance grades are 9 and 12, product grades are A and B, Class A is used for specifications less than or equal to M16; Class B for specifications larger than M16! (A here; B refers to accuracy level);
Because the performance level of type 2 is very high, so the thickness of the nut is thicker and the bolt performance grade is 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9 and other more than 10 grades, of which 8.8 and above bolts are made of low carbon alloy steel or medium carbon steel and heat treatment (quenching, tempering). Generally referred to as high strength bolts, the rest commonly referred to as ordinary bolts.
How do screw and nut grades match? If the bolt is rated 8.8, what grade should be used for the nut?
Metric bolt grades are :4.6; 4.8; 5.8; 6.8; 8.8; 9.8; 10.9; 12.9.
The grades of metric nuts are :4; 5; 6; 8; 10; 4 of 12; 5 is the strength level of thin nuts.
So: Does it make sense to use 8.8 for bolts and 4 for nuts?
First of all: 8.8 and level 4 are too unscientific.
Generally, the decimal behind the bolt grade is the grade of the nut, 8.8 grade bolts with 8 grade nuts, 10.9 grade bolts with 10 grade nuts.
That is, the bolt and nut have the same strength grade (bolt 8.8 and nut 8 have the same strength grade, because the “.8 “behind the bolt indicates that the yield strength is 80% of the tensile strength).
But consider the use of the situation if the nut is not good to replace, the bolt is relatively easy to replace, and it can also be changed into bolt 4.8 nut 5.
In some cases, the strength of the nut is a little lower than that of the bolt, the main reason is that the failure of the bolt is usually not perceptible, sometimes sudden, and the failure of the nut is more likely to be found. There are also safety concerns. This can ensure that the life of the screw is longer than the nut when used together, and the cost will be cheaper and easier when removing and replacing fasteners in this way.