Introduction
During World War I, the mismatch of screw shapes and threads in different countries created many problems for the allied forces, which intensified during World War II. In the late 1940s, the ISO standard screw was defined to improve the production of screws.
In 1948, the United Kingdom, the United States, and Canada defined the DIN in 1919 as having unified forms of agreement, while Germany had already standardized.
Undoubtedly, in today’s modern world, the use of different standards is inevitable for the design of any engineering structure. The bolt and nut connections are of course no exception to this rule.
Bolt standards.
The design of bolt and nut connections is based on various standards such as ANSI, ASME, ISO, EN, ASTM, DIN and so on.
In the industry of metal structures and other industries of the country, DIN standard has traditionally been used in the past. However, with the increase in the level of knowledge of designers and familiarity with other standards, the use of standards such as EN and ASTM in the country is also increasing.
The world of bolts and nuts and its standards are very diverse and only the DIN standard has over 150 standards for bolts and nuts.
A variety of different standards used for bolt and nut connections
Standard: ASTM
The ASTM standard specifies various bolts and nuts of various sizes and grades for use in metal structures. These bolts are usually available in both quenched and tempered modes and have two types 1 and 3: type1 and (type 3).
ASTM A325 or ASTM A325M Metric:
Includes hardened steel screws with normal strength
ASTM A490 or ASTM A490M Metric:
Includes high-strength hardened steel screws
ASTM F2280:
Stretch and bolt control bolts {bolt control voltage (TCB or TC bolt بالا high strength)
ASTM F1825:
Stretch and tension bolts {bolt control tension (TCB or TC bolt معم normal strength)
The ASTM F16 Committee, led by the American Metal Institute and the Metal Fittings Research Council, has considered a proposal to integrate these standards and create a single standard. Therefore, the ASTM F3125 standard is presented as a combination of the above standards:
| grade | The lowest strength | Type | Model | |
| A325 | 120xi (830 MPa) | 1 | 3 | Large hex wrench |
| A490 | 150xi (1040 MPa) | 1 | 3 | Large hex wrench |
| F2280 | 150xi (1040 MPa) | 1 | 3 | TCB |
| F1852 | 120xi (830 MPa) | 1 | 3 | TCB |
Today, hundreds of millions of bolts and nuts are manufactured based on specifications set by ASTM standards.
Given this high volume, using a single standard called ASTM F3125 will be a great help in designing.
Definition of screw resistance categories:
There are 11 types of mechanical grades for screw screws in ISO 989-1 standard:
If the nominal diameter of the screws is less than 5 mm, there is no need to insert grades on them.
Different grades are defined in ISO and DIN standards, which are shown in the DIN 297-3 standard.
The DIN screw resistance is defined by three 8.8, 10.9 and 12.9.
Of course, this regulation also defines the resistance categories 4.6, 5.6 and 5.8 for ordinary screws.
Since according to paragraph 10-3-5-3 of the tenth topic in seismic connections, only high-strength screws should be used, this type of screws is not useful in designing friction connections, while the production of this type of screws in the country. We are very limited.
This number represents the amount of stress flow and the stress of the screw rupture.
for example :
In the resistance range of 8.8, the mean of the first 8 is the minimum final strength equal to 8000 Kg / cm2 and the mean of .8 is the amount of stress of screw flow equal to Kg / cm2 6400 = 8000 × 0.8.
Similarly, we have 10.9 for rejection:
Fy = 0.9 × 10000 = 9000Kg / cm2 – and FU = 10000 Kg / cm2 Introduction
Standard DIN :
The DIN standard used in large metal structures is DIN6914, DIN6915 and DIN6914 high strength.
DIN6914 : Large hexagonal screw screws, especially large metal fittings, high strength friction connections (grade HV-10..9)
DIN6915 : Large hexagonal nut, for large metal structures High strength friction connections (grade HV-10.9)
DIN6916 : Gasket for large steel structures High strength friction connections (grade HV-10.9)
DIN931 : This screw has two types of half-threaded and all-threaded and can be used in different structures – reliability connections (produced in different grades.)
DIN933: This screw is all threaded and can be used in a variety of structures – relying joints.
DIN934 : The bead used in various structures of reliability connections (produced in different grades.)
Dry screw 8.8.
What is called a dry screw is a type of steel (low strength) screw that determines the number of 8.8 screw grades and the degree of stiffness of this type of bolt and nut.
Steel screws are used in many industries. All manufacturers of metal sheds and skeletons are well acquainted with the type of hexagon, which also deals with resistances above 8.8.
In the automotive industry, steel screws are used in different parts of the car to connect different parts of the car.
The 8.8 steel screw is also widely used in the connection of industrial and construction valves.
A variety of standards for these screws include:
Hexagon steel screw head
DIN 931 – ISO4014
DIN 933 – ISO 4017
Dry screw 10.9.
The 10.9 grade steel screw is a high-carbon steel with a higher hardness than the 8.8 grade and lower than the 12.9 grade.
Steel screws 10.9 with MO40 wire are produced in small sizes up to M24 size by cold forging method and high size M24 by hot forging method.
High-strength steel screws are very suitable for metal and shed structures.
These screws have higher torque and prestressing strength than the 8.8 steel screw, which plays an important role in the design of friction steel structures.
The 10.9 steel screw may be the head of the Allen hexagon or even the entire thread, or it may be produced as an anchor bolt.
10.9-HV high-strength screw .
The high-strength HV hexagon head screw is one of the most widely used screws in the construction industry and metal frame.
The HV hex screw is known as ASTM A490, DIN6914 and ISO7412.
The nut required for this type of screw must be DIN6915, ASTM A536 and ISO7414.
The standard for washers is DIN6916, ASTM F436 and ISO4716.
HV hex screws are compatible with the 10.9 DIN931 grade in terms of raw materials, but the difference is more in the dimensions and type of tensile strength.
The HV screw has a larger flower and shorter threads, so it is used in friction metal structures where the screw is fastened with more force and actually stores the prestressing force itself.
For example, an M30 screw with DIN931 has a wrench size of 46, while an M30 screw with a DIN6914 HV has a wrench size of 50.
The standard thread length for the M30x100 on the ISO4014, which is the same as the DIN931, is 66mm, but for the same size on the ISO7412, which is the same as the DIN6914, it is 42mm.
Grade 8 steel bead grade 8.8 :
Steel beads are mostly used as hexagons.
Two numbers are used to determine the resistance of the screws, for example, 8.8, but only one number is used to determine the resistance of the nut. For example, a hexagon nut of grade 8 or class 8.
What defines the dimensional standard of the nut emphasizes the wrench holders with the abbreviations S, the height of the nut with the letter M, and the diameter of the hole with the letter m.
The letter e indicates a large hexagonal diameter.
The millimeter step nut is closed for the millimeter screws and the inch nut for the screw bolts. In fact, the type of nut thread should be selected based on the screw thread.
Today, the use of millimeter beads is more than other standards.
Steel bead class 10 grade 10.9 :
Class 10 dry nuts, which are often fastened to 10.9 grade screws, are among the most durable industrial nuts.
But the raw materials used in the production of these beads are the same as MO40. The beads are engraved on the upper part of the bead resistance category and the abbreviation mark or mark on the bead manufacturer is engraved on the lower part.
Class 10 beads should not be covered with hot melt galvanized plating as it will reduce its resistance and create hydrogen brittleness on the surface of the bead.
The most widely used dimensional standards for this nut:
DIN 934 – ISO 898–2 Grade 10 steel bead
DIN 555 – ISO 898–2 Grade 10 steel bead
DIN 6915 – ISO 7414 Grade 10 steel bead
ASTM A 563 Grade 10 Steel Bead
HV-10 flat washer:
In general, it is widely used in steel fittings.
This type of washer is used with all kinds of hard and semi-hard steel screws.
In the types of washers, the outer diameter, inner diameter and thickness of the washer are discussed.
This washer is often used in metal structures.
Especially if the structure is friction type, this type of washer should be used on both sides under the screw flower and under the nut, because in this type of screw structure, a pre-tensioning force is applied and the washer distributes this force well at the contact surface. .
Therefore, structural designers must consider a double washer for each screw.
Of course, the type of hole also plays an important role in using the washer.
Another application of this washer is for all types of B7 and L7 bolts. Usually, HV washers can be used on both sides of these bolts.
The standard defined for this washer is as follows:
Flat washer HV DIN6916
Flat washer HV F436