GOST 1759.0-87

4.2. Assembly of connections on high-strength bolts when it is necessary to completely disassemble the connections

If necessary, complete disassembly
connections should be removed from the design load on a specially designed
project and restore the node using the following technology:

4.2.1. Technological process
assembly of connections provides:

- inspection of structures and verification
their compliance with the requirements of the project and the chapter of SNiP III-18-75 (in terms of the accuracy of manufacturing structures);

- alignment of holes and fixation
in the design position of elements and connection parts using mounting plugs
(10% of the number of holes, but not less than 2 pcs.);

- installation in free from traffic jams
high-strength bolt holes;

- checking geometric
dimensions of assembled structures;

- tight screed of the package;

- tension of the supplied
high-strength bolts for the force provided by the project;

- removal of plugs, setting in
the freed holes of high-strength bolts and their tension to the design
efforts.

4.2.2. Difference in thickness
elements overlapped by overlays, determined before placing overlays with
using a ruler and a feeler gauge, should not exceed 0.5 mm.

4.2.3. With a difference in planes
parts to be joined from 0.5 to 3 mm to ensure smooth bending of the lining
the edge of the protruding part must be smoothed with an emery stone at a distance of up to
30 mm from the cut of the part with a slope not steeper than 1:10. With a plane difference of more than 3
mm, gaskets should be used. The use of gaskets must be agreed
with a design organization.

4.2.4. Bolt lengths
are selected depending on the thickness of the package ().

4.2.5. When drilling holes
in elements with treated surfaces, cooling must not be used
liquids containing oil.

4.2.6. The assembly of connections should be
produce directly on high-strength bolts and plugs. Applying other bolts to
assembly quality is prohibited.

4.2.7. Every bolt
is installed in connection with two high-strength washers (one is placed under
the head of the bolt, the other under the nut).

4.2.8. Nuts tightened on
design effort, nothing is additionally fixed.

4.2.9. At the time of installation
high-strength bolts, the nuts should be free (by hand) screwed on the thread.
Otherwise, the nut or bolt should be replaced, and rejected bolts and
send nuts for re-preparation.

8.1 Proof load test

The trial load test should always be carried out when the test equipment allows it, and this test is critical for thread diameters of M5 or larger.

The nuts are screwed onto the hardened mount as shown in Figures 1 and 2. In auxiliary cases, the tensile test is decisive (see Figure 1).

1) D11 - to GOST 25346

Drawing 1
—Trialonaxialstretching

Drawing 2
—Trialonaxial compression

The test load is applied to the nut in an axial direction and held for 15 s. The nut must withstand the load without breaking or breaking the thread and can be unscrewed manually after the load is removed. If the repair thread is damaged during the test, the test is invalid. When unscrewing the nut, it is allowed to use a wrench to turn the nut 1/2 turn, and then it must be unscrewed by hand.

The hardness of the test fixture must be at least 45 HRC.

The thread tolerance is 5h6g, except for the outer diameter of the thread, which must be 1/4 of the 6g tolerance of the lower part.

How to decipher the marking of steels?

To decipher the designation of various types of steels did not cause difficulties, you should know well what they are.Certain categories of steels have special markings. It is customary to designate them with certain letters, which allows you to immediately understand both the purpose of the metal in question and its approximate composition. Let's consider some of these brands and understand their designation.

Properties and purpose of structural alloy steels

Structural steels specially designed for the manufacture of bearings can be recognized by the letter "Ш", this letter is placed at the very beginning of their marking. After it, the name of the brand is followed by the letter designation of the corresponding alloying additives, as well as numbers by which the quantitative content of these additives is recognized. So, in steels of grades ShKh4 and ShKh15, in addition to iron with carbon, chromium is contained in an amount of 0.4 and 1.5%, respectively.

The letter "K", which stands after the first digits in the name of the brand, indicating the quantitative content of carbon, denote structural unalloyed steels used for the production of vessels and steam boilers operating under high pressure (20K, 22K, etc.).

High-quality alloy steels with improved casting properties can be recognized by the letter "L" at the very end of the marking (35ХМЛ, 40ХЛ, etc.).

Some difficulty, if you do not know the specifics of the marking, can be caused by the decoding of grades of construction steel. Alloys of this category are designated by the letter "C", which is put at the very beginning. The numbers following it indicate the minimum yield strength. Such brands also use additional letter designations:

• letter T - heat-strengthened rolled stock;
• letter K - steel, characterized by increased corrosion resistance;
• letter D - an alloy characterized by a high copper content (S345T, S390K, etc.).

Unalloyed steels belonging to the category of tool steels are designated with the letter "U", it is affixed at the beginning of their marking. The number following this letter expresses the quantitative content of carbon in the alloy under consideration. Steels of this category can be of high quality and high quality (they can be identified by the letter "A", it is affixed at the end of the brand name). Their marking may contain the letter "G", which means an increased content of manganese (U7, U8, U8A, U8GA, etc.).

containing alloying elements in their composition are marked in the same way as alloyed structural ones (KhVG, 9KhVG, etc.).

Alloy tool steel composition (%)

The marking of those steels that are included in the high-speed category begins with the letter "P", followed by numbers indicating the quantitative content of tungsten. Otherwise, the brands of such alloys are named according to the standard principle: letters denoting an element, and, accordingly, numbers reflecting its quantitative content. In the designation of such steels, chromium is not indicated, since its standard content in them is about 4%, as well as carbon, the amount of which is proportional to the vanadium content. If the amount of vanadium exceeds 2.5%, then its letter designation and quantitative content are affixed at the very end of the marking (Z9, P18, P6M5F3, etc.).

Effect of some additives on steel properties

Unalloyed steels belonging to the category of electrical engineering (they are also often called pure technical iron) are marked in a special way. The low electrical resistance of such metals is ensured due to the fact that their composition is characterized by a minimum carbon content of less than 0.04%. There are no letters in the designation of grades of such steels, only numbers: 10880, 20880, etc. The first number indicates the classification by the type of processing: hot-rolled or forged - 1, calibrated - 2. The second number is associated with the category of the aging coefficient: 0 - non-standardized, 1 - standardized. The third digit indicates the group to which this steel belongs according to the standardized characteristic taken as the main one. The fourth and fifth digits determine the value of the standardized characteristic itself.

The principles by which the designation of steel alloys is carried out was developed back in the Soviet period, but to this day they are successfully used not only in Russia, but also in the CIS countries. Having information about a particular steel grade, one can not only determine its chemical composition, but also effectively select metals with the required characteristics.

Symbols and markings

All nuts have a reference designation, it shows the specialists the basic information about the products. They are also marked with information about the parameters and properties of hardware.

The symbol is divided into three types:

• full - all parameters are indicated;
• short - not very significant characteristics are described;
• simplified - only the most important information.

The designation includes the following information:

• type of fastener;
• accuracy and strength class;
• view;
• step;
• thread diameter;
• coating thickness;
• designation of the standard according to which the product was manufactured.

In addition, the nut is marked to help identify the fastener. It is applied to the end face and, in some cases, to the side. It contains information about the strength class and the manufacturer's mark.

The inscription is applied by the method of deepening into the surface with a special automatic machine. Information about the manufacturer is indicated in any case, even if there is no strength class. Complete data can be obtained by examining the relevant sources. For example, information for high strength nuts can be found in GOST R 52645-2006. Or in GOST 5927-70 for ordinary ones.

By appointment

Based on the purpose, the following types of bolts can be distinguished:

1. Plowshare - designed for fastening heavy suspended structures. Based on the name, it can be understood that it is widely used in agriculture, usually for attaching plowshares to tillage equipment.
2. Furniture - differs from other types in that the thread is not cut along the entire length of the rod. The third part remains the usual integral part. The head of such fasteners is usually smooth, which is necessary so that the bolt does not protrude above the surface of the furniture. Despite the names, such elements are used in other areas, especially in construction.
3. Road - widespread in the installation of fences. It features a semicircular head, under which there is a square head. This design allows you to firmly fix the fence elements to the posts. It is used in all areas where there is a need to fix thin sheets of metal, wood, plastic and other materials.
4. Machine-building bolts are the most common type of bolts used in machine-building. It is distinguished by increased strength properties and resistance to aggressive external environment.
5. Track - used in the railway sector, most often to connect separate parts of the rail. They differ in that the thread can be less than half the length of the rod.

You can understand what type of hardware is based on the symbol of the bolts.

Strength classes

GOST 1759.4−87, depending on the mechanical characteristics of the parts, assumes the division of these products into eleven categories. The rules for decoding their designations should not cause serious problems - multiplying the number in front of the point by 100 allows you to determine such an indicator as the tensile strength of the bolt material. For its measurement, the unit is used - N / mm 2. For example, the designation 4.6 assumes that the product has a tensile strength parameter equal to 400 N / mm 2.

Multiplying the second digit by 10, allows you to find out the indicator of the yield stress parameter (the stress at which the alloy becomes susceptible to plastic deformation). For example, for category 3.6 it will be 60%.

When calculating loads in threaded connections, it is customary to lay a certain margin of safety in terms of the yield point.

Bolts belonging to the group of high-strength products must have a tensile strength of at least 800 MPa. They have found wide application in those industries in which strict reliability requirements are imposed on structures. This group includes all parts from category 8.8. High-strength nuts, in turn, should be considered products of class at least 8.0.

It should be noted that the strength category of threaded parts depends not only on their material, but also on the manufacturing technology. Almost all bolts included in the group of high-strength products are made by the upsetting method, and special rolling semiautomatic devices are used to form the thread. After machining, the products undergo appropriate heat treatment. The final step in the production of high strength bolts is the coating.

The technological equipment used for the release of parts by the upsetting method is very diverse. There are models that can produce about 200 units of products in one minute. The main material for their production is low-carbon and alloy steel alloys. The main requirement for them is the amount of carbon. According to the documentation, this parameter should not exceed 40%.

An excellent example of such materials can be steel 20KP, 40X, 20G2R and others. Thanks to the use of various types of heat treatment, it is possible to produce parts belonging to different strength categories from the same material. As an example, it is worth considering steel 35, from which the following products can be made:

• 5.6 - achieved by processing the product on a turning and milling machine.
• 6.6, 6.8 - die forging.
• 8.8 - after machining, the product is hardened.

The classification of high strength bolts assumes the presence of highly specialized products used in some industries. All their characteristics are described in special regulatory documents. And also highly specialized bolts may differ in design options, for which the letters are used:

• U - indicates the possibility of using the fastener at temperatures down to -40 ° C. It is worth noting that the letter is most often not indicated in the designation.
• CL - the temperature conditions are tougher, and such a product can be used at -65 ° C.

What classes are there?

Strength classes for nuts are approved in GOST 1759.5-87, which is currently not relevant. But its analogue is the international standard ISO 898-2-80, it is on it that manufacturers all over the world are guided. This document applies to all metric nuts except fasteners:

• with special parameters (work in extreme temperatures - 50 and +300 degrees Celsius, with high resistance to corrosion processes);
• self-locking and locking type.

According to this standard, nuts are divided into two groups.

• With a diameter of 0.5 to 0.8 mm. Such products are called "low" and serve in places where a high load is not expected. Basically, they protect against loosening a nut with a height of more than 0.8 diameter. Therefore, they are made from low-grade low-carbon steel. For such products, there are only two strength classes (04 and 05), and they are designated by a two-digit number. Where the first says that this product does not hold a power load, and the second shows one hundredth of the effort at which the thread can break.
• With a diameter of 0.8 or more. They can be of normal height, high and especially high (respectively Н≈0.8d; 1.2d and 1.5d). Fasteners above 0.8 diameters are designated by one number, which indicates the greatest degree of reliability of the bolts with which the nut can be connected. In total, there are seven strength classes for nuts of a high group - this is 4; 5; 6; eight; nine; 10 and 12.

The normative document specifies the rules for the selection of nuts to bolts in terms of strength level.For example, with a class 5 nut, it is recommended to use a bolt section less than or equal to M16 (4.6; 3.6; 4.8), less than or equal to M48 (5.8 and 5.6). But in practice, it is advised to replace products with a low level of strength with a higher one.

Manufacturing technology

In the modern world, several technologies are used with the help of which nuts are manufactured. Some of them are used for the production of large quantities of fasteners with a minimum amount of scrap and optimal material consumption. The process takes place practically without human participation, in automatic mode. The main methods for the production of nuts in large volumes are cold stamping and hot forging.

Cold stamping

It is a fairly progressive technology that allows the production of fasteners in large quantities with small losses of no more than 7% of the total number of products. Special automated machines allow you to receive up to 400 products within a minute.

Stages of manufacturing fasteners using cold technology.

1. Bars are prepared from the desired type of steel. Before processing, they are cleaned of rust or foreign deposits. Then phosphates and a special lubricant are applied to them.
2. Slicing. Metal blanks are placed in a special mechanism and cut into pieces.
3. The blanks of the nuts are cut off with a movable cutting mechanism.
4. Stamping. After all the previous manipulations, the blanks are sent to a hydraulic stamping press, where they are shaped and a hole is punched.
5. The final stage. Cutting threads inside parts. This operation is carried out on a special nut-cutting machine.

After completing the work, some nuts from the batch must be checked for compliance with the preset parameters. These are dimensions, threads and the maximum load that the product can withstand. For the production of hardware using this technology, a certain steel is used, intended for cold stamping.

Hot forging

Hot nut technology is also very common. The raw material for the production of hardware in this way is also metal rods, cut into pieces of the required length.

The main stages of production are as follows.

• Heat. The cleaned and prepared rods are heated to a temperature of 1200 degrees Celsius so that they become plastic.
• Stamping. A special hydraulic press forms hexagonal blanks and punches a hole inside them.
• Thread cutting. Products are cooled, threads are applied inside the holes. For this, rotating rods resembling taps are used. To facilitate the process and prevent rapid wear during cutting, machine oil is supplied to the parts.
• Hardening. If the products require increased strength, they are hardened. To do this, they are heated again to a temperature of 870 degrees Celsius, cooled at high speed and immersed in oil for about five minutes. These actions harden the steel, but it becomes brittle. To get rid of fragility, while maintaining strength, the hardware is kept in an oven for about an hour at a high temperature (800-870 degrees).

After the completion of all processes, the nuts are checked on a special stand for compliance with the strength requirements. After checking, if the hardware has passed it, they are packed and sent to the warehouse. The production facilities still have outdated equipment in need of repair and maintenance work. For the production of fasteners to such equipment, turning and milling machines are used. However, such works are characterized by very low productivity, huge consumption of materials. But they are needed in any case, and therefore, for small batches of fasteners, this technology is still relevant.

See the following video for the manufacturing process of nuts and other hardware.

8.2 Hardness test

During the control, the hardness test is carried out on one of the support surfaces of the nut at three points offset relative to each other by 120 °, the hardness values ​​are determined as the arithmetic mean of the three measurements.

In auxiliary cases, the hardness is checked at the longitudinal section passing through the nut, at points located as close as possible to the outer diameter of the nut thread.

Vickers hardness testing is critical, wherever possible, a load of 30HV should be applied.

In the case of Brinell and Rockwell tests, translations should be used.

Hardness test according to Vickers-according to GOST 2999.

Hardness test according to Brinell-according to GOST 9012.

Rockwell hardness test according to GOST 9013.

9.1 Symbols

The markings are shown in Tables 12 and 13.

Table 12 - Marking designations of strength class nuts in accordance with 3.1

The endingtables
12

Table 13 - Marking of nuts in strength class in accordance with 3.2

Material

The strength of the fastener is one of the most important parameters characterizing hardware. The strength of the fastening directly depends on the type of material from which it is made. Depending on the strength characteristics, the metal for production and the mode of its heat treatment are selected.

Most commonly, standard bolts are made from alloy and non-alloy steel. But in some cases, they can be made of other metals, such as copper, aluminum, which are used in cases where there is no need to achieve high strength of fasteners. For jewelry making, fasteners can be made from precious metals such as gold, silver, and others.

Very often, to improve the technical characteristics of hardware, a special coating is used that protects the fastener from environmental influences (humidity, water, temperature range, chemicals).

Among the most common coatings, the following varieties can be distinguished:

1. Zinc. The thickness of the zinc coating can vary depending on requirements. For bolts used in household appliances, the thickness is usually small. But for industrial fasteners, it is usually up to 25 microns.
2. Cadmatized. Not a very common type of bolt due to the toxic properties of cadmium. Therefore, they are used in rare cases when the coating from other materials does not allow achieving the desired performance characteristics.
3. Nickel. Typically used for fasteners used in furniture manufacturing. Nickel increases only decorative characteristics, practically does not change the operational characteristics.
4. Phosphating and oxidation. They are used to create a protective layer on the surface of bolts due to the oxidation of the base metal.
5. Zinc lamellar. They are becoming widespread, as they can increase the service life of fasteners several times.

8.1 Proof load test

The trial load test should always be carried out when the test equipment allows it, and this test is critical for thread diameters of M5 or larger.

The nuts are screwed onto the hardened mount as shown in Figures 1 and 2. In secondary cases, the tensile test is decisive (see Figure 1).

1) D11 - by GOST 25346

Drawing 1
—Trialonaxialstretching

Drawing 2
—Trialonaxial compression

The test load is applied to the nut in an axial direction and held for 15 s. The nut must withstand the load without breaking or breaking the thread and can be unscrewed manually after the load is removed. If the repair thread is damaged during the test, the test is invalid. When unscrewing the nut, it is allowed to use a wrench to turn the nut 1/2 turn, and then it must be unscrewed by hand.

The hardness of the test fixture must be at least 45 HRC.

The thread tolerance is 5h6g, except for the outer diameter of the thread, which must be 1/4 of the 6g tolerance on the lower part.

By the shape of the heads

Given the variety of bolts and their designs, there is an additional classification. Depending on the shape of the head, the following are distinguished:

1. The universal (hex head) is the most common type and can be tightened with conventional wrenches. It is used in almost all areas, most often this type can be found in mechanical engineering and construction.
2. Anchor - a special type that is used to fasten heavy elements. These bolts come in a wide variety of shapes and sizes. Typically used in construction for the installation of window frames, doorways, suspended ceilings and lighting fixtures.
3. The eyebolt is a high strength fastener that has a fastening ring instead of a head. This design allows you to lift large loads. Some models provide a pivot ring that allows it to rotate freely on the base.
4. With a semicircular head - often used in the manufacture of furniture and the installation of fences.
5. Countersunk head - commonly used in radio and electrical engineering. After tightening, the bolt head does not protrude above the surface, the bolt head is completely flat and has internal slots for tightening with a screwdriver. The head diameter is slightly larger than the rod diameter.
6. Bolt without a head - this type does not have a part called a head, the slots for screwing are cut directly into the rod.