How To Choose A Quality Screw. The Concept Of The Quality Of Bolts According To GOST 7798-70, DIN Marking, Types, Designation

2023 Author: Philip Bishop | [email protected]. Last modified: 2023-08-25 07:54

• Bolt as an important structural element
• Strength characteristics
• Using bolts as directed
• Application of bolts taking into account the shape of the heads
• Differences between bolts by bar shape
• Division of products by accuracy class
• Designation and marking of bolts

In the construction of civil and industrial facilities, in mechanical engineering, shipbuilding, the production of agricultural machinery and equipment, special fasteners are used in the form of bolts with nuts of various sizes, which have certain characteristics in accordance with the requirements for strength.

Bolt as an important structural element

A bolt is a metal rod that can have different lengths and diameters of a rod with a thread and a head of a restrictive set shape.

The variety of bolts produced is classified according to certain criteria and is displayed in GOST according to the quality and type of steel (carbon, alloy).

To increase the service life and shelf life of bolts, products are processed with special protective compounds that prevent the appearance of metal corrosion:

• using zinc (coating up to 25 microns thick);
• nickel, for bolts used in furniture production;
• containing cadmium (it is toxic, therefore it is used only in special cases);
• using the technology of oxidation and phosphating;
• according to the technology of coating with a zinc-cadmium shell.

Strength characteristics

For bolts, 11 strength classes are established, which are regulated by the requirements shown in GOST 7798-70 with indicators from 3.6 to 12.9. The bolts with the highest level of strength are made from alloy steel, which is minimized in phosphorus and sulfur.

Bolts that are manufactured in accordance with GOST 1488-84 of the Russian Federation are analogous to DIN 478 screws, but there may be some differences in size between the products of these standards.

The current DIN 478 standard contains information on the parameters of set screws with a square head and a collar. Due to the presence of a shoulder, the bearing area of the screw is increased, which avoids pushing the material through the head and, in some cases, makes it possible to do without washers.

Bolt DIN 478 is produced with metric thread M6-M24 with partial thread length 18-38 mm. The square screw head has a side of 6-16 mm. The height of the collar of the product is 2-4 mm, and the diameter of the screw head along the collar is 13.5-25 mm. The total height of the collar head is 10 to 20 mm.

According to DIN 478, the set screws are available in strength class 5.8 / 8.8 / 10.9. The material for the production is ordinary steel or corrosion-resistant steel, as well as galvanized steel.

A very important parameter is the strength of the bolt under load!

Using bolts as directed

The bolts produced are divided according to the requirements:

• for mechanical engineering used in the production of equipment, machinery, machine tools;
• furniture ones have a smaller thread notch; smooth head;
• on roadways used for the manufacture of metal fences. The head of the bolt is made in the shape of a semicircle, and the headrest is square;
• on plowshares with a scope of application in agriculture;
• on track - specially designed for use during construction work on railway transport.

Application of bolts taking into account the shape of the heads

The use of bolts with different shapes and head fit sizes of the product are divided into groups:

• anchor used for the installation of heavy structures, used for the installation of doors, suspended ceilings, various lamps, window frames;
• universal (with a hex head) are used in production, construction;
• equipped with a semicircular head are used in the manufacture of furniture;
• eyebolts - products with fastening rings instead of a head:
• with countersunk head - used in the production of radio engineering products.

Differences between bolts by bar shape

The bolt shank can have different lengths, thread incisions, different diameters of the head and threaded part of the bolt. The type of such bolts is marked - stepped.

Use of different thread pitches.

The bolts are threaded depending on the technical conditions of use:

• inch made in the form of a triangular profile;
• metric, isosceles triangle thread;
• trapezoidal has an identical shape, creating strong friction during rotation, which prevents spontaneous unscrewing of the nut;
• rectangular is used for bolts subject to high loads;
• Thrust is designed for bolts in structures experiencing strong lateral displacement.

Division of products by accuracy class

The bolts produced are classified into 3 accuracy classes:

• class A - is characterized by almost the same diameter of the bolt rod and the bore, and the hole can have a smaller diameter by no more than 0.3 mm;
• class B (normal accuracy) - the difference between the diameter of the bolt and the hole should not exceed 3 mm, and the diameter of the rod should have a deviation of more than 0.52 mm;
• class C - the difference between the bolt rod and the bore hole should not exceed a tolerance of 11 mm.

Example

GOST 7808-70 Bolt of accuracy class A with a reduced hexagonal head, metric thread size from M8 to M48, length from 10 to 300 mm. Used to connect and fasten parts, assemblies, structures experiencing dynamic loads.

Legend:

d - nominal thread diameter

d1

- rod diameter P - thread pitch

D - head diameter

k - head height

b - thread length

S - turnkey size

e - diameter of the circumscribed circle, not less than

hw- width, not less than

dw- diameter, at least

l is the length of the bolt rod.

Designation and marking of bolts

To apply certain information characterizing the quality and strength indicators, the bolt head is most often used, on which can be applied:

numbers - indicating the strength class, indicating the permissible load that the bolt can withstand.

thread designation - indicated only when using a left-hand notch in the product.

letters - indicating the type of metal used to make the bolt. The applied designation system of fasteners in wide practice is used in 3 variants:

• full designation;
• abbreviated designation;
• simplified notation,
• which are regulated by GOST 1759 - 87, GOST 18126 - 94.

Correct reading of the information printed on the bolts will help to avoid an emergency situation in the event of a violation of the selection of bolts necessary for fastening.

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