All about bolt strength

Fasteners represent a large assortment on the market. They can be used both for the usual connection of various parts of structures, and in order for the system to withstand increased loads, to be more reliable.

The bolt is a cylindrical fastener with a thread on the outside. Usually has a hex head made for a wrench. The connection is made with a nut or other threaded hole. Before the creation of screw fasteners, bolts were called any products in the form of a rod.

The design of the bolt is as follows.

With its help, the rest of the fastener is transmitted torque... It can have a hexagonal, semicircular, semicircular with a screw, cylindrical, cylindrical with a hexagonal recess, countersunk and countersunk with a screw.

It is divided into several types:

• standard;
• for installation in a hole with a gap;
• for mounting into a reamer hole;
• with a shank of reduced diameter without thread.

It can be of the following forms:

• round;
• wing nut;
• hex (with chamfers low / high / normal, crown and slotted).

There are many types of bolts, it all depends on what qualities the structure should have during operation. The strength class of bolts describes their mechanical properties.

Strength is a property of a product characterized by resistance to destruction from external factors. Any manufacturer must indicate the strength of the product so that during installation or assembly it is clear whether the fasteners are suitable for certain cases. Strength is measured in two numbers, separated by a dot, or a two-digit and single-digit number, also separated by a dot:

• 3.6 - connecting elements made of unalloyed steel, additional hardening is not applied;
• 4.6 - used for production of carbon steel;
• 5.6 - are made of steel without final tempering;
• 6.6, 6.8 - hardware made of carbon steel, without impurities;
• 8.8 - components such as chromium, manganese or boron are added to the steel; additionally, the finished metal is tempered at a temperature above 400 ° C;
• 9.8 - has a minimum of differences from the previous class and a higher strength;
• 10.9 - for the production of such bolts, steel is taken with additional additives and tempering at 340-425 ° C;
• 12.9 - stainless or alloy steel is used.

The first number means the tensile strength (1/100 N / mm2 or 1/10 kg / mm2), that is, 1 millimeter of a square bolt 3.6 will withstand a break of 30 kilograms. The second number is the percentage of yield strength to tensile strength. That is, the 3.6 bolt will not deform up to a force of 180 N / mm2 or 18 kg / mm2 (60% of the ultimate strength).

Based on the strength values, the connecting bolts are divided into the following options.

• Tensile-rupture on the inner diameter of the bolt. The higher the strength of the fastener, the more likely it is that the bolt will deform under load, that is, it will stretch.
• Functioning to cut the bolt in two planes. The lower the strength, the higher the likelihood that the mount will fail.
• Tensile and Shear - Shears the bolt head.
• Frictional - here there is a crushing of the material under the fasteners, that is, they function for a cut, but with a high tension of the fasteners.

Yield point - this is the greatest load, with an increase in which deformation occurs, which cannot be restored in the future, that is, the screw connection will increase in length after certain actions. The heavier the structure can withstand, the higher the flow rate. When calculating the load, usually take 1/2 or 1/3 of the yield strength. Consider a kitchen spoon as an example - bending it to one side creates a different object. The fluidity was broken - this led to deformation, but the material itself did not break. It can be concluded that the elasticity of steel is higher than its yield.

Another object is a knife, which will break when bent. Consequently, the strength of strength and yield is the same. Products with such characteristics are also called fragile. Tensile limit - a change in the size and shape of a material under the influence of external factors, while the product is not destroyed. In other words, it is the percentage of elongation of the material compared to the original sample. This characteristic shows the length of the bolt before breakage. Size classification - the larger the area, the greater the torsion resistance.

Fasteners are also divided by such an indicator as accuracy. Various methods of threading and surface treatment are used in production. It can be elevated, normal and rough.

• C is rough accuracy. These fasteners are suitable for holes 2-3 mm larger than the rod itself. With such a difference in diameters, the joints can move.
• B is normal accuracy. The connecting elements are installed in holes 1-1.5 mm wider than the rod. They give in to less deformation in comparison with the previous class.
• A - high accuracy... The holes for this bolt group can be 0.25-0.3 mm wider. Fasteners have a fairly high cost, since they are produced by turning.

For fasteners made of stainless steel, they indicate not the class, but the tensile strength, their designation is different - A2 and A4, where:

• A is the austenitic structure of steel (high-temperature iron with a crystalline GCC lattice);
• the numbers 2 and 4 are the designation of the chemical composition of the material.

Stainless bolts have 3 strength indicators - 50, 70, 80. In the production of high-strength bolts, alloys with higher hardness and strength are used. Such materials are more expensive than carbon steel. Strength class varies - 6.6, 8.8, 9.8, 10.9, 12.9. Also, to increase the performance, a stage of heat treatment is carried out, which changes the chemical composition and structure of the material. Possible operation at temperatures below 40 ° C - has the designation U. 40-65 ° C is marked as HL.

Bolt hardness Is the ability of a material to resist penetration of another body into its surface. Bolt hardness is measured by Brinell, Rockwell and Vickers. Brinell hardness tests are carried out on a hardness tester, a hardened ball with a diameter of 2.5, 5 or 10 millimeters serves as an indeter (pressed object). The size depends on the thickness of the material being tested. The indentation takes place within 10-30 seconds, the time also depends on the tested material. The resulting print is then measured with a Brinell magnifier in two directions. The ratio of the applied load to the surface of the indentation is the definition of hardness.

Rockwell's method is also based on indentation. A diamond cone acts as an indeter for hard alloys, and a steel ball with a diameter of 1.6 millimeters for softer alloys. In this method, the test is carried out in two phases. First, a preload is applied to make the material and tip come into close contact. Then the main load goes on for a short time.After the work load is removed, the hardness is measured. That is, the calculations will be performed according to the depth at which the indeter remains, with the applied preload. In this method, 3 groups of hardness are distinguished:

• HRA - for extra hard metals;
• HRB - for relatively soft metals;
• HRC - for relatively hard metals.

Vickers hardness is determined by the width of the print. The pressed-in tip is a diamond pyramid with four faces. Measured by calculating the ratio of the load to the area of ​​the resulting mark. Measurements are made under a microscope mounted on the equipment. This method is characterized by increased accuracy and hypersensitivity. The measurement methods used in accordance with GOST in Soviet times did not allow determining all the maximum allowable loads on fasteners, therefore, the materials produced were of poor quality.

• Lemeshny... With its help, suspended heavy structures are attached. Most often used for agriculture.

• Furniture. The main difference is that the thread is not applied over the entire rod. The head is smooth - this is done so that the bolt does not protrude above the plane. In addition to the production of furniture, this fastener has found its application in construction.

• Road. Used when installing fences. It is distinguished by a semicircular head, under which there is a square headrest. Thanks to this design, the elements are firmly fixed.

• Mechanical engineering... The most popular type used in car manufacturing.

• Travel. Used in the construction of railways, it is usually used to connect rail parts. The thread is applied to less than half of the shank.

All fasteners are marked according to the standards:

• GOST;
• ISO is a system introduced in most states since 1964;
• DIN is a system created in Germany.

Taking into account all the requirements and standards, the following designations are applied to the bolt head:

• the strength class of the raw material from which the fasteners were made;
• manufacturer's plant sign;
• thread direction (usually only the left direction is indicated, the right one is not marked).

The applied marks can be either in-depth or convex. Their size will be determined by the manufacturer itself.

In accordance with GOST standards, the following designations are applied to the bolts.

• Bolt - the name of the fastener.
• Bolt precision. It has a letter decoding A, B, C.
• The third is the performance number. It can be 1, 2, 3 or 4. The first performance is not always indicated.
• Letter designation of the type of thread. Metric - M, conical - K, trapezoidal - Tr.
• The size of the thread diameter is usually indicated in millimeters.
• Thread pitch in millimeters. It can be large or basic (1.75 millimeters) and small (1.25 millimeters).
• The LH thread direction is left-handed, the right-hand thread is not indicated in any way.
• Precision carving. It can be fine - 4, medium - 6, rough - 8.
• Fastener length.
• Strength class - 3.6; 4.6; 4.8; 5.6; 5.8; 6.6; 6.8; 8.8; 9.8; 10.9; 12.9.
• Letter designation C or A, that is, the use of calm or free-cutting steel. This designation is only suitable for bolts with strength up to 6.8. If the strength is higher than 8.8, then the steel grade will be applied instead of this marking.
• Number from 01 to 13 - these numbers indicate the type of coating.
• The last is also the digital designation of the coating thickness.

The main parameters for measuring the dimensions of fasteners are length, thickness and height. To determine these parameters, you must first visually understand what type of bolt is available. The diameter of the fastener can be measured with a vernier caliper or ruler. Accuracy measurement is carried out with the PR-NOT calibration kit - pass-not pass, that is, one component is screwed onto the anchor, the second is not.Length is also measured with a caliper or ruler.

Screw measurements are indicated:

• M - thread;
• D is the size of the thread diameter;
• P - thread pitch;
• L - bolt size (length).

The thread diameter is measured in the same way as for bolt measurements. The thread diameter of the nuts is more difficult to determine. Typically, the marking characterizes the outer diameter of the bolt that will be screwed into the nut, that is, the nut hole will be smaller. The accuracy of the diameter can also be measured using the PR-NOT kit. It is worth remembering here that the size of the nut can be reduced, normal and increased.

During construction, the connection of structures is mainly carried out using bolted connections. Their main advantage is easy installation, especially if we take welding joints for comparison. The formulas used to calculate tensile joints depend on the substrate material (concrete, steel, mortars and material combinations).

The main condition for installing fasteners is holding the bolts of the general structure... Highest load-bearing capacity of hanging grade alloy steel anchors. The force of additional impacts can be dynamic, static and maximum. The additional load mass does not exceed 25% of the breaking force of the bolt shank.

The bolting method has become very popular in the modern world. Based on all the characteristics, you can highlight the points that you should pay special attention to when choosing:

• the field of activity where the fastening will be applied;
• head design;
• used material;
• strength;
• is there an additional protective coating;
• marking according to GOST.

In the next video, you will find more information on the strength grades in bolt marking.