Tightening bolts with a torque wrench: tables, methods for determining forces

To increase the strength and service life of threaded connections, as well as to increase their resistance to various external factors, it is necessary to correctly tighten the fasteners, calculating the screwing force. Each connection has its own specific degree of tightening depending on the seat. The tightening torque is calculated depending on the temperature conditions, the properties of the material and the load that will be exerted on the threaded connection.

For example, under the influence of temperature indicators, the metal begins to expand, and under the influence of vibration, an additional load is placed on the element. Accordingly, to minimize the influencing factors, the bolts must be tightened with the correct force calculated. We suggest that you familiarize yourself with the bolt tightening force table, as well as the methods and tools for performing the work.

What is lingering force and how do you know it?

The tightening torque is an indicator of the force that must be applied to threaded connections during the process of screwing them. If the fastener was tightened with a little force than was necessary, then under the influence of various mechanical factors the threaded connection may not withstand, the tightness of the fastened parts is lost, which entails serious consequences. Also, with excessive force, the threaded connection or fastened parts can simply collapse, which will lead to thread failure or the appearance of cracks in structural elements.

Each size and strength class of threaded connections has a certain tightening torque when working with a torque wrench, which is indicated in a special table. In this case, the designation of the strength class of the product is located on its head.

Required axial bolt force

Essentially, the tightening torque of a bolt creates a pressing force against the surfaces. Force is very important, since connections are different, in some cases it is important to press the surfaces, for example, in metal-to-metal contact, and in some cases, excessive force can damage the connection, for example, installing a cover through a rubber gasket, or installing a plastic part on a metal frame.

First, the designer determines the required surface pressing force, then determines the diameter of the bolts or their number. I talked about how to determine the diameter and quantity in the lesson “Calculating bolts.” Then the tightening torque is assigned. There is a little trick here: When a little force is required (gasket or plastic), it is better to assign a little more bolts of a smaller diameter, which will allow them to be positioned with a smaller pitch and press the surfaces more evenly. And, the closer the bolt tightening torque is to the recommended value, the less likely it is that spontaneous unscrewing will occur.

Marking and strength class of parts

The numerical designation of the strength parameter of a metric bolt is indicated on the head, and is presented in the form of two numbers separated by a dot, for example: 4.6, 5.8 and so on.

1. The number before the dot indicates the nominal size of the ultimate tensile strength, is calculated as 1/100, and is measured in MPa. For example, if the product is marked as 9.2, then the value of the first number will be 9*100=900 MPa.
2. The number after the point is the ultimate yield in relation to strength; after calculation, the number must be multiplied by 10, as indicated in the example: 1 * 8 * 10 = 80 MPa.

Strength Class Designation for Metric Bolts
The yield strength represents the maximum load on the bolt structure. Elements that are made from stainless steel types are designated directly by the type of steel itself (A2, A4), and only after that the ultimate strength is indicated.

For example, A2-50. The value in such markings indicates 1/10 of the strength limit of carbon steel. At the same time, products for the manufacture of which carbon steel is used have a strength class of 2.

The strength designation for inch bolts is marked with notches on its head.

Designation of the strength class of inch bolts

Tightening a bolt without a torque wrench

If necessary, you can obtain the required tightening force without a torque wrench. To make a tightening, you will need a regular wrench, an open-end or a spanner wrench is best, a tape measure or ruler and a cantor with a measuring limit of up to 40 kg, an electronic device is optimal.

First, in the table you need to find out the tightening torque of the threaded connection. Next, you need to measure the length of the key from the central part of the bolt head to the point where the rotating force is applied in centimeters. After this, we calculate the value that should be on the cantor using the formula F=M/(0.1·L), where M is the moment of force from the table, and L is the length of the key in centimeters. For example, if you need to tighten an M6 bolt of strength class 8.8 with a 25 cm long wrench, you need to proceed as follows:

• In the table we find the tightening torque value, which is 10.5 Nm.
• We calculate using the formula F = 10.5/(0.1•25) = 4.2 kg•m or kilogram of force per meter.
• At a distance of 25 cm from the bolt head, at an angle of 90 degrees to the longitudinal axis of the key, we place the cantor and tighten it until it shows 4.2 kg.

Obviously, this method is not ideal and cannot be used in critical installations. But still, this is much better than tightening structural elements based only on your own feelings.

Despite the existence of such “folk” methods for tightening bolts, it is advisable to buy a high-quality torque wrench that will avoid such inconveniences. This will guarantee the quality, strength and durability of the installation, which will last the entire service life of structures, machines or equipment.

How is traction force measured?

The basic measurement of bolt tightening force is Pascal (Pa). The international SI system assumes that this unit measures both pressure and mechanical stress. Accordingly, Pascal is equal to the pressure value that is caused by a force equal to one Newton and is uniformly distributed over a plane measuring 1 m2.

To understand how you can convert one unit of measurement to another, let's look at an example:

• 1 Pascal = 1 Newton/m2;
• 1 MPascal = 1 Newton/mm2;
• 1 Newton/mm2 = 10 kgf/cm2.

Tightening force values ​​for different types of bolts (table)

For a more convenient and accurate perception, a table is presented for tightening bolts with a torque wrench.

We will also present a table of tightening torques for inch threads according to the standard used in the United States.

Torque application method

The most common and probably the easiest method for tightening threaded connections. It consists of creating a torque on the nut that provides the necessary pre-tightening force. And its main advantage is that it is very simple, takes a minimum of time and the tool used is relatively inexpensive.

Torque (μr, in Nm) is the moment of force applied to the nut at a certain distance from its center (the product of force on the arm), the action of which causes the nut to rotate around its axis.

The bolt in the threaded connection is under constant mechanical stress and is resistant to fatigue. However, if the initial force is too small, the bolt will quickly become damaged under changing loads. If the initial force is too great, the tightening process may cause the bolt to break. Consequently, reliability depends on the correct choice of the initial force and, accordingly, control of the torque on the nut is necessary.

A critical factor when tightening a threaded connection is the pre-tightening force of the parts being connected. Torque indirectly characterizes the magnitude of the pre-tightening force.

The pre-tightening force (Q, in H) , to which the threaded connection is tightened, is usually taken within 75-80%, in some cases 90%, of the test load.

The proof load (N, in N) is the reference value that a rod fastener must withstand when tested. The test load is approximately 5%-10% less than the product of the yield strength of the rod fastener and the nominal cross-sectional area.

The test load, in accordance with GOST 1759.4, for fasteners with strength class 6.8 and higher is 74-79% of the minimum breaking load (P, in H).

The minimum breaking load corresponds to the product of the tensile strength (temporary tensile strength) of the rod fastener and the nominal cross-sectional area.

Accordingly, the pre-tightening force should not lead to a transition of the rod fastener from the elastic region to the plastic deformation region of the material.

The question often arises: why “preliminary”. The fact is that tightening connections implies the creation of some stress in all parts - both fastening and connecting ones. At the same time, in elastically stressed bodies, some mechanisms of plastic deformation appear, leading to a decrease in stress over time (the phenomenon of stress relaxation). Therefore, after some time, the tightening force of the connection decreases slightly without any additional force acting on it.

The required tightening torque for a particular connection depends on several variables:

1. The coefficient of friction between the nut and the rod fastener;
2. The coefficient of friction between the surface of the nut and the surface of the part being connected;
3. Thread quality and geometry.

The most important is the friction in the thread between the nut and the rod fastener, as well as the nut and the surface of the part being connected, which depend on factors such as the condition of the contact surfaces, type of coating, presence of lubricant, pitch and thread profile angle errors, deviation from the perpendicularity of the supporting thread end and axis, screwing speed, etc.

Friction losses can be quite large. With almost dry friction, a rough surface and shrinkage of the material, the losses can be so large that when tightening directly to the tension of the connection, no more than 10% of the torque will remain (see figure above). The remaining 90% is spent on overcoming frictional resistance and shrinkage.

To illustrate, let's show the following example: when the equipment is installed, the connections are new and clean. After a few years of operation, they become dirty, recoded, etc. Thus, when unscrewing and tightening, there is more “parasitic” friction. And although the impact wrench will show the required torque, the required compression of the connection will not be achieved. And when, during operation, the threaded connection is exposed to loads or vibration, there is a high risk of the connection self-loosening and, as a result, an accident.

The coefficient of friction can be reduced by using oil, but not excessively, since there is a high risk of excessive drop in resistance and excessive joint tension, which can lead to destruction of the rod fastener.

The friction coefficient values ​​under real assembly conditions can only be predicted. As numerous experiments show, they are not stable. In table their reference values ​​are given.

Table Values ​​of friction coefficients in the thread of a steel rod fastener µр and between the surface of the nut and the surface of the connected part µt

For fasteners made of stainless steel A2 and A4, friction coefficients:

1. Without lubricant: µр – 0.23-0.50 µt – 0.08-0.50
2. With lubricant including chloroparaffin: µр– 0.10-0.23 µt – 0.08-0.12

The rated torque is calculated using the formula:

Micro = 0.001 Q*(0.16*P + µр *0.58* d2 + µt *0.25*(dt + d0)),

where µр is the coefficient of friction in the thread between the nut and the rod fastener;

µt is the coefficient of friction between the surface of the nut and the surface of the part being connected;

dt – diameter of the supporting surface of the bolt or nut head, mm;

d0 – diameter of the hole for the fastener, mm;

P – thread pitch, mm;

d2 – average thread diameter, mm;

Q – pre-tightening force.

To simplify the calculations of Mkr, the friction coefficients are averaged. The average coefficients of friction of steel fasteners correspond to the following surface conditions:

- 0.1 - phosphated or galvanized bolt, well-lubricated surface -0.14 - chemically oxidized or galvanized bolt, poor quality of lubrication -0.2 - uncoated bolt, no lubricant

The pre-tightening force is determined by the requirements for the connection, therefore our recommendations for choosing the pre-tightening force and torque given in the tables are for reference only and cannot be taken as a guide to action, taking into account the many factors that affect the quality of the connection.

To select the pre-tightening force of threaded connections and torque for different strength classes, you can use the tables below. The tables are given for connections having an average coefficient of friction of 0.14.

Pre-tightening force and torque of a threaded connection with a coarse thread thread and a coefficient of friction of 0.14

Pre-tightening force and torque of a threaded connection with a fine pitch thread and a coefficient of friction of 0.14

UNLOCKING

When unscrewing nuts, a greater torque is required than when tightening. This is explained by corrosion of the threaded connection, mutual penetration of the materials of the bolt and nut in the thread zone under the influence of prolonged load.

The general rule is that when unscrewing, a torque is required 1.3-1.5 times greater than when tightening!

When unscrewing corroded and painted connections, a tool with a torque 2 times greater than when tightening is often required. But in such cases it is better to use special means to destroy corrosion products. This will reduce friction and, accordingly, the forces acting on the thrust part of the tool, extending its life.

Torque values ​​for band clamp with worm clamp

The table below contains a number of information about the initial installation of band clamps on a new hose, as well as about re-tightening already crimped hoses.

Determining the tightening torque

Torque wrench

The selection of this tool should be carried out so that the tightening torque on the fastening element is 20-30% less than the maximum torque value on the wrench used. If you try to exceed the permissible limit, the tool can easily break.

The tightening force and grade of material must be present on each product; methods for deciphering the markings are described above.

To perform secondary tightening of bolts, you should adhere to the following recommendations:

1. Know exactly the required tightening force.
2. When performing a control check of tightening, it is necessary to set the force and check each fastener in a circle.
3. It is forbidden to use a torque wrench as usual; it should not be used to tighten parts, nuts and bolts to obtain only an approximate force. It should be used to perform control broaching.
4. The torque wrench must have a margin to measure the torque.

Control over the tightening of fasteners

We recommend that you perform controlled tightening of fasteners. By using dynamometer devices you get several advantages:

• The precise load on the fastener elements allows you not to worry about the integrity of the hardware, nuts and thread reliability.
• The load distribution during screwing becomes uniform. This ensures uniform compression in fastening joints and increases the reliability of the structure as a whole.
• Eliminates the risk of injury at work. The device helps to avoid excessive force and makes working with fasteners easier and safer.
• Save time. It takes much less time to tighten the nut than without torque tools.
• No defects when making fastening connections.

To ensure that anyone can tighten the bolts with the required force, torque wrenches are used. Dynamometric devices are in demand in all areas of construction, in the repair and production of automobiles, in the assembly of furniture, household appliances and in many other areas. There are several varieties of this tool:

• A click-type torque wrench is the most common type of tool. When the required bolt tightening torque is reached, the wrench clicks and stops transmitting torque to the fastening joint. The maximum value of the twisting force is set in advance.
• Arrow torque wrench - requires control over the applied force during use. The main disadvantage is that the required force value cannot be set in advance. This is especially inconvenient if the fasteners need to be installed in a hard-to-reach place. The principle of operation of the tool: the handle with the scale moves to a certain angle. The key pointer remains fixed. An arrow wrench is not suitable for a person without experience - it requires professionalism and the ability to “feel” the force when tightening the nuts.
• A digital torque wrench works in the same way as a limit wrench. The difference is that the bolt tightening force is measured using an electronic mechanism. When the required torque when screwing the nut is reached, a sound signal sounds. You can track the change in twisting force over time on the digital display of the device.

When high-strength bolts need to be tightened, an additional tool may be required to increase the torque. For these purposes, it is customary to use a multiplier key. This tool is also useful for tightening nuts in hard-to-reach places. The multiplier should be selected taking into account the characteristics of the torque wrench. Experts recommend buying a torque wrench with a force that is 5 times less than that of a multiplier. The multiplier can have any form - the choice depends on personal preferences and ease of use. You cannot use a multiplier wrench without a torque tool. This is equivalent to applying a lever of considerable length without controlling the torque force. The result may be an overtightened fastening joint.

If you need to calculate how much to tighten bolts when changing tires on a car or truck, you can simply install a special app on your smartphone. Suitable software for gadgets was released by Bridgestone. The application works very simply: the user enters the make of the car and receives the torque value of the bolts with the necessary tolerances. Now you don’t need to save tables to the cloud or carry paper instructions with you - the program will tell you how to tighten the hardware in accordance with the manufacturer’s recommendations.