Influence of Friction Coefficient on Stability of Bolted Connection

Summary:

Bolt connection makes two or more connected parts closely fit through the clamping force between connected parts;

The friction coefficient of the bearing surface and thread are two important factors affecting the clamping force;

In this article, the assembly process of high-strength bolts for slewing ring of a large crane is taken as the research object.

First, the relationship between installation torque, friction coefficient, torque coefficient and the clamping force is theoretically analyzed.

Then, through the comparative test of tightening under the full lubrication condition of thread lubrication and thread and bearing surface, it is finally shown that the lubrication condition has a great impact on the reliability of fastener installation and the dispersion of torque coefficient and friction coefficient.

The dispersion of friction coefficient and torque coefficient under full lubrication is smaller, and the stability and reliability of bolted connections are higher.

Influence of Friction Coefficient on Stability of Bolted Connection 1

Preface

As a typical mechanical connection mode, bolt connection is widely used in hoisting machinery.

Its reliability is related to the overall performance of hoisting machinery, especially the high-strength bolt connection of key parts such as slewing ring.

Its reliability is directly related to the normal operation of products.

Once the connection fails, it may even cause serious safety accidents.

The purpose of bolt connection is to make two or more connected parts closely fit.

In order to bear certain motion load, sufficient clamping force is required between connected parts to ensure reliable connection and normal operation of connected parts;

Not enough clamping force may cause lateral sliding between two parts, which will subject the bolt to unnecessary shear stress, and finally lead to bolt fracture.

Therefore, how to improve the reliability and stability of high-strength bolt connection has attracted more and more attention.

Improper tightening parameters or tightening process control will lead to poor reliability of threaded connection and failure.

From the point of view of reducing the dispersion of clamping force and optimizing the tightening process, through theoretical analysis and comparison of test data, this article determines the installation torque and tightening process of high-strength bolts for slewing ring of large hoisting equipment.

1. Theoretical analysis

1.1 Use the following formula to determine the torque coefficient according to the relationship between tightening torque and clamping force.

Influence of Friction Coefficient on Stability of Bolted Connection 2

Where:

  • K – torque coefficient;
  • T — Tightening torque;
  • F — Clamping force;
  • D – Nominal diameter of the thread

1.2 According to GBT16823.3, the torque of bolts shall meet

Influence of Friction Coefficient on Stability of Bolted Connection 3

Where,

  • P — screw pitch;
  • μth — thread friction coefficient;
  • μb – friction coefficient of nut or bolt bearing surface;
  • d2 – pitch diameter of thread;
  • dh — inner diameter of bearing surface contacted;
  • d0 – outer diameter of bearing surface.

In bolt installation, the tightening torque T can be divided into three parts:

Bearing surface friction torque Tb consumed by bearing surface friction between nut and washer plane;

Thread torque Tth  and clamping force F consumed by friction between bolt thread and nut (body) thread;

When installing the fastener connecting pair, most of the installation torque is lost in the first two friction;

Therefore, the end friction coefficient μb and thread friction coefficient μth are the main factors affecting the clamping force F.

The clamping force may vary greatly with different friction coefficients;

The dispersion of end face friction coefficient μb and thread friction coefficient μth directly determines the stability of clamping force F. See Fig. 1.

Influence of Friction Coefficient on Stability of Bolted Connection 4
Fig.1

1.3 According to the relationship between thread torque and clamping force, the thread friction coefficient is calculated and determined by the following approximate formula.

Influence of Friction Coefficient on Stability of Bolted Connection 5

1.4 According to the relationship between the friction torque of the bearing surface and the clamping force, the friction coefficient of the bearing surface is calculated and determined by the following approximate formula.

Influence of Friction Coefficient on Stability of Bolted Connection 6

It can be seen that the torque coefficient, thread friction coefficient and bearing surface friction coefficient can be calculated by measuring the tightening torque, thread torque, bearing surface friction torque and clamping force.

2. Test analysis

By testing the tightening process of different series of slewing ring mounting bolts, the influence of torque coefficient, thread friction coefficient and bearing surface friction coefficient of bolted connection on the clamping force in bolted connection under different conditions is analyzed and studied.

The test equipment is a vertical friction coefficient tester (as shown in Fig. 2) of Schatz, Germany, and the test method is in accordance with GB/T 16823.3.

The total torque, thread torque, bearing surface friction torque and clamping force are detected through the torque/angle sensor and clamping force sensor.

According to formulas (1), (3) and (4), the test equipment can automatically calculate the torque coefficient, thread friction coefficient and end face friction coefficient.

Influence of Friction Coefficient on Stability of Bolted Connection 7
Fig.2

The bolts used for the slewing ring installation of a large crane are selected for the test to simulate the actual assembly conditions.

The test nuts are replaced by nuts, and their materials, processing equipment and assembly process are consistent with those of the products.

The bolt specifications are DIN931, M48-10.9;

Material: 40CrNiMo;

Surface state: Dacromet;

Corresponding washer: DIN6919, inner diameter 49  outer diameter 82;

Test nut replacement, made of domestic 960;

The lubricant is a silver-based high-temperature anti-seize agent.

In order to compare the influence of the lubrication state on friction coefficient, torque coefficient and connection dispersion, we have conducted two groups of comparative tests.

One group is to evenly apply grease on the threaded part of the bolt and not on the bearing surface of the washer;

The other group is to apply grease on the threaded part of the bolt and the upper surface of the bearing surface of the washer. See Table 1 for test data.

Table 1

Lubrication method

Number

F(KN)

T (N.m)

K

1th

1b

1tot

Full lubrication

1#

1000.3

5389.39

0.11

0.08

0.09

0.08

2#

1000.1

5185.81

0.11

0.07

0.09

0.08

3#

1000.24

5515.26

0.11

0.08

0.09

0.09

4#

1000.1

5683.1

0.12

0.09

0.09

0.09

5#

1000.1

5238.65

0.11

0.08

0.08

0.08

6#

1000.24

5394.05

0.11

0.08

0.09

0.08

7#

1000.37

5578.98

0.12

0.09

0.08

0.09

8#

1000.1

5768.57

0.12

0.08

0.1

0.09

Thread lubrication only

1#

1000.1

6568.71

0.15

0.09

0.13

0.1

3#

1000.4

5998.86

0.13

0.07

0.13

0.09

4#

1000.1

6716.1

0.15

0.09

0.14

0.11

5#

1000.3

5733.3

0.12

0.07

0.1

0.08

6m

1000.1

5982.98

0.13

0.08

0.12

0.09

7#

1000.2

5356.29

0.11

0.07

0.1

0.08

8#

1000.1

5990.22

0.13

0.09

0.13

0.09

Figures 3 and 4 show the curves of the friction coefficient μ_b of the bearing surface and the friction coefficient μ_th of the thread under the condition of thread lubrication only and full lubrication respectively.

The results show that the friction coefficient μ_th of the thread under the lubrication condition is between 0.07 and 0.09, and the values are relatively stable;

The friction coefficient μ_b of the bearing surface is distributed between 0.08 and 0.1 in the eight groups of samples under lubrication, which is relatively stable, while it is distributed between 0.1 and 0.14 in the eight groups of samples under no lubrication, which is relatively large, has poor stability, and has a large dispersion.

Influence of Friction Coefficient on Stability of Bolted Connection 8
Fig.3
Influence of Friction Coefficient on Stability of Bolted Connection 9
Fig.4

Fig. 5 shows the curve of torque coefficient K under thread lubrication only and full lubrication conditions.

The results show that the K coefficient of 8 groups of samples under full lubrication conditions is 0.11-0.12, with stable values and small dispersion;

However, the K value of the thread-lubricated sample is distributed between 0.11-0.15, which is relatively large, has poor stability and has a large dispersion.

Influence of Friction Coefficient on Stability of Bolted Connection 10
Fig.5

Fig. 6 and Fig. 7 show the torque clamping force curves of the bolt tightening process under thread lubrication only and full lubrication respectively.

The results show that, with the same tightening torque, the clamping force under full lubrication is higher than that under thread lubrication only, and the dispersion of the curves in the same group of samples is smaller.

Influence of Friction Coefficient on Stability of Bolted Connection 11
Fig.6
Influence of Friction Coefficient on Stability of Bolted Connection 12
Fig.7

From the data obtained from the above two groups of comparative tests, it can be seen that the friction coefficient and torque coefficient have significantly decreased by about 15% due to good lubrication after the gasket plane is coated with lubricant, and the stability has also been significantly improved;

Therefore, applying lubricant on the bearing surface is very effective for the reduction of torque coefficient and stability.

3. Industry status

At present, in construction machinery, fasteners are generally assembled by torque method, which is intuitive, simple and easy to operate.

The installation torque is determined by the friction coefficient, and the numerical value is accurate. However, if the dispersion of the friction coefficient, namely standard deviation, is ignored, the reliability of bolted connection may be greatly affected.

At present, the national standard GB/T 1231-2000 specifies the standard of high-strength bolt connection pairs for steel structures.

The torque coefficient K=0.11-0.15, and the standard deviation ≤ 0.01.

The user pays more attention to the torque coefficient and thinks that the installation torque can be determined immediately after the torque coefficient is determined, so that the fastener connection pairs can be installed, but the standard deviation is easy to ignore.

If the standard deviation is greater than 0.01, the pre-tightening force of individual bolts will deviate during installation.

If the standard deviation is too large or too small, the connection pairs of individual fasteners will be over-tightened or not tightened, which may pose a hidden danger to the installation’s reliability.

If the torque coefficient is too large and the standard deviation is less than 0.01, the dispersion of the connection pair is very small, and the force on each pair of fasteners is relatively uniform;

The torque coefficient is too large, and some torque values can be increased during installation, but no over-tightening or looseness will occur, and the reliability and safety of installation will be increased.

Therefore, from a certain point of view, the standard deviation is more important than the torque coefficient.

4. Conclusion

The thread friction coefficient, bearing surface friction coefficient and torque coefficient are several important technical parameters that must be understood and mastered during the installation of fastener connecting pairs.

At present, they have been widely understood and accepted by the majority of users, and are generally concerned during the installation.

The smaller the friction coefficient, the smaller the torque coefficient;

When the installation torque is determined, the smaller the torque coefficient is, the greater the clamping force is;

The bigger the torque coefficient is, the smaller the clamping force is;

When the torque coefficient is small to a certain extent, under the action of a certain torque, the clamping force exceeds the strength limit of the bolt, and the high-strength bolt will stretch or even fatigue fracture;

On the contrary, the larger the torque coefficient is, the smaller the clamping force generated is, and the whole bolt connection pair cannot achieve the locking function, which may lead to the looseness of the connection pair.

The lubrication state has a great influence on the thread friction coefficient, bearing surface friction coefficient and torque coefficient values.

In general, the torque coefficient and friction coefficient values of the fastener connection pair with rough product surface and obvious machining marks will increase.

After lubrication, the torque coefficient and friction coefficient can decrease significantly;

More importantly, the lubrication state also has a great impact on the dispersion of thread friction coefficient, bearing surface friction coefficient and torque coefficient.

The dispersion of thread friction coefficient, bearing surface friction coefficient and torque coefficient under lubrication state is smaller, and the stability and reliability of bolted connection are more guaranteed.

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