Analysis of tightening torque of high strength bolts for wind turbines

introduction

In the assembly process of wind turbines, the high-strength bolt connection between the components is the most used connection method. Because of its simple structure, convenient assembly and disassembly, and no looseness under dynamic load, it is used in the assembly process. The application is very extensive. However, if the assembly is improper and the tightening torque is not well controlled, it is easy to cause loose bolting, slippery wire, broken failure, etc., which affects the normal operation of the unit and sometimes even causes serious consequences.

When the threaded connection pair does not have the friction torque to ensure reliable connection, its self-locking ability is poor. When the unit is subjected to variable loads such as vibration and impact during the operation of the unit, the bolt will loosen. Therefore, how to ensure the reliability of the bolt connection has always been a concern of the wind turbine assembly work.

We know that the friction torque of the threaded coupling pair is obtained by applying a tightening torque to the bolt to cause the thread pair to generate a preload. Therefore, to ensure reliable connection, it is necessary to ensure that the appropriate tightening torque is applied to the bolts, so that a suitable pre-tightening force is generated in the thread pair to achieve the best self-locking capability. In the assembly process of wind turbines, high-strength bolts are used for key parts and load-bearing parts, so controlling the tightening torque of high-strength bolts is one of the keys to preventing loose bolts.

1. Selection of high strength bolts

Since the wind turbine is subjected to large loads of vibration, shock, etc. during operation, the clamping force between the components is achieved by applying a pre-tightening force to the bolts, so the bolts must be made of high-strength steel. This is also the reason why wind turbines use high-strength bolts. In addition to the high strength of the high-strength bolt, after the tightening torque is applied, the bolt produces a large and controlled pre-tension, and the pre-tension is generated by the nut and the gasket to the same size of the connected piece. Pre-stress. Under the pre-pressure, a large frictional force is generated along the surface of the connected member. As long as the axial force is less than the frictional force, the member will not slip and the connection will not be damaged. In general, the high-strength bolts used on wind turbines are mostly in the 10.9 class.

2, high-strength bolt connection performance

According to the characteristics of the bolt, the high-strength bolt connection is divided into two types: friction type connection and pressure type connection. The essential difference between the two is that the limit state is different. In the seismic code, the calculation formula of the bearing capacity of the high-strength bolt limit is determined. Although the pressure-bearing type has an advantage in the design value, the bolt hole is a pore-type bolt hole similar to a normal bolt because it belongs to the shear-destroy type. The deformation when subjected to load is much larger than the friction type, so the high-strength bolt bearing type is mainly used for the connection of non-seismic members and the connection of non-bearing dynamic load members.

For the connection between the components of the wind turbine, slippage is not allowed. The high-strength bolt connection relies on the friction between the contact surfaces of the joints to prevent them from slipping each other. Therefore, the connection between the components of the wind turbine is Friction type connection. That is to say, when the high-strength bolt obtains a large pre-tensioning tension, a large frictional force is generated between the connecting members, thereby improving the integrity and rigidity of the joint. For the high-strength bolt friction type connection, the shear force is transmitted by the frictional resistance between the connected parts, and the shear force is equal to the frictional force as the limit state of the bearing capacity, and the pre-tensioning force cannot exceed the yield limit of the bolt material.

In actual work, the bearing capacity of the bolted joint is affected by factors such as pre-tension, anti-slip coefficient and steel type.

3. Pre-tensioning torque determination and control method for high-strength bolts

3.1 Determination of pre-tension

For high-strength bolted connections, a large pre-tightening force must be used. According to VDI2230-2003, the general pre-tightening force exceeds 70% of the yield limit of the bolt material, which will result in bolt damage during tightening. Therefore, the pre-tightening stress of the control bolt is about 70% of the yield limit of the bolt material.

Namely: maximum preload (N)

F0max = 0.7Aefu

Ae - thread nominal stress cross-sectional area, mm2;

Fu—the minimum tensile strength of the bolt after heat treatment, N/mm2

3.2 Determination of tightening torque

The tightening torque is calculated by the usual calculation formula. The calculation method here is in accordance with GB/T16823.2-1997.

Namely: tightening torque T (Nm)

T=kF0max d

K― is the torque factor. According to the test data of the bolt manufacturer, the average value of the torque coefficient is in the range of 0.09~0.126, and the standard deviation is less than or equal to 0.01. Here, the torque coefficient is K=0.11.

F0max - the maximum bolt preload, kN;

D― is the outer diameter of the bolt, mm

3.3 pre-tension control method

Large hexagonal high-strength bolts and torsion-shear high-strength bolts are pre-tensioned by tightening the screw to cause the screw to be stretched, thereby creating a pressing force between the connected plates. Therefore, to control the pre-tension, the tightening torque applied to the bolt must be controlled.

1 Torque method - suitable for large hexagonal high strength bolts

A specific torque wrench that can directly display the torque value is used. In the assembly work of the wind turbine, an electro-hydraulic torque wrench is often used to control the pre-tension by controlling the tightening torque.

In order to overcome the deformation of components and washers, the gap between the plates is basically eliminated, the tightening torque coefficient has a good linearity, and the accuracy of the pre-tension value of the construction control is improved. The tightening of the bolts should be based on the initial screwing, the double screwing and the final screwing. The steps are carried out. The initial tightening torque is about 50% of the construction torque, the double tightening torque is equal to the initial tightening torque, and the last 100% final screwing. When the bolts are screwed, twisted and screwed, they are twisted in the order of the diagonal and should be completed on the same day.

Advantages: simpler, easier to implement, and less expensive, but due to the difference in surface quality and tightening speed of the connecting piece and the connected piece, the measured pre-tension value error is large and scattered, and the general error is ±10%.

2 corner method - suitable for torsion shear type high strength bolt

Torsional shear type high-strength bolts have the advantages of high strength, easy installation, easy quality assurance, single-sided tightening, and no special requirements for operators. Unlike large hexagonal high-strength bolts, the torsion-shear high-strength bolt head is a pan head. The end of the threaded section has a dodecagonal body that is subjected to a tightening counter-torque and a broken neck groove that can be sheared at a specified torque.

The method of applying the screw is to firstly use the ordinary wrench to perform the initial screwing, and the connected plates are closely attached to each other, and then the starting screwing position is used as the starting point, and the nut is rotated by a long wrench or a pneumatic wrench according to the final tightening angle.

4, anti-slip coefficient

When the high-strength bolt is tightened, a large pre-tension is generated in the screw, and a large pre-stress is generated between the connected plates. The frictional force on the contact surface after the force is applied can prevent the relative slip between the plates under a considerable load, and thus the elastic working phase is long. When the external force exceeds the friction between the plates, the relative sliding between the plates occurs, and the friction type connection is a state in which the sliding between the plates is the shear bearing capacity limit state.

The bearing capacity of the friction type connection depends on the frictional force of the contact surface of the component, and the magnitude of this friction force is related to the pre-tensioning force of the bolt and the anti-slip coefficient of the friction surface and the number of transmitted friction surfaces of the joint. The magnitude of the anti-slip coefficient is related to the processing method of the contact surface of the member and the material of the member. This coefficient value decreases as the pressing force between the contact faces of the connected members decreases, so it is different from the friction coefficient in physics.

In the high-strength bolt connection, the magnitude of the friction coefficient has a great influence on the bearing capacity. In order to increase the friction coefficient of the contact surface, the contact surface is treated. In the assembly and construction of wind turbines, the contact surfaces of the components in the connection range are often treated by sandblasting, spraying Dacromet zinc paint, and cleaning with a wire brush. After the surface of the steel is sprayed, rusted and sprayed with zinc, the surface appears smooth and flat. In fact, the surface of the steel still has microscopic irregularities. The high-strength bolts are connected under the action of a high pressing force and are meshed with each other by the surface of the connecting member. The higher the strength and hardness of the steel, the greater the force to cause the meshing surface to slip. Therefore, the anti-slip coefficient μ is related to the material of the steel.

Tests have shown that the anti-sliding system after the rubbing surface is red <0.15, even after the treatment is still very low, it is strictly prohibited to apply red dan and anti-rust lubrication materials on the friction surface. In addition, if the assembly is carried out under wet or rainy conditions, the anti-slip coefficient μ value is also lowered, so effective measures should be taken during assembly to ensure the surface of the joint is dry.

5, summary

Wind turbines are assembled from various components. Because they operate under harsh environmental conditions, to ensure reliable operation of the unit, the connection of the components must be guaranteed first. By analyzing the above-mentioned high-strength bolt tightening torque, it can be seen that only the tightening torque of the high-strength bolt can be controlled to make the bolt produce the optimal pre-tension, so as to produce the best pre-stress of the connected part, and at the same time take Measures to increase the friction coefficient of the contact surface of the connected member can improve the integrity and reliability of the connection.

references

Chen Youquan;Discussion on the application of high strength bolted joints[J];Steel Structure;2004-04

Li Zhangyin;Design and Research of New Bolt Fastening Test System[D];Hefei University of Technology;2010

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