0 Preface

The stability, tightness, anti-loose ability, and bolt strength and rigidity of the bolted joint structure are directly related to the bolt preload. The magnitude of the preload determines the properties of the bolted structure. In theory, the greater the pre-tightening force of the bolt, the longer the service life of the connecting structure, and the more stable the connection structure is. However, if the pre-tightening force is too large in actual work, it will cause certain damage to the joint structure. The aeronautical structure contains many precise joint structures. Therefore, it is necessary to strengthen the research on the bolt pre-tightening force in the aviation structure to make the connection stable. Without being destroyed, the overall connection level is improved.

1 Bolt preload in aviation structures

1.1 The role of preload in the aeronautical structure

Appropriate control of the preload is an important measure to improve the quality of aeronautical structure connection. Bolt preload has the following effects in aviation structures: First, there are often some structures that need to be sealed in the aircraft, and the appropriate preload can be To achieve a better sealing effect of these structures; secondly, the appropriate bolt pre-tightening force can effectively avoid the loose, offset or even separation of the bolt structure caused by the vibration of the body; third, the rigidity of the connecting structure will be preloaded. The force increases and increases. When the number of bolted joints in the same structure increases, the rigidity of the structure is stronger, thereby increasing the bearing capacity of the structure. Fourth, the bolt connection does not produce a larger joint like welding. Stress, therefore, the fatigue resistance and load carrying capacity of the bolted joints are effectively improved.

1.2 The adverse consequences of excessive pre-tightening

In theory, the aircraft connection structure will have a certain yield strength. When the bolt connection does not reach the yield point of the aircraft structure, the greater the preload force, the more stable the connection structure. However, the actual situation is not the case. If the pre-tightening force is too large, it will lead to inconvenient disassembly and assembly of the connecting structure. Secondly, some sealing structures will place a sealing ring between the bolt and the connected part. The ring causes damage and affects the sealing effect; finally, the pre-tightening force excessively causes the thread of the bolt to be deformed, and the connecting structure may fall off due to the thread deformation in the later stage. In the actual bolting process, the pre-tightening force is often determined by the experience of the constructor, which will cause a large preloading error and affect the connection effect. Therefore, when bolting, it is necessary to make scientific calculations to determine the pre-tightening force, and then adjust the pre-tightening force with professional tools, so that the aviation structure is more firm.

2 Determination of bolt preload

2.1 Tightening torque calculation formula

Boeing, a world-renowned aircraft manufacturer, studied the preload in the aerospace structure and concluded that for bolts used in general aviation structures, the preload should be about half of its yield load. , the maximum can not exceed 70%, but because the aviation structure will be affected by the dynamic load of the aircraft, the bolt pre-tightening force should be properly adjusted, and it is most suitable to choose about 35% of its yield load. The calculation formula for the preload of the aviation structural bolts is as follows:

Where: F is the bolt pre-tightening force; K ₁ is the pre-tightening force coefficient. Generally, the coefficient is selected between 0.35 and 0.7; A _s is the stress cross-sectional area of ​​the bolt; σ _s is the yield point of the bolt material.

This formula can only theoretically calculate the bolt pre-tightening force. It is impossible to accurately measure the bolt pre-tightening force in actual work, and thus it is impossible to achieve precise control. Therefore, in the actual work, the pre-tightening formula is usually converted to obtain a calculation formula of the tightening torque, thereby controlling the pre-tightening force by controlling the tightening torque. The tightening torque mainly refers to the following three types: first, the torque required to produce the expected pre-tightening force of the bolted joint structure; second, the torque required to overcome the frictional resistance generated during the tightening process; and third, overcoming The torque required for the self-locking mechanism resistance. Finally, according to the type of tightening torque, the theoretical calculation formula of the bolt tightening torque is compiled:

Where: F is the expected pre-tightening force; M is the final tightening torque of the bolt; d ₂ is the thread diameter; P is the pitch; μ ₁ is the friction coefficient between the joint and the nut; μ ₂ is the friction between the threads Coefficient; β is the thread half angle; λ is the thread angle; ρ is the friction angle between the Bolt And Nut threads; R is the outer radius of the nut bearing surface; r is the inner radius of the nut bearing surface. However, if the tightening torque of the bolt is calculated according to the formula (2) in actual work, it will be very troublesome. Therefore, in order to facilitate the calculation of the actual project, (2) is simplified, and finally the following formula is obtained:

(3) where: K ₂ is the tightening torque coefficient; F is the pre-tightening force, F is calculated by the formula (1); d is the nominal diameter of the thread.

2.2 Tightening torque test method

After studying a large number of experimental results of the above formula, one conclusion can be drawn: the torque calculated by formula (3) can meet the relevant engineering requirements under normal circumstances. However, since the aviation structure requires high stability and firmness, in order to obtain accurate torque, it is necessary to verify the calculation results through experiments. The specific methods are as follows: First, calculate the connection structure according to formula (1). Preload force F; second, according to the calculation result of formula (1), calculate the moment of the joint structure; third, strictly follow the requirements of the relevant national documents on the torque in the joint structure, carry out experiments, verify the torque and bolt theory preload The error between forces.

3 The main method of controlling the bolt preload

3.1 Corner method

The corner method is to tighten the bolts in a normal way, so that the connected parts are attached to the nuts, and then the professional wrench is used to further tighten the bolts according to the mechanical calculation, thereby increasing the bolt extension. To improve the preload. The size of the tightening angle is determined by the bolt material, the material of the connected piece and the related dimensions and preload force. In addition, the elastic deformation of the bolt and the connected part and the pre-tightening of the initial tightening are also considered. Force size. Therefore, when the pre-tightening force of the bolt is controlled by the corner method, in addition to the influence of the rigidity of the material of the connecting member, the frictional force of the thread and the manufacturing quality of the bolt will not affect the control precision, and the method can preload the pre-tightening force. The accuracy error is controlled below 15%, which is the highest precision of the pre-tightening force currently achievable.

3.2 Torque method

The main feature of the torque method is to use the torque of the wrench to control the pre-tightening force. In the actual work, the worker uses the professional torque wrench to complete the bolt tightening work. When tightening, the pre-tightening force can be precisely adjusted by the torque displayed on the wrench. . During the tightening process of the thread, friction between the nut and the connected member is generated, and a certain frictional resistance is also generated between the threads, and the torque wrench shows the sum of the two forces. Therefore, the torque method is used to control the pre-control. The tightening force can intuitively understand the resistance generated when the bolt is tightened, and then accurately adjust the pre-tightening force. However, since the thread may be damaged, the friction force is a variable. Therefore, the pre-tightening force that can be achieved by the method is low, and the connection structure with high requirements is not applicable.

3.3 Stretching method

The stretching method refers to the fact that the bolt is heated to have a certain ductility, and then it is stretched to the length required for the connection, and the nut is tightened before cooling to complete the connection. According to the principle of thermal expansion and contraction, after the bolt is cooled, the pre-tightening force is greatly increased due to the shortened length. This method can produce a huge preload, which is very suitable for some occasions where the preload force is very high. In addition to being affected by the "stretching amount", the stretching method is not affected by other external factors, so the pre-tightening effect is also very obvious. For example, when connecting the hydraulic nut of the M30, the pre-tightening force achieved by the stretching method is about 37kN, which far exceeds the pre-tightening force generated by the general tool tightening.

4 Conclusion

Through the above research, we can find that many parts of the aviation structure are connected by bolts, and the bolt pre-tightening force is closely related to the overall level of the aviation structure. In the actual work, the relevant staff needs to adopt the corresponding bolt connection for the aviation structure with different strength requirements. After scientific and rigorous calculation, the pre-tightening force is obtained. Do not blindly pursue the stability of the structure and increase the bolt. Pre-tightening, sometimes it will not achieve the expected effect, affecting the stability of the aviation structure.

references:

[1]SHAN Xijun, QIN Lihua, SHAO Chunshou. Research on tightening torque of threaded joint structure of aerospace products[J].Science & Technology Prospect,2017,27(26):66.

[2]SHI Pengfei, SUN Rui, HU Bin.The role of bolt preload in aircraft structural connection[J].Science & Technology Innovation and Application,2017(20):39-40.

[3]TAN Jian. Analysis of the influence of bolt pre-tightening force on the docking structure of single and double ears[J]. Dual-use technology and products, 2017(14).

[4] Zhu Hong, Chen Hailong, Zhu Haibing, et al. Discussion on control method of bolt pre-tightening force in modern aircraft structure [J]. Science and Informatization, 2017(22).

BUTTERFLY VALVE

Butterfly Valve is either can be a shut-off valve or a control valve, the open and close action is controlled by rotarying the round disc, it is mainly for cutting off the pipeline. Butterfly valve has the characteristics of simple structure, small size, light weight, quick switch and small torque. Plastic butterfly valve is mainly used for corrosive liquid, body and disc material is made by PVC, CPVC, PVDF, PP. seat material is usually made by EPDM, NR, NBR, VITON and PTFE.

Butterfly valve is mainly used in industrial water treatment, fish farm, mining, steel, chemical and other industries. Ningbo RMI Plastic Co.,Ltd offer different kind plastic valves include PVC Butterfly Valve, CPVC Butterfly Valve, PVDF Butterfly Valve, PP butterfly valve, PPH Butterfly Valve, PTFE lined butterfly valve.

PVC Butterfly Valve,PP Butterfly Valve,CPVC Butterfly Valve,PVDF Butterfly Valve

Ningbo RMI Plastic Co.,Ltd , https://www.rmiplast.com