At present, the safety of screw connection has been paid great attention to in the manufacture, production and maintenance of engine in aviation field. Especially for self-locking nuts, it has been widely used in aviation industry because of its good anti-seismic and loose resistance and reuse.
The self-locking nut adopts plastic deformation to form friction force in the thread pair to prevent the thread from loosening. But in the repeated use, the self-locking torque between the thread pairs will gradually decrease, especially in the case of impact or significant load change of the engine, this phenomenon is more obvious.
In this study, the influence of internal and external factors on the loose thread connection is analyzed, hoping to provide guidance for the development, use and maintenance of the engine in the following aviation field.
1、 Service life of self-locking nut
In principle, to ensure the function and spinability of thread connection, it is necessary to keep a certain gap between the mating surfaces of the thread.
According to the research of structural loosening mode, the relative vibration between threads under dynamic load will reduce the friction coefficient of thread pair, make the nut rotate or even loose. In aviation components, thread connecting line is an important component. The design of thread locking is of great significance to the products.
The common ways to prevent loosening include three types:
Firstly, the anti loose mode which destroys the movement relationship of screw pair can make the thread of nut and bolt deform locally by welding or filling, and realize the effect of thread anti loose.
Second, mechanical relaxation refers to the use of additional locking elements after tightening the nut to prevent the relative rotation of the nut and bolt. The cotter pin, stop washer, etc. are often used, which can increase the weight of the thread pair and the difficulty of disassembly.
Third, the friction force is loose, which can be achieved by increasing the friction of the contact surface.
For self-locking nut, radial deformation can be realized through the locking area of nut to achieve the effect of anti loosening. When the bolt is screwed into the deformation position of the self-locking nut, the deformation position of the nut will form radial extrusion force on the bolt, thus forming static friction force on the thread, and make the thread surface have anti loosening torque.
It is also an important measure to prevent the loosening of the components in aviation field. However, with the use of self-locking nuts, the distance of self-locking force will decrease.
Combined with the existing research, the limit value of self-locking force distance will be close to the limit value after 15 repeated experiments. In the process of use, due to the problems of locking area and equipment status of thread pair, processing quality, the products produced by the self-locking nut can not reach the expected life, so effective measures should be taken to reduce the attenuation of locking torque in use of self-locking nut, so as to improve its service life.
2、 Analysis of the factors influencing the torque attenuation of self-locking nut
（1） Material selection and heat treatment method
For the fastener in aviation field, the design condition of engine assembly position can be selected from stainless steel, structural steel or titanium alloy to fastener materials, and the selection gradually shows the trend of high strength and high temperature resistance. Some superalloy materials are widely used in the design of engine self-locking nuts.
Reasonable selection of materials is the key to ensure the quality of products. The service life of self-locking nut and the existing experimental research prove that the material used and heat treatment method of nut will affect the torque attenuation of self-locking nut, and also pay attention to the material, hardness and coating matching of thread.
（2） Locking structure
The selection of suitable materials and controlling the heat treatment method have significant influence on the locking system of the lifting nut. However, since the self-locking nut mainly relies on friction force to prevent loosening, the influence of the movement of thread connecting parts on the thread loosening is obvious in the process of force, Therefore, the service period of self-locking nut will be affected by the structural characteristics of the nut lock and the deformation of the lock area.
First, the typical lock structure, which is often used in aviation field, is elliptical flattening structure, non-metallic insert structure and slotted closure structure.
The non-metallic insert uses the elastic properties of the nonmetallic material to achieve the film anti loosening effect. The rest uses the self locking action of the locking area to prevent the looseness.
Through the actual use of self-locking nuts in the engine, it is found that the reuse stability of self-locking nuts in different lock areas will be different. The simulation analysis of different locking structures of screw pairs is realized by using finite element software, The results show that the deformation of ligands in the screw pair will affect the contact area and the uniform performance of the locking area, and it is directly related to the stability of the locking performance.
3、 Experimental analysis
（1） Material of self-locking nut
In order to further analyze the influence of structural deformation in the lock area on the attenuation of the locking torque, the performance analysis of the locking torque was carried out in this study with 10 products as a group. The torque experiment analysis was carried out by selecting the same specification of non-metallic insert nuts, elliptical, slotted and three-point self-locking nuts under indoor conditions. The experimental results are as follows.
It is found that the self-locking nut of insert has a large tolerance area in the whole process by the elastic properties of nonmetal materials, and the theoretical degree of self-locking torque attenuation is small. However, in the actual use process, the self-locking system of nylon insert self-locking nut is better in the first and second experiments, which is greatly affected by temperature, The torque will decay rapidly after assembly heating, and the service temperature should be lower than 100 ℃, so the effect of the performance temperature of non-metallic materials on this type of self-locking nut is relatively large.
For the metal self-locking nut, the main rely on the closure deformation in the locking area, and the interference amount of the extrusion contact of the screw pair. Compared with the non-metal self-locking nut, the service life of the material is less affected by the temperature, and the influence of the deformation and deformation control in the locking area is relatively large.
For self-locking nuts, there are two deformation modes, namely slotted and elliptical flat, which are simple in technology and are also commonly used in some domestic self-locking nuts. Because the deformation mode only depends on two-point contact force, the deformation amount has a great influence on the attenuation of self-locking moment.
Slotted end mainly refers to the radial closure deformation after 6-8 uniform notches are opened at the tail of the nut, so that it can achieve multi-point interference contact with the external thread. Through the analysis, the deformation method is more uniform and the locking moment is more stable than that of the elliptic compression deformation method.
The design method of the new self-locking nut is the way of three-point non slotting, which can overcome the traditional slotted nut process defects and the overall force is uniform. Through the existing research, it is found that the three-point deformation method is very important for the service life of the lifting nut.
（2） Lock structure dimensions
Combined with typical structural nuts, the self-locking area can be thin-walled cylindrical, so its size includes the axial length dimension of the locking area, the diameter of the outer circle and the deformation of the locking area.
It is found that the contact area of thread pair can be increased by increasing the axial length of locking area properly under certain deformation condition. The slope of torque attenuation after screw in the bolt several times decreases, which can improve the service life of self-locking nut, and increase the weight of screw pair to a certain extent. This method is not in conformity with the concept of actual product quality control.
Secondly, the radius of the outer circle of the locking area can be changed properly under the same variable condition, but the attenuation slope is basically the same as that of the whole. It is necessary to increase the wall thickness of the closing area properly to ensure that the first tightening torque does not exceed the rated range, and the service cycle of the nut can be increased. The deformation of the locking area can reflect the radial interference in the thread pair to a certain extent, It is also an important factor that affects the lock performance of the whole nut.
To obtain the minimum locking torque, the self-locking nut must have enough interference. When the interference is small, the locking torque test is unqualified, in this case, the deformation of the closure can be increased to improve the product qualification rate.
It is found that the increase of the tightening torque and the deformation of the closure is not only linear. Especially, if the deformation of the nut is larger, the slope of the moment attenuation will increase and the service life of the nut will be reduced.
From the angle of structural optimization, although the self-locking nut has different connection structure and method, it can be designed in different forms, but for aeroengine, the structure of the components is compact, the conventional nut structure is the twelve angle plate screw structure, and the self-locking closure area can realize the function of self-locking nut.
In order to ensure the normal use of the plate screw structure, the locking area is required to have a large bearing area and the axial dimension of the nut is long. The existing technology can use simple elliptical closure to achieve self-locking effect. After the optimization design, the original independent closure deformation ellipsoid section is cancelled and the twelve corner plate screw area with the axis position extended appropriately, In this area, the tightening area of the nut plate can be closed by three ports to form a self-locking area.
The design of this structure can improve the assembly stability of the plate screw structure properly, and also increase the stability of the self-locking area.
After optimization, the nut closure area is a 12 angle assembly surface, which can ensure its self-locking performance without affecting the use of plate screw structure. The three-point method is adopted for the closure position. The deformation is smaller than that of the elliptical closure. It contacts the bolt and the self-locking area increases. Because the closing area is under uniform force, the stability of the self-locking performance will be improved to a certain extent.
4、 Conclusion
In a word, as an important anti loose product, the stability of self-locking performance of self-locking nut will affect the connection performance of the product.
In this study, the paper analyzes the use cycle of self-locking nut, and expounds that the optimization of self-locking nut cycle is the goal. After the experimental optimization, the locking performance of self-locking nut has been significantly improved. It is further demonstrated that the method is of great significance for improving the maintenance reliability of fasteners.
