The intersection frame arm receives the bending moment M, the shear force Py, and the pressure Pz transmitted from the wing. Under the combined action of these loads, the pressure on the joints on the joints, the upper and lower forks, the tension force P, and the outer edge of the arm, the horizontal reference line, the horizontal ears, the shear force Py and the pressure Pz, the frame The specific stress characteristics are as shown. The structural form of the joint zone and the structural form of the force-bearing arm determine that the load P is not in the same plane as the No. 1, No. 2, No. 3 bolt holes on the P. The load P and P act on the 16-frame axis, while the shear force Py and pressure Pz act on the 16-frame axis 16 mm. The No. 1, No. 2, and No. 3 bolts are installed 20 mm before the 16-frame axis. Tensile load P The bottom and the No.1, No.2 and No.3 bolt axes do not coincide, causing the bolt to withstand the tensile load and also bear a certain additional bending moment My; at the same time, the shearing force Py does not pass the bolt center, so the bolt is subjected to the shear load. At the same time, it also bears a certain bending moment Mx and torque T. The bolt is completely in the state of composite stress. Which kind of load causes the bolt to break must be determined according to the broken part of the bolt in the whole machine fatigue test. It can be seen from the results of the aircraft fatigue test that the fracture points of No. 1, No. 2, No. 3 bolts are concentrated at the root of the bolt (the joint between the bolt head and the bolt polished rod) and the first engagement thread. The two fracture sites are all size abrupt zones, and there is obvious stress concentration in the local range; and the bolt material is 30CrMnSiA, although its strength is high, but the fracture performance is poor. Therefore, the bolt is in a tensile state under the tensile bending load, and the fracture site belongs to a high stress concentration zone, and the formation, expansion and fracture of the bolt crack are caused at such a high stress level. Improved design of bolts to improve the life of bolts is how to reduce their stress levels. In this paper, the bolt life is improved by reducing the tensile bending load of the bolt and improving the structure of the bolt. Reduce the bolt tensile load (1) Increase the pre-tightening force of the frame web connecting bolt. It can be seen from the structural form of the joint zone that the direction of the static friction between the bolt heads No. 4, No. 5 and No. 6 and the gasket and the connected parts is consistent with the tensile load direction of the No. 1, No. 2 and No. 3 bolts. Increasing the static friction will inevitably reduce the tensile load of the No. 1, No. 2, No. 3 bolts and reduce the stress level. The static friction force is directly related to the bolt preload force, the pre-tightening force is large, and the static friction force is also large. In order to improve the service life of No. 1, No. 2 and No. 3 bolts, the pre-tightening force of No. 4, No. 5 and No. 6 bolts should be increased to give full play to the pre-tightening force. Therefore, increasing the pre-tightening force of bolts No. 4, No. 5 and No. 6 can reduce the tensile load of bolts No. 1, No. 2 and No. 3 and improve the service life of the three bolts. (2) Add pad. The aircraft was tested to 10237h in the whole machine. The left part of the No. 2 bolt was added with a rubber pad of 1mm thick. The right part was tested to add 1250h rubber pad of the same thickness. The rubber pad was added to the nut side. After 2 bolts and rubber mats, the fatigue life has been significantly improved. In particular, the right part of the No. 2 bolt was loaded with rubber mat after 2363h loading test. After the padding, it was tested by 3430h. The padding bolts were not broken until the test was completed, indicating that the rubber pad greatly increased the service life of the bolts. The above situation can be explained by using the No. 1, No. 2, No. 3 bolt tensile stiffness calculation formula. Bolt tensile stiffness <1> is expressed as C=FK where: F is the tensile load of the bolt; K is the tensile deformation of the bolt; C is the tensile stiffness of the bolt. Since the rubber mat is a small amount <2> compared with the tensile stiffness of the bolt, the No. 2 bolt plus the rubber mat is equivalent to an axial tensile deformation of the bolt by an amount of $x, and the bolt deformation $x is the thickness of the rubber mat, such as Shown. Under the action load, if the thickness (L1) of the connected parts is kept equal, the tensile load P2 of the bolt after the padding is inevitably smaller than the tensile load P1 of the bolt before the padding. In fact, the padding is to unload the tensile load of No. 2 bolt, and the rubber pad is equivalent to the initial deformation of the bolt, and the bending load of the bolt is also reduced to some extent. However, after the No. 2 bolt is padded, the shear load transmitted is not improved. Therefore, the bolt breakage is related to the tensile load that the bolt is subjected to. Adding a certain thickness of the rubber pad will unload the bolt tensile load, reduce the stress level of the bolt and increase the service life of the bolt. Conclusion By analyzing the causes of bolt breakage, the following conclusions are drawn: (1) The bolt break is mainly related to the tensile bending load of the bolt. (2) Different methods are proposed for new aircraft and active aircraft to improve the fatigue life of bolts. The new machine needs to redesign the structural form and parameters of the bolt; the active aircraft reduces the stress of the bolt by increasing the pre-tightening force of No. 4, No. 5 and No. 6 bolts and adding rubber pads to No. 1, No. 2 and No. 3 bolts. Level to increase the life of the bolt. Green Houses,Greenhouses For Winter,Greenhouses For Home,Backyard Greenhouse Jilin Yidao Technology Co., Ltd , https://www.ydgreenhouses.com
Study on the analysis and change of the damage of the bolts