Analysis of the influence of preload on bolt strength
0 Preface
Bolted joints are easy to assemble and disassemble and are therefore widely used in mechanical construction. When installing the bolt, the bolt joint must be tightened so that the joint is compressed when the bolt is stretched, and the pre-energizing force acting on the bolt is called preload [1]. The purpose of preloading is to enhance the reliability and compactness of the connection [2]. In the assembly process, the selection of a reasonable pre-tightening force is a necessary condition to ensure the stable operation of the assembly for a long time [3].
In an air suspension system, the damper is usually bolted to the airbag bracket. During the operation of a truck, the bolts at this location broke, causing the suspension system to fail to work properly.
In this paper, Hypermesh software is used to establish the relevant finite element model, and the actual situation is simulated. The model is constrained and the boundary conditions are loaded. The influence of the preload force on the bolt strength is analyzed by finite element simulation.
1 Pre-tightening force and limit force selection
The mounting point under the damper is mounted on the airbag bracket by bolts and nuts. As shown in Figure 1, the pre-tightening force of the bolt is 54000N according to the design requirements. During the actual installation and use of the bolt, there may be insufficient pre-tightening or attenuating conditions. Therefore, two working conditions are used in this paper to load the bolts with 54000N and 5400N pre-tightening forces for comparative analysis.
When the suspension system jumps to the limit of the damper, the bolt receives a shear force of 35000N.
2 finite element analysis
2.1 Finite Element Modeling
To evaluate the bolt strength, it is necessary to consider the bolt preload, the relative movement between the bolt and the nut and the joint. Therefore, the bolt and nut use the solid element to establish a detailed mesh model, as shown in Figure 2.
2.2 Model material parameters
In the model, the bolt material is 40Cr.
2.3 Boundary conditions
Install the point under the damper to load the downward impact force, load the pre-tightening force on the bolt, establish contact between the bolt and the gasket and the nut, and restrain the airbag bracket, as shown in Figure 3.
2.4 Analysis results
The preload of 54000N and 5400N was applied to the bolts, and the vonMises stress of the bolts was compared. The analysis results are shown in Fig. 4. When the bolt pre-tightening force is 54000N, the maximum stress of the bolt is 367MPa, which is lower than the tensile strength of the material, and the bolt meets the strength requirement.
When the pre-tightening force of the bolt is 5400N, the maximum stress of the bolt reaches 795 MPa, and the stress concentration is obvious, which is easy to cause fatigue damage.
Therefore, the pre-tightening force is insufficient, which may cause relative misalignment and sway of the mounting member, thereby causing stress concentration of the bolt, causing the bolt to be cut during operation, as shown in FIG.
It can be seen from Fig. 5 that the finite element analysis results are in good agreement with the actual fracture position, and the analysis method can effectively verify the influence of the bolt preload on the bolt strength.
3 Conclusion
(1) The finite element model of the bolt assembly is established. The influence of the preload force on the bolt strength is analyzed by finite element simulation, and compared with the actual failure parts. The results show that the finite element analysis results are in good agreement with the actual ones, and then verified. The finite element model and the rationality of the boundary conditions.
(2) When the pre-tightening force of the bolt is insufficient, the bolt stress exceeds the tensile strength of the material and the damage occurs; therefore, in actual work, sufficient pre-tightening force is required to ensure the stable operation of the bolt assembly.
references
[1] Yu Q, H Zhou, LWang.Finite element analysis of relationship between
Tightmg torque and initial load of bolted connections[J].Advances
In Mechanical Engineering, 2015: 7(5).
[2] Wang Mingyue. Zhai Bingrong et al. Analysis and optimization of bolting preload force of a weighing instrument sensor. Mu. 2017(1): 22~26.
[3] Zhao Feng, Luo Feng. Selection of bolt pre-tightening force and specification design of bolt specifications. [J] Automotive Practical Technology. 2016(9): 236~250.
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