Therefore, certain measures must be taken to improve it to ensure that no breakage occurs during its fatigue life. We first thought of improving the neck structure of the connecting rod bolt so that the connection between the head of the connecting rod bolt and the rod is smooth, but this piece is made abroad. Design, production requirements must not change the original structure. If you start with materials, use materials with higher strength levels, which will increase the production cost of the products and reduce the profit margin of the products. In order to solve this problem, the fatigue strength of the transition arc of the connecting rod bolt head and the rod portion can only be improved by improving the process, so local rolling of the transition arc is attempted.
At the same time, according to the qualitative analysis of the stress concentration phenomenon at the transition arc of the step, the tangential tensile stress of each point on the neck contour line of the connecting rod bolt is shown as shown in the figure, and the point where the tangential tensile stress is the largest on the contour line. The angle with the axis (in the range before and after this angle should be the key strengthening zone. Therefore, it is preliminarily proposed to adopt the normal rolling method at the transition arc of the head bolt and the rod of the connecting rod bolt. First, we use the finite element analysis method to determine the rolling pressure, and then use the fatigue test to verify its correctness. The rolling pressure in the local rolling process is determined by using the finite element analysis method to determine the rolling pressure in the local rolling process. The size of the first, the correct model is first established, and then the fatigue life analysis of the connecting rod bolts is carried out by the finite element method, and the results are compared with the results of the fatigue test. If the results are consistent, the correctness of the finite element model can be proved. Based on this, the size of the rolling pressure is further determined.
The finite element analysis of the original connecting rod bolt is established by the finite element model: considering that the rolling process of the connecting rod bolt is completed after the process, the thread is strengthened, so the influence of the thread can be completely ignored. In order to better simulate the actual working condition of the bolt, we assembled the nut on the bolt and ignored the influence of the thread, and regarded the bolt and the nut as a whole to establish the geometric model. After repeated practice, the three-dimensional node tetrahedral element +, 0.1 is selected, and the intelligent division is adopted when dividing the grid. The accuracy defaults to 2. The finite element model after meshing is shown in the figure).
Finite element analysis: Theoretically, on the tight surface of the bolt and the nut, displacement parallel to the direction of the tight surface is allowed, but in this analysis, the bolt and the nut are considered as a whole, so the bolt The head is bound by 34, 35, and 36 directions. The connecting rod bolts are subjected to an alternating load 7 as a surface load distributed on the upper surface of the nut, with a maximum load of 28/9:, an average load of /9: and a minimum load of 28/9:. The stress distribution of the connecting rod bolt corresponding to the maximum load is as shown.
It can be seen from the stress distribution diagram that the stress at the transition arc of the connecting rod bolt head and the rod portion is the largest, so the fatigue life calculation is performed from the point where the stress value is large. The calculation results show that when the ideal number of cycles is secondary, the consumption life coefficient is, that is, the number of actual cycles can be secondary.
This is completely consistent with the test results. This verifies the correctness of the finite element model and the reliability of the calculation results.
The determination of the rolling pressure and the test verification are only due to the improvement of the process of the connecting rod bolt, so the finite element model and the constraint conditions are unchanged, only a normal rolling pressure is added on the basis of the load of the original connecting rod bolt. The specific fatigue life analysis steps are the same as above.
Through many attempts and calculations, it is finally concluded that when the rolling pressure is A, the consumption life factor of the connecting rod bolt is . This value is less than, indicating that the link bolt does not suffer fatigue damage after being subjected to a given number of stress cycles. In order to verify the correctness and practicability of the calculation results, the fatigue test of the link bolts which were only partially rolled was performed, and the results were all qualified. Conclusion By analyzing the results of fatigue testing of connecting rod bolts, a process improvement scheme is proposed, which is to locally roll the transition arc of the connecting rod bolt head and the rod to improve the fatigue life of the connecting rod bolt.
Through the finite element analysis of the connecting rod bolt, the rolling pressure is determined. When the rolling pressure is A, the connecting rod bolt can meet the fatigue life requirement, and there is no early fatigue fracture phenomenon and fatigue. The test is proved.
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