Fracture failure analysis of the coupling bolt 1. The macroscopic and micro fracture morphology of the coupling bolt is shown in Figure 1. All the broken bolts have the same fracture position, and are the joints between the bolt head and the thread. The bolt section is not flat, but is divided into several steps. The origin and divergence direction of the broken fiber of each step are different. It can be seen that the bolt breakage does not occur at the same time. Obviously, several smaller fracture faces on the outer side of the bolt are the source of the fracture that causes the overall fracture of the bolt (see Figure 2).
The cracked step (Fig. 3a) has friction marks and shallow scales on the surface (Fig. 3b, 3c). The fracture properties of the bolts are quasi-cleavage fractures (Fig. 4), which are ductile fractures (Fig. 5).
2. The surface morphology of the bolt pickling is shown in Fig. 6. After pickling, there is a crack on the surface of the bolt.
Fracture Failure Analysis of Joint Bolts Shengli Oilfield Company Oil Production Technology Research Institute/Chen Wei Feng Xiaohong Wang Jinjing 3. Analysis of Assembly Structure of Bolts 1 Raw Data Rated Power: 1 2 3 2 k W; Rated Speed ​​of Diesel Engine: 2400r/min; Worm Gear Worm Pair The transmission ratio is 7.17; the pin diameter: 19m m; the pin length: 31m m; the pin diameter: diameter: development and application 298mm; pin number: 18; bolt basic diameter: 12m m; bolt distribution circle diameter: 305mm The number of bolts: 9.
2 Load distribution The input power is transmitted to the crankshaft via the inner and outer ring gears and distributed to both sides. One side of the crankshaft drives three plungers, the other side of the crankshaft drives two plungers, and the cranks of the five plungers are The phase difference on the circumference is 72°.
The load distribution is shown in Figure 7.
It can be seen from Fig. 7 that there are always three crank bearing moments at the maximum load, wherein when one crank is subjected to the maximum torque, the other two received torques are cos 72° of its value. The load distribution of the three cranks is 1:2, respectively. Since the matching clearance between the pin and the pin hole is increased, the torque is shared by the pin and the bolt.
3 The clearance between the pin and the pin hole is excessively increased, and all the moments are only borne by the bolt. The following three cases will be checked separately.
3 Only the condition of the pin force is checked. It can be seen from Fig. 1 that the original intention of the designer should be to transmit the torque mainly by 18 pins. Therefore, we first assume that the torque of 5439.02Nm is all borne by the 18 pins. The torque of each pin is 302.18Nm, and the distribution circle diameter of the pin is d.298m, so the force on the pin. F=2027.97N is the pin shear stress: when the crank force of the crankshaft is the maximum value, then the maximum value of the torque of the crankshaft is: when one crank of the crankshaft is subjected to the maximum force At this time, the torque received by the crankshaft is: Therefore, when the crankshaft is working normally, the maximum torque appears on one side of the crankshaft of the crankshaft, that is, the coupling pin and the connecting bolt on the side of the crankshaft connecting the crankshaft are more stressed. Large, here only the force on this side is the benchmark.
From the fixed structure of the bolt and the pin shown in Fig. 1, we can see that there are three possibilities for transmitting the torque of 5439.02 Nm: 1 The torque is only received by the pin bearing.
Squeeze stress on the pin: In this case, both the bolt and the pin can meet the strength requirements.
4 Check the joint force of the bolt and pin. In the second case, all the torque will be shared by the pin and the bolt. Due to the torque, the torque ratio of the single bolt and the pin shaft is 305:298; therefore, the torque value to be transmitted by the single bolt is: the torque received by the single pin shaft is: the check result of the formula (3), (4) is known. The pin must meet the strength requirements. Check the strength of the bolt: the shearing force of the bolt: the shear stress on the bolt: in this case, the shear strength of the bolt obviously cannot meet the strength requirement, so for the third case, the bolt is even more It is not possible to meet its strength requirements, so it is no longer necessary to check its strength for the third case.
Conclusion Through the above analysis, we can see that all the bolts are broken is not an accidental factor. The reason is not only the material does have problems, but also the structural problems. In the beginning, due to the interference fit between the pin and the pin hole, the pin can be subjected to the load alone, but with the wear and tear, the second or even the third case will occur. Under these conditions, the bolt will definitely occur. Shear fracture. Therefore, in addition to improving the material of the bolt itself, the unreasonable positioning relationship of the parts should be completely changed, so that the assembly positioning diameter of the bolt is smaller than the positioning diameter of the pin.
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