When the engine balance shaft bolt running condition is found, the camshaft phase deviation is found. After the disassembly, the balance shaft bolt is broken. The bolt is tightened with a fixed value torque wrench. The actual assembly torque is 60Nm. The bolt quality is tested and the fracture reason is analyzed.
Figure 1 Schematic diagram of the connection structure
The failed bolt breaks at the unthreaded thread, and the fracture has been necked at the break; the bolt head side has a collision with the connected member due to the loss of the constraint after the break.
Figure 2 Failure sample morphology
Figure 3 Macroscopic appearance of the failed bolt fracture
As can be seen in Figure 3, there is a ratchet-shaped tearing rib at the fracture of the failed bolt, indicating that the bolt is subjected to a certain torsional load when it is broken ;
It can also be seen in Figure 2 that the bolt breakage has apparently caused necking, indicating that the bolt is subjected to tensile loads and excessive plastic deformation.
The intrinsic quality of the bolt was tested. The hardness of the bolt was (34.9, 35.1, 35.8) HRC. The decarburization test met the technical requirements in GB/T3098.1, and the metallographic structure was tempered sorbite (Fig. 4).
Figure 4 Metallographic structure of the bolt
in conclusion:
According to the macroscopic shape of the failure bolt and the fracture morphology analysis, the bolt is subjected to both tensile and torsional loads in the event of failure. The failure is caused by excessive tightening of the bolt caused by excessive tightening during assembly and excessive plastic deformation. And broken;
The cause of over-tightening of the bolt may be related to factors such as setting the assembly torque too much, the uncontrolled friction coefficient of the bolt, and the accuracy of the assembly tool . The specific reasons need to be further analyzed in combination with other samples and information.
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