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Six reasons why crankshafts break

2022-09-26

 The fracture of the crankshaft usually starts from the smallest crack, and most of the cracks appear in the connecting part of the crank arm at the fillet of the connecting rod journal of the head cylinder or the end cylinder. During the operation, the crack gradually expands and suddenly breaks when it reaches a certain level. The brown part is often found on the broken surface, which is obviously an old crack, and the shiny and shiny tissue is the trace that developed to the sudden breakage later. Today, let's take a look at what causes the crankshaft to break!
 
1. The rounded corners at both ends of the crankshaft journal are too small
When grinding the crankshaft, the grinder failed to correctly control the journal fillet of the crankshaft. In addition to the rough camber processing, the fillet radius was too small, so a large stress concentration was generated at the fillet when the crankshaft was working, and the crankshaft was shortened. fatigue life.
 
2. Offset of crankshaft main journal axis
The axis of the crankshaft main journal is offset, which destroys the dynamic balance of the crankshaft assembly. When the engine runs at high speed, a strong inertial force is generated, which causes the crankshaft to break.
 
3. The cold competition of the crankshaft is too large
The crankshaft will be greatly bent after long-term use, especially after the accident of burning tiles or pounding the cylinder, and should be removed and corrected by cold pressing. Due to the plastic deformation of the metal inside the crankshaft during calibration, a large additional stress will be generated, thereby reducing the strength of the crankshaft. If the cold competition is too large, the crankshaft may be damaged or cracked, and this crankshaft will break soon after installation.



 
4. The flywheel is loose
If the flywheel bolt is loose, the crankshaft assembly will lose its original dynamic balance, and the engine will vibrate after running, and at the same time, a large inertial force will be generated, resulting in fatigue of the crankshaft and easy breakage at the rear end.
 
5. Poor quality of crankshaft itself
Buying crankshafts should not be greedy for cheap, and must be purchased from regular channels. It should also be checked carefully before installation, and if there is any problem, it should be replaced or returned in time. In addition, when the engine is overhauled, the crankshaft should be subjected to magnetic flaw detection or oil immersion percussion inspection. If there are radial or axial cracks extending to the shoulder fillet on the surface of the journal, the crankshaft cannot be reused.
 
6. The main bush is different from the shaft
When the crankshaft is assembled, if the centerlines of the main bearing bushes on the cylinder block are not on the same axis, the accident of burning the bearing bushes and holding the shaft will easily occur after the engine works, and the crankshaft will also break under the strong action of alternating stress.