The fracture of the crankshaft usually starts from the smallest crack, and most of the cracks and fractures occur at the connection part of the crank arm at the rounded corners of the connecting rod journal of the head cylinder or the end cylinder. During operation, the crack gradually expanded and suddenly broke when it reached a certain level. The brown part is often found on the fracture surface, which is obviously an old crack, and the shiny and shiny structure is the sign of the sudden breaking later. Let’s take a look at the cause of the broken crankshaft with everyone today! Engine crankshaft fracture problem

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 crankshaft's axial fillet. In addition to the rough surface machining, the fillet radius is also too small. Therefore, when the crankshaft is working, a large stress concentration occurs at the fillet, and the crankshaft is shortened. Fatigue life.

2. Offset of crankshaft main journal axis

The axis of the main journal of the crankshaft is deviated, which destroys the dynamic balance of the crankshaft assembly. When the diesel engine runs at high speed, it generates strong inertial force, which causes the crankshaft to break.

3. The crankshaft's cold contest is too large

After long-term use of the crankshaft, especially after the occurrence of tile burning or cylinder ramming accidents, larger bends will occur, and should be removed for cold press correction. Due to the plastic deformation of the internal metal of 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. This kind of crankshaft will break shortly after being used.

4. The flywheel is loose

If the flywheel bolts are loose, the crankshaft assembly will lose its original dynamic balance, and the diesel engine will shake after running, and a large inertial force will be generated at the same time, which will cause the crankshaft to fatigue and easily break at the tail end.

5. The quality of the crankshaft itself is poor

Don’t buy crankshafts for cheap, you must buy them from regular channels. It should be carefully checked 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 inspected by magnetic inspection or oil immersion tapping. If the journal surface has radial cracks or axial cracks extending to the rounded corners of the shoulder, the crankshaft can no longer be used.

6. Different axis of main bearing

When assembling the crankshaft, if the center lines of the main bushes on the cylinder block are different from the axis, the diesel engine will be prone to bush burning and axle holding accidents after operation, and the crankshaft will also break under the strong action of alternating stress.

7. The crankshaft assembly clearance is too large

If the matching clearance between the crankshaft journal and the bearing bush is too large, the crankshaft will impact the bearing bush after the diesel engine is running, but the alloy will fall off and the bush will burn and hold the shaft, and the crankshaft will also be easily damaged.

8. The oil supply time is too early or the oil volume of each cylinder is uneven

If the fuel injection pump fuel supply time is too early, the piston will burn and work before the top dead center, which will cause the diesel engine to knock and the crankshaft will be impacted by alternating stress. If the oil supply of each cylinder is uneven, the force of each journal of the crankshaft will be uneven due to the inconsistency of the explosion case of each cylinder, which will cause premature fatigue and cracks.

9. Poor crankshaft lubrication

If the oil pump is severely worn, the lubricating oil channel is dirty, and the flow is not smooth, the oil supply will be insufficient and the oil pressure will drop, resulting in the failure of a normal lubricating oil film between the crankshaft and the bearing bushes, which will cause dry friction and cause burning of the bushes and shafts. , Breaking the crankshaft and other major accidents.

10. Crankshaft fracture caused by improper operation

If the throttle is too high or too low, braking is frequent or overloaded for a long time during operation, the crankshaft will be damaged by excessive torque or impact load. In addition, the crankshaft is also prone to fracture when the diesel engine has accidents such as runaway, ramming and valve ejection. Diagnosis and troubleshooting of engine crankshaft fracture

In order to prevent the crankshaft from breaking, the following measures can be taken during maintenance:

First of all, before repairing the crankshaft, carefully check the crankshaft for cracks, pay special attention to the fillet transition part, if there is a crack, the shaft should be scrapped. When grinding the journal, keep a certain fillet radius between the journal and the crank arm. Do not reduce the size of the fillet arbitrarily. Pay attention to the surface finish of the fillet, otherwise it will cause stress concentration and cause the crankshaft to break.

Secondly, when the wear of the journal size exceeds the limit, it is necessary to choose a method that has less influence on the fatigue strength of the journal to recover. The intensity reduction is greater.

Then, the matching clearance and end clearance of each journal and the bearing should be in accordance with the standard. If the clearance is too large, it is easy to damage the crankshaft due to impact. If the clearance is too small, the crankshaft may be broken due to shaft holding. In terms of assembly, the ignition time must be adjusted accurately, not too early or too retreat, pay attention to the balance of the crankshaft, flywheel and clutch.