pubdate：2019-08-16 14:57:56 source： 美心翼申
Crankshaft fracture usually starts at the smallest crack, and most of the cracks occur at the fillet of the head cylinder or the end cylinder connecting rod journal and the connecting part of the crank arm. In the course of operation, the crack gradually enlarges and suddenly breaks when it reaches a certain extent. The Brown part is often observed on the fracture surface, which is obviously an old crack, and the glossy structure is the trace of the development to a sudden break later.
Crankshaft Fracture of Engine
1. Too small fillets at both ends of crankshaft journal
When grinding a crankshaft, the grinder fails to correctly control the fillet of the crankshaft's axle stiffness. Besides the rough processing of the arc surface, the radius of the fillet is too small, so the stress concentration at the fillet of the crankshaft will be greater and the fatigue life of the crankshaft will be shortened.
2. Axis deviation of crankshaft main journal
The offset of crankshaft main journal axis destroys the dynamic balance of crankshaft components. When diesel engine runs at high speed, it produces strong inertia force, which leads to crankshaft fracture.
3. Excessive cold competition of crankshaft
After long-term use of crankshaft, especially after the accident of burning tile or tamping cylinder, large bending will occur, which should be unloaded for cold pressure correction. Because the plastic deformation of the metal inside the crankshaft will produce a great additional stress during correction, which will reduce the strength of the crankshaft. If the cold competition is too large, the crankshaft may be damaged or cracked, and the crankshaft will rupture soon after installation.
Flywheel Loosening
If the flywheel bolts are loosened, the crankshaft assembly loses its original dynamic balance, and the diesel engine will flutter after running. At the same time, a great inertia force will be produced, which will cause the crankshaft to be fatigued and easy to break at the end.
5. The poor quality of crankshaft itself
Buying crankshaft can't be greedy and cheap. It must be purchased from regular channels. Careful inspection should be conducted before installation, and timely replacement or return of goods should be made if there are any problems. In addition, when the engine is overhauled, the crankshaft should be inspected by magnetic force inspection or oil immersion percussion inspection. If there are radial or axial cracks on the surface of the journal, the crankshaft can not be used any more.
6. Different axles of main bearing bush
When the crankshaft is assembled, if the center line of the main bearing bush on the cylinder block is different from the axis, the accident of burning bush and holding Bush will easily occur after the diesel engine works, and the crankshaft will also break under the strong action of alternating stress.
7. Excessive crankshaft assembly clearance
If the clearance between crankshaft journal and 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 bearing bush will be burned, and the crankshaft will be vulnerable to damage.
8. premature oil supply or uneven oil supply in each cylinder
If the fuel supply time of the injection pump is too early and the piston burns before the top dead point, it will cause the diesel engine detonation and the crankshaft will be impacted by the alternating stress. If the fuel supply of each cylinder is not uniform, the stress of crankshaft journal will be uneven due to the inconsistency of the explosion cases of each cylinder, which leads to premature fatigue and cracking.
9. Poor lubrication of crankshaft
If the oil pump is worn seriously, the lubricating oil passage is dirty and the flow is not smooth, the oil supply will be insufficient and the oil pressure will drop, which will cause the normal lubrication film between the crankshaft and the bearing bush to be unable to form, thus causing dry friction and other accidents such as burning bush holding axle and breaking crankshaft.
10. Crankshaft Fracture Less than Operated
If the throttle is too large or too small, braking frequently or overloading for a long time, the crankshaft will be damaged by too large torque or impact load. In addition, the crankshaft is also liable to break when accidents such as flying car, cylinder tamping and top valve occur in diesel engines.
Fault Diagnosis and Elimination of Engine Crankshaft Fracture
In order to prevent the crankshaft from breaking, the following measures can be taken during maintenance:
First, before repairing the crankshaft, we should carefully check whether there are cracks in the crankshaft, and pay special attention to the fillet transition part. If there are cracks in the crankshaft, it should be scrapped. When polishing the journal, the journal and crank arm should keep a certain radius of roundness, and the radius of roundness should not be arbitrarily reduced. The surface finish of roundness should be paid attention to, otherwise, stress concentration will occur and the crankshaft will break.
Secondly, when the size of the journal wear exceeds the limit, the method that has little influence on the fatigue strength of the journal should be chosen as far as possible to restore it. For example, the reduction of the fatigue strength of the journal by spray plating method is less, while the reduction of the fatigue strength by surfacing method is larger.
Then, the matching clearance and end clearance of each journal and bearing should be in accordance with the standard. If the clearance is too large, the crankshaft may be damaged by impact. If the clearance is too small, the crankshaft may be broken by holding the axle. In assembly, the ignition time should be corrected accurately, not too early or too backward, pay attention to the balance of crankshaft, flywheel and clutch.
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