Do you know what is the most expensive and precise single component of an automobile engine? Many people may not imagine that it is the engine crankshaft. It is a very important part of the engine. It can convert the linear motion of the piston into rotary motion, and convert the gas pressure from the piston and connecting rod into torque for outward output. At the same time, it drives the vehicle's transmission system, the engine's valve train, and other auxiliary devices.

The force on the crankshaft is extremely complex. It works under the combined action of periodically changing gas pressure, reciprocating inertial force and torque, and bears huge alternating bending and torsion loads. At the same time, it is also a slender high-speed rotating part, so strict dynamic balance is required, and bending and twisting are not allowed to exceed a certain value.

The manufacturing process of the crankshaft

Therefore, the crankshaft needs to be made of very high-quality materials and undergo a strict surface treatment process. Generally, 45, 40Cr, 35M,n2, and other medium carbon steel and medium carbon alloy steel are used for die forging. In recent years, ductile iron is also used more. After forging or casting, to improve the fatigue strength of the crankshaft and eliminate stress concentration, it is also necessary to carry out shot peening on the journal surface, and the fillets shall be rolled. To improve the surface hardness of the journal, high-frequency quenching or nitriding treatment is also required on the journal surface, and finally finishing is required. The crankshaft treated in this way has sufficient bending and torsion resistance fatigue strength and stiffness, and the journal has sufficient pressure-bearing surface and wears resistance.

Oil passages are running through the main journal, crank, and connecting rod journal on the crankshaft, from which the pressure oil from the main oil passage of the lubrication system enters the main journal and connecting rod journal to lubricate the two friction pairs of the main journal and the main bearing bush, and the connecting rod journal and the connecting rod bearing bush. However, due to the huge force on these two parts, there must be a certain degree of wear in use. If the lubrication is poor or the engine load is too large, other types of damage will occur.

Common damage to the crankshaft

The common damage forms of crankshaft include journal wear, crack, ablation, bending, distortion, fracture, etc. Generally speaking, the crankshaft with severe ablation, cracks, or fractures cannot be repaired and can only be scrapped; For the crankshaft with excessive bending or twisting deformation, we can use the pressure correction method to repair it. For the excessively worn crankshaft, we can grind it to the specified repair size, and then select the corresponding oversized bearing bush to restore the fit clearance; If the wear is serious, vibration surfacing or electric spraying can also be used to fill the defective parts and then machining to the specified repair size. However, there is a special point to be noted here: the crankshaft must be nitrided again after grinding, otherwise, the crankshaft may break.

However, in the actual engine repair process, the crankshaft is rarely repaired, and the defective crankshaft is replaced directly. On the one hand, because the repair of the crankshaft is difficult, the process is complex, the required precision is high, and the cost is expensive, it is not cost-effective either economically or technically. On the other hand, the vast majority of models now do not provide oversized repair bearing shells. After the crankshaft is ground, no matching parts can be found.

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