How the crankshaft work?

The crankshaft is an important part of the automobile engine. Its function is to convert the gas force from the piston and the connecting rod into torque, and convert the linear motion of the piston into rotational motion, which is used to drive the transmission system of the automobile, engine valve train and other auxiliary devices. To put it bluntly, it is the power output component of the car. The crankshaft basically consists of several unit cranks. A crank pin, two left and right crank arms and two left and right main journals constitute a unit crank. The relative position or arrangement of the cranks depends on the number of cylinders, the arrangement of the cylinders and the sequence of engine operation.

The force of the crankshaft is extremely complex. It works under the combined action of periodically changing gas force, inertial force and its moment, and bears alternating loads of bending and torsion. Therefore, the crankshaft is required to have sufficient fatigue strength and rigidity against bending and torsion; the journal should have sufficient bearing surface and wear resistance.

The crankshaft is generally made of 45, 40Cr, 35Mn2 and other medium carbon steel and medium carbon alloy steel by die forging. The surface of the journal is subjected to high frequency quenching or nitriding treatment, and finally finishing. In order to improve the fatigue strength of the crankshaft and eliminate stress concentration, the surface of the journal should be shot peened, and the rounded corners should be processed by rolling. It should be noted that the nitrided crankshaft must be re-nitrided after grinding, otherwise the crankshaft will be in danger of breaking.

The fracture of the crankshaft usually starts from the smallest crack, and most of the cracks occur in the connection part with 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, I will share with you what is the key reason for the broken crankshaft.

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 axial fillet of the crankshaft. In addition to the rough surface processing, the fillet radius was too small, so a large stress concentration occurred 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 diesel engine runs at high speed, a strong inertial force is generated, which leads to the fracture of the crankshaft.

3. The cold competition of the crankshaft is too large

 After long-term use of the crankshaft, especially after the accident of burning tiles or pounding the cylinder, a large bending will occur, and it 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 diesel engine will vibrate after running, and a large inertial force will be generated at the same time, 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 bushes on the cylinder block are not on the same axis, the accident of burning the bushes and holding the axles will easily occur after the diesel engine works, and the crankshaft will also break under the strong action of alternating stress.

7. Crankshaft assembly clearance is too large

If the clearance between the crankshaft journal and the bearing bush is too large, the crankshaft will impact the bearing bush after the diesel engine runs, but the alloy falls off and the bush is burned to hold the shaft, and the crankshaft is also easily damaged.

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

If the fuel injection pump supplies fuel too early, the piston will burn before reaching the top dead center, which will cause the diesel engine to knock, and the crankshaft will be impacted by alternating stress. If the fuel supply of each cylinder is not uniform, the crankshaft journals will be unevenly stressed due to the inconsistency of the explosion cases of each cylinder, resulting in premature fatigue and cracks.

9. Poor crankshaft lubrication

If the oil pump is seriously worn, the lubricating oil channel is dirty and the circulation is not smooth, the oil supply will be insufficient and the oil pressure will drop, resulting in the failure to form a normal lubricating oil film between the crankshaft and the bearing bush, resulting in dry friction and causing the burning bush to hold the shaft, broken crankshaft and other major accidents.

10. The crankshaft has been broken after operation

If the accelerator is too large or too small, frequent braking or overloaded operation for a long time, the crankshaft will be damaged by excessive torque or shock load. In addition, when the diesel engine has accidents such as flying, ramming the cylinder and top valve, it is also easy to break the crankshaft.

How to Diagnose 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 whether the crankshaft has cracks, pay special attention to the transition of the fillet, if there are cracks, the shaft should be scrapped. When polishing the journal, the journal and crank arm should keep a certain radius of fillet, and the size of the fillet should not be reduced arbitrarily. Attention should be paid to the surface finish of the fillet, otherwise it will cause stress concentration and cause the crankshaft to break.

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

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 easily damaged due to impact. If the clearance is too small, the crankshaft may be broken due to the shaft. In terms of assembly, the ignition time should be calibrated accurately, not too early or too backward, and pay attention to the balance of the crankshaft, flywheel and clutch.

A crankshaft replacement is a major repair job requiring a full day or more of a qualified mechanic’s time. If the crankshaft itself is where the problem lies, then the cost can be hefty.  If the problem is with a peripheral component, then the time and cost involved will be less.