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Cause Analysis of Engine Crankshaft Damage (2)

Pulished on Nov. 02, 2020


5. Crankshaft deformation


The deformation of the Crankshaft is usually bending deformation and torsion deformation. A crankshaft that is too deformed will cause increased wear of itself and connected parts, accelerated fatigue, crankshaft fracture and excessive mechanical vibration.


The causes of crankshaft deformation are non-human and man-made. Non-human causes include: the crankshaft is deformed due to periodic gas pressure, reciprocating inertial force, centrifugal force of rotating motion, and mechanical braking force.


Man-made reasons are mainly caused by violation of driving operation regulations and accidental mechanical failure:


①Starting too hard under heavy load.


②The diesel engine works rough, causing the crankshaft to be subjected to sudden shock load.


③In order to stop the diesel engine when speeding or running at speed, take excessive braking measures.


④Diesel engine suffers from bush-burning and axle-holding accidents due to lack of lubricating oil or too small bearing gap, which causes the crankshaft to receive a large torque.


⑤The main bearing housing hole of the cylinder body is severely deformed and out of concentricity.



6. Crankshaft fracture


Crankshaft fracture is an accidental damage to a diesel engine, and its causes are more complicated. The broken parts are:


①At the crank arm where two adjacent round corners of the Heavy Crankshaft journal meet.


②At the 45° angle along the oil hole of the connecting rod journal.


③At the root of the connecting rod journal or the root of the main journal.


④At the keyway of the tapered surface where the flywheel is installed.


All the causes of cracks on the surface of the crankshaft journal and the bending and twisting of the crankshaft are the causes of crankshaft fracture. In addition, there are several reasons:


①The material of the crankshaft is poor, the manufacturing is defective, the heat treatment quality cannot be guaranteed, and the processing roughness cannot meet the design requirements.


②The flywheel is unbalanced. The flywheel and the crankshaft are connected in different axes, which damages the balance between the flywheel and the crankshaft, and causes the crankshaft to generate a large inertial force, resulting in fatigue fracture of the crankshaft.


③The weight difference of the replaced piston and connecting rod group exceeds the limit, causing the explosive force and inertial force of each cylinder to be inconsistent, and the force on each journal of the crankshaft is unbalanced, causing the crankshaft to break.


④ During installation, the torque of the flywheel bolts or nuts is insufficient, which causes the connection between the flywheel and the crankshaft to become loose, which makes the flywheel run out of balance, and generates a large inertial force, causing the crankshaft to break.


⑤ Bearings and journals are severely worn, the fit clearance is too large, and the speed changes suddenly, and the Forged Crankshaft is subjected to impact loads.


⑥When the crankshaft is used for a long time, and it is refurbished for more than three times, the crankshaft will easily be broken due to the corresponding reduction in the journal size.


⑦The oil supply time is too early, causing the diesel engine to work rough; the throttle is not well controlled during work, and the speed of the diesel engine is unstable, which causes the crankshaft to be subjected to a large impact load and easy to break.


7. Damage to other parts of the crankshaft


Crankshaft oil blocking thread or shaft shoulder wear, front keyway wear, starting claw thread damage or sliding buckle, etc.


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