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The crankshaft is the most important part of an engine. It takes the force from the connecting rod and converts it into torque which is output through the crankshaft and drives the other accessories on the engine to work.
The crankshaft is subjected to the combined action of centrifugal force of rotating mass, gas inertia force of periodic change and reciprocating inertia force, which makes the crankshaft bear the action of bending and torsional load.
Therefore, the crankshaft is required to have enough strength and stiffness, and the journal surface should be wear-resistant, uniform and balanced.
◆Definition of crankshaft
The crankshaft is the most important part of an engine. It takes the force from the connecting rod and converts it into torque which is output through the crankshaft and drives the other accessories on the engine to work.
The crankshaft is subjected to the combined action of centrifugal force of rotating mass, gas inertia force of periodic change and reciprocating inertia force, which makes the crankshaft bear the action of bending and torsional load.
Therefore, the crankshaft is required to have enough strength and stiffness, and the journal surface should be wear-resistant, uniform and balanced.
◆Common crankshaft damage
①Crankshaft journal wear
After the crankshaft journal is worn, the clearance between the crankshaft and the bearing bush increases, and abnormal sound occurs during operation, and the working state deteriorates. The main reasons are:
▶Too little oil or oil in the presence of hard abrasives, oil deterioration containing acid.
▶The clearance between the journal and the bearing bush is too large or too small, resulting in the difficult formation of oil film, dry friction will wear early.
▶The bending and twisting of the connecting rod and the deflection of the cylinder liner cause the uneven distribution of the force acting on the crankshaft and also produce the taper.
②Crankshaft journal surface scratch or strain
The main reasons are:
▶Assembly does not pay attention to cleaning, so that the diesel engine into the slag, metal and other abrasive particles.
▶The lubricating oil in the oil pan is not replaced on time, so that the abrasive particles contained in the lubricating oil, such as large metals, mix into the gaps between the bearing bush and the journal to scratch and strain the friction surface.
▶Improper maintenance of the air filter will increase the wear clearance of the cylinder liner, piston and piston ring, and abrasive materials such as sand and impurities will be sucked into the cylinder with the air and then burned into the oil pan and circulated into the matching clearance of the journal and bearing.
③Crankshaft journal burns
▶Bluish ablative marks on the journal burn. The burn wear of the crankshaft journal was caused by the burning of the tile.
▶In this case, there will be intense friction between the journal and the bearing bush, the lubricating oil film will be destroyed and the scratching will occur, the temperature will rise sharply and the surface of the journal will be oxidized blue, the surface hardness of the journal will decrease, and the bearing alloy will often be glued down.
④The crankshaft journal surface cracks
Crankshaft cracks mostly occur at the rounded corners of the transition between the crank and the journal and the oil holes.
The former is the radial crack, the harm is great, easy to cause the crankshaft fracture, the occurrence of major crash accident; The latter is the axial crack, along the oil hole along the axial development.
Cracks are mainly caused by defects in manufacturing and repair and improper use:
▶In use, the journal surface roughness is poor, there are indentation, scratch, corrosion, grinding pit and other defects.
▶Insufficient lubrication will cause serious tile burning, which will cause axial cracks.
▶After long-term use, the metal fatigue transition on the journal surface will cause the circumferential cracks.
▶When surfacing repair the journal, excessive residual stress is generated in the surface surface, which leads to cracks.
⑤Fracture of crankshaft
Crankshaft fracture is an accident damage of diesel engine. The parts of the fracture are:
▶At the crank arm where the two adjacent fillets of the crankshaft journal meet.
▶In the connecting rod journal through the oil hole along the 45° Angle.
▶In the connecting rod neck root or the main shaft neck root.
▶In the conical surface keyway for mounting the flywheel.
All causes of cracks on the surface of the crankshaft journal and causes bending and distortion of the crankshaft are causes of fracture of the crankshaft.
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