Analysis and judgment of the ablative cause of the

2022-07-27
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Cause analysis and judgment of crankshaft ablation

crankshaft ablation is mainly caused by the direct friction between crankshaft journal and bearing bush due to the failure to form an effective lubricating oil film between crankshaft journal and bearing bush. When the crankshaft of the diesel engine is ablated, white smoke will be emitted, metal impact noise will be emitted, and even the crankshaft and bearing bush will be locked together and unable to rotate. Therefore, it is necessary to correctly analyze and judge the cause of the fault

1. Poor oil quality

1) poor oil quality. The quality grade and viscosity grade of the selected engine oil are low, or the engine oil of different brands is mixed, resulting in the service performance of the engine oil not meeting the requirements; The engine oil is mixed with a lot of dust during use, and the engine oil is oxidized and deteriorated due to the high operating temperature of the diesel engine. The oxidized and deteriorated engine oil is black and lusterless, and the concentration increases. Twist the engine oil with two fingers, and feel that there are dust, impurities, etc. in the engine oil. If the oil used shows the above phenomena, it must be replaced in time

2) engine oil becomes thin. As some diesel fuel injection pumps are lubricated by pressure lubrication, once the seal between the fuel injection pump and the lubricating oil passage fails. Diesel oil entering the lubricating oil chamber will dilute and deteriorate the diesel engine lubricating oil. In addition, if the water jacket has cracks or sand holes, the cooling water will penetrate into the engine oil, which will also make the engine oil thinner

check the upper parts of the diesel engine (upper plane of cylinder head, air valve mechanism, cylinder head cover) for water vapor or water drops. If it is found that the engine oil capacity increases, the concentration decreases, the viscosity decreases, and there is a smell of oil, it means that the engine will have the official installation of elium blades next year and diesel will be mixed into the oil

2. Insufficient oil stock and low oil pressure

insufficient oil stock and low oil pressure will result in insufficient lubricating oil flow of the diesel engine, which can not guarantee the formation of lubricating oil film between the crankshaft journal and bearing bush of the diesel engine

the quantity of oil in the oil pan and the reading of the oil pressure gauge can be checked to further check whether there are abnormal phenomena such as blockage and leakage in the oil pipeline

3. The clearance between crankshaft journal and bearing bush is too large or too small

1) the clearance between crankshaft journal and bearing bush is too large, resulting in low oil pressure of diesel engine, and insufficient lubricating oil film can not be formed

2) due to the small clearance between crankshaft journal and bearing bush, the thickness of oil film between crankshaft journal and bearing bush is insufficient or there is no lubricating oil film

when the clearance is too large or too small, the dimensions of journal, bearing hole and bearing bush can be checked with micrometer and cylinder gauge to calculate the fitting clearance for the advantages of stable performance, stable operation and low noise of all systems. The fitting clearance can also be measured directly with feeler gauge. After the new machine arrives at the factory,

4. The geometric dimensions of the crankshaft or cylinder block are out of tolerance

1) the radial runout of the crankshaft is large, resulting in small or no clearance between the journal and the bearing bush, insufficient thickness of the lubricating oil film or no lubricating oil film

2) the coaxiality of the main bearing hole of the cylinder block is out of tolerance, resulting in too small or no clearance between the main journal and the bearing bush, insufficient thickness of the lubricating oil film or no lubricating oil film

3) the perpendicularity of cylinder and main bearing hole is out of tolerance, resulting in too small or no clearance between connecting rod journal and main journal, insufficient lubricating oil film thickness or no lubricating oil film

the above phenomenon often occurs when the diesel engine is put into service for a short time after a new engine or overhaul. To determine the position where the geometric dimension exceeds the tolerance, first rotate the diesel engine crankshaft with a hand crank (hard rotation), then loosen the diesel engine main bearing cap and connecting rod bearing cap cylinder by cylinder, and then rotate the crankshaft. It shall be specifically determined according to the torque of the rotating crankshaft and the variation difference (but if the difference is determined, the diesel engine shall be disassembled and measured with a special inspection tool)

5. The shape and dimension of crankshaft main journal and connecting rod journal are out of tolerance

the cylindricity or taper difference of crankshaft main journal, connecting rod journal or cylinder block main bearing hole and connecting rod big end hole is large, which will make the gap between journal and bearing bush intermittent (intermittent) or conical. Good lubricating oil film cannot be guaranteed

the diesel engine will not be put into use for a long time after the new engine or overhaul, which is prone to the above phenomena. The journal size can be measured. If the measured journal is ablated, the adjacent corresponding journal can be measured (because the adjacent corresponding journal is processed with the same equipment and the same cutting tool under normal conditions, it can represent the shape error of the ablated Journal)

6. Tensile strength, compressive strength, elongation strength at random point, load extension at random point, elastic modulus, elongation, comparison of stripping interval process or stop curve, curve amplification journal, connecting rod journal surface roughness out of tolerance

due to poor roughness, there are many metal ridges on the journal surface, which destroys the integrity and continuity of lubricating oil film between journal and bearing bush. The metal ridge directly rubs with the bearing bush, causing the crankshaft to be ablated

the fault often occurs when the diesel engine is put into service for a short time after a new engine or overhaul. If the measured journal is ablated, the roughness detector can be used to measure the adjacent corresponding journal (because in general, the adjacent corresponding journal is processed with the same equipment and the same cutting tool, so the measured roughness can represent the roughness of the ablated Journal)

7. The dynamic balance of crankshaft, flywheel and clutch is out of tolerance

when the dynamic balance is out of tolerance, the high-speed rotation of the crankshaft will generate inertia force, which will damage the fit clearance between the crankshaft journal and the bearing bush. In serious cases, it will cause direct friction between the journal and the bearing bush, resulting in crankshaft ablation

the fault usually occurs after the machine has driven for 10000km. After the crankshaft is ablated, the residual sundries on the journal can be removed, and the dynamic balance can be re calibrated. The unbalance basically indicates that the dynamic balance of the ablated journal is out of tolerance

8. Improper maintenance

if the diesel engine is not properly maintained after a period of operation, it will cause wear, failure and deformation of parts such as the pressure limiting valve of the oil pump. The oil filter element will be blocked by oil dirt and sludge, which will reduce the oil pressure and cause crankshaft ablation

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