Hardware knowledge: causes and countermeasures of crankshaft ablation

Causes and countermeasures of crankshaft ablation: Crankshaft ablation is mainly caused by the failure to form an effective lubricating film between the crankshaft journal and the bearing bush, which makes the crankshaft journal and the bearing bush directly rub. When the crankshaft of a diesel engine is ablated, it will emit white smoke, make a sound of metal impact, and even cause the crankshaft and the bearing bush to lock together and cannot rotate. Therefore, the cause of its failure must be correctly analyzed and judged.

1. Poor quality of engine oil: 1) Inferior quality of engine oil. The selected engine oil has a low quality grade and viscosity grade, or the blending of different brands of engine oil, resulting in the performance of the engine oil not meeting the requirements; a large amount of dust is mixed in the engine oil during use, and the engine oil is oxidized and deteriorated due to high operating temperature . The oxidized and deteriorated engine oil is black and dull, and the concentration increases. Twist the engine oil with two fingers, and feel that there are dust and impurities in the engine oil. If the engine oil used shows the above phenomenon, it must be replaced in time. 2) The engine oil becomes thinner. As part of diesel engine fuel injection pump lubrication adopts pressure lubrication, once the fuel injection pump and lubricating oil passage seal fails. Diesel entering the lubricating oil chamber will dilute and deteriorate the diesel engine lubricating oil. In addition, if the water jacket has cracks or blisters, allowing cooling water to penetrate into the engine oil will also make the engine oil thinner. The upper parts of the diesel engine (the upper plane of the cylinder head, the valve mechanism, the cylinder head cover) can be checked for water vapor or water droplets. If it is found that the oil capacity increases, the concentration decreases, the viscosity decreases, and there is a smell of oil during use, it means that diesel oil is mixed into the oil. Import pump valve

2. Insufficient oil inventory and low oil pressure: Insufficient oil inventory and low oil pressure will cause insufficient lubricating oil flow of diesel engine, which can not guarantee the formation of lubricating oil film between diesel engine crankshaft journal and bearing bush. You can check the amount of oil in the oil pan and the reading of the oil pressure gauge, and further check whether there is any blockage or leakage in the oil pipeline.

3. The gap between the crankshaft journal and the bearing bush is too large or too small: 1) The gap between the crankshaft journal and the bearing bush is too large, resulting in low oil pressure of the diesel engine and insufficient lubricant film formation. 2) The gap between the crankshaft journal and the bearing bush is too small, and the thickness of the oil film between the crankshaft journal and the bearing bush is insufficient or there is no lubricating oil film. When the gap is too large or too small, you can use a micrometer or a cylinder gauge to check the dimensions of the journal, bearing holes, and bushes, and calculate the fit gap. You can also use a feeler gauge to directly measure the fit gap.

4. The geometric dimensions of the crankshaft or cylinder block are out of tolerance: 1) The radial runout of the crankshaft is too large, so that the gap between the journal and the bearing bush is small or no gap, and the thickness of the lubricating oil film is insufficient or there is no lubricating oil film. 2) The coaxiality of the main bearing hole of the cylinder block is too poor, which causes the gap between the main journal and the bearing bush to be too small or no gap, and the thickness of the lubricating oil film is insufficient or there is no lubricating oil film. 3) The verticality of the cylinder and the main bearing hole is too poor, causing the gap between the connecting rod journal and the main shaft journal to be too small or no gap, and the lubricating oil film thickness is insufficient or there is no lubricating oil film. In the case that the diesel engine has not been put into use for a long time after a new engine or an overhaul, the above phenomenon often occurs. To determine the out-of-tolerance part of the geometrical size, you can first crank the crankshaft of the diesel engine by hand (the rotation is laborious), then loosen the diesel engine main bearing cover and connecting rod bearing cover cylinder by cylinder, and then rotate the crankshaft. It is determined according to the torque of the rotating crankshaft and the difference of the change (but to determine the size of the deviation value, the diesel engine needs to be disassembled and measured with a special inspection tool).

5. The shape and size of crankshaft main journal and connecting rod journal are out of tolerance: large difference in cylindricity or taper of crankshaft main journal, connecting rod journal or main bearing hole of cylinder block, big end hole of connecting rod will cause the journal The gap with the bearing bush is intermittent (intermittent) or tapered. Can not guarantee the formation of a better lubricating oil film. After a new engine or overhaul, the diesel engine has not been put into use for a long time, and the above phenomenon is prone to occur, so the journal size can be measured. If the measured journal is ablated, the adjacent journals can be measured (because the adjacent journals are processed with the same equipment and the same cutting tool under normal circumstances, it can represent the shape error of the ablated journals).

6. The surface roughness of the crankshaft main journal and connecting rod journal is too poor: Due to the poor roughness, there are many metal edges on the journal surface, which destroys the integrity and integrity of the lubricating oil film between the journal and the bearing bush. Continuity. The metal ridges directly rub against the bearing bush, causing ablation of the crankshaft. Failures often occur when the diesel engine has not been put into use for a long time after a new engine or overhaul. If the measured journal is ablated, the roughness detector can be used to measure the adjacent journals (because in general, the adjacent journals are processed with the same equipment and the same cutting tool, so the measured roughness can represent the ablated journals Roughness).

7. The dynamic balance of crankshaft, flywheel, and clutch is out of tolerance: when the dynamic balance is out of tolerance, the crankshaft will rotate at high speed to generate inertial force, which will damage the clearance between the crankshaft journal and the bearing bush, and in severe cases, it will cause direct friction between the journal and the bearing bush. And cause ablation of the crankshaft. The failure generally occurs after the machine has traveled 10000km. After the crankshaft is ablated, the residual debris on the journal can be removed, and the dynamic balance can be rechecked. The imbalance can basically indicate the excess of the dynamic balance of the ablated journal.

8. Improper maintenance: If the diesel engine is not properly maintained and maintained after running for a period of time, it will cause wear, failure and deformation of the oil pump pressure limiting valve and other parts. The filter element of the oil filter will be blocked by oil dirt and sludge, which will reduce the oil pressure and cause ablation of the crankshaft.