Analysis and Judgment of the Causes of Crankshaft Ablation

1. Poor quality of engine oil

The quality of the engine oil is poor. The quality grade and viscosity grade of the selected engine oil are relatively low, or different grades of engine oil are mixed for use, resulting in the performance of the engine oil failing to meet the requirements. During the use of engine oil, a large amount of dust was mixed in, and the oil oxidized and deteriorated due to the excessively high working temperature of the diesel engine. Oxidized and deteriorated engine oil appears black and lackluster, with an increased concentration. When you pinch the oil with two fingers, you can feel that there are dust, impurities, etc. in it. If the engine oil used shows the above phenomena, it must be replaced in time.

2) The engine oil has become thin. Since some diesel engine fuel injection pumps are lubricated by pressure, once the seal between the fuel injection pump and the lubricating oil passage becomes ineffective. When diesel enters the lubricating oil chamber, it will dilute and deteriorate the lubricating oil of the diesel engine. In addition, if the water jacket has cracks or sand holes and air holes, allowing the coolant to seep into the engine oil will also make the oil thinner.

It is possible to check whether there is water vapor or water droplets on the upper parts of the diesel engine (the upper surface of the cylinder head, the valve mechanism, and the cylinder head cover). If it is found that the oil volume increases, the concentration decreases, the viscosity drops and there is a raw oil smell during use, it indicates that diesel has been mixed into the oil.

The oil level is insufficient and the oil pressure is low

Insufficient oil stock and low oil pressure can lead to inadequate flow of lubricating oil in diesel engines, which in turn cannot ensure the formation of a lubricating oil film between the crankshaft journal and the bearing bush of the diesel engine.

You can check the amount of engine oil in the oil pan and the reading of the oil pressure gauge, and further inspect whether there are any abnormal phenomena such as blockage or leakage in the oil pipeline.

The clearance between the crankshaft journal and the bearing bush is either too large or too small

Due to the excessive clearance between the crankshaft journal and the bearing bush, the oil pressure of the diesel engine is relatively low, and it is impossible to form a sufficient lubricating oil film.

2) Due to the excessively small clearance between the crankshaft journal and the bearing bush, the oil film thickness between them is insufficient or there is no lubricating oil film.

When the clearance is too large or too small, the dimensions of the journal, bearing hole and bearing bush can be checked with a micrometer and a cylinder gauge, and the fit clearance can be calculated. Alternatively, the fit clearance can be directly measured with a feeler gauge.

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

1) The radial runout of the crankshaft exceeds the tolerance significantly, resulting in a small or no clearance between the journal and the bearing bush, and insufficient or no lubricating oil film thickness.

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

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

The above-mentioned phenomenon often occurs when a new engine or a diesel engine that has undergone major overhauls has not been in use for a long time. To determine the out-of-tolerance parts of the geometric dimensions, you can first rotate the diesel engine crankshaft with a hand crank (which is laborious to rotate), then loosen the main bearing cover and connecting rod bearing cover of the diesel engine cylinder by cylinder, and then rotate the crankshaft. The specific determination is based on the magnitude and variation of the rotating crankshaft torque (but to determine the extent of the deviation, the diesel engine needs to be disassembled and measured with a special inspection tool).

The shape and size of the crankshaft journal and the connecting rod journal are out of tolerance

If the cylindricity or taper difference between the crankshaft journal, connecting rod journal or the main bearing hole of the cylinder block, or the big end hole of the connecting rod is significant, it will cause the clearance between the journal and the bearing bush to be intermittent or conical. It cannot guarantee the formation of a good lubricating oil film.

When a new engine or a diesel engine that has undergone major overhauls has been in use for a short period of time, the above phenomena are prone to occur. The dimensions of the journal can be measured. If the measured journal is ablated, the adjacent corresponding journal can be measured (as the adjacent corresponding journal is processed by the same equipment and the same cutting tool under normal circumstances, it can represent the shape error of the ablated journal).

The surface roughness of the crankshaft journal and the connecting rod journal exceeds the tolerance

Due to poor roughness, there are many metal peaks on the surface of the journal, which damages the integrity and continuity of the lubricating oil film between the journal and the bearing bush. The metal peaks directly rub against the bearing bush, causing the crankshaft to burn.

Faults often occur in new machines or diesel engines that have been in use for a short time after major overhauls. If the measured journal is ablated, the roughness detector can be used to measure the adjacent corresponding journal (as in general, the adjacent corresponding journal is processed by the same equipment and the same cutting tool, so the measured roughness can represent the roughness of the ablated journal).

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

When the dynamic balance is out of tolerance, the high-speed rotation of the crankshaft generates inertial force, which will damage the fit clearance between the crankshaft journal and the bearing bush. In severe cases, it may cause direct friction between the journal and the bearing bush, leading to crankshaft erosion.

Faults usually occur after the machinery has traveled 10,000 kilometers. After the crankshaft is burned, the residual debris on the journal can be removed, and the dynamic balance can be recalibrated. The unbalance amount can basically indicate the over-tolerance of the dynamic balance of the burned journal.

8. Improper maintenance and care

If a diesel engine is not properly maintained and serviced in a timely manner after running for a period of time, it will cause wear, failure and deformation of parts such as the oil pump pressure limiting valve. The filter element of the oil filter can be clogged with oil stains and sludge, reducing the oil pressure and causing the crankshaft to burn.