A bulldozer equipped with Cummins NT855 diesel engine often encounters a problem of burning the bush and holding the shaft. Why does this happen? It is because in the actual use process, if the oil is deteriorated or added too much, how much will make the bush There is incompatibility, repulsion and friction. Under the condition of high temperature and high speed, the bearing alloy will be damaged, resulting in deformation of the crankshaft and scrapping of the bearing, which will cause greater damage to the diesel engine. If white smoke appears at the fuel port of the diesel engine, the crankshaft temperature is higher than the temperature on the other surface, which indicates that the bearing of the diesel engine is burnt out; the decrease in oil pressure fluctuation may also be due to the burnt out of the bearing.

　　Cause of burning bush and shaft of NT855 diesel engine

　　1: Unreasonable assembly

　　Improper torque of the bearing, unreasonable height of the bearing bush, and too small gap between the bearing bush and the journal are all caused by unreasonable assembly.When tightening the crankshaft main bearing, the staff worried about loosening the bolt and intentionally tightened it. The reason is that the mismatch between the connecting rod bearing cover and the bearing is caused by improper torque, which results in different degrees of damage to the crankshaft and changes in the lubricating oil film. In severe cases, it may cause fractures, resulting in bearing bushing shaft failure [1]. When the bearing bush is installed in the seat hole, in order to ensure that the bearing bush fits the seat hole and improves the heat dissipation effect, the upper and lower ends of the bearing bush should be higher than the end surface of the bearing bush seat. If the height is not enough, the bearing bush will not fit with the bearing seat, affecting the heat dissipation and causing the bearing pad seat. Abrasion, clogged oil holes cause the bush to burn out. If the bearing bush assembly gap is too small, there will not be enough oil to form an oil film, which will cause the bearing bush to appear wire fuzzing or even melt.

　　2: Unreasonable lubricant

　　Oil quality, low pressure, and unreasonable temperature will cause bushing shaft failure, heavy load on the diesel engine, and harsh working environment. If unqualified low-quality lubricating oil is used, the damage of the bearing alloy layer will be accelerated. Because the poor quality lubricating oil is doped with more water and has poor corrosion resistance, it is easy to accelerate the aging of components. Under high-speed operation, the viscosity of lubricating oil will drop rapidly, which will accelerate the wear of bush and the failure of burning bush. If the pressure of the oil is too low, the correct lubricant film will not be formed, and the acceleration bush will be damaged. It is believed that the factor is a major cause of engine failure. During the cleaning process, the maintenance personnel use cotton yarn to wipe the surface of the part, which will leave some thread wool. During the work, they will be adsorbed in the filter and enter the oil passage, which will cause the oil hole to be blocked. The oil pressure will be reduced and the tile will be damaged. When the engine is working, the temperature of the engine oil will increase with the increase of the operating temperature. The thickness of the lubricating oil film between the journal and the bearing pad will gradually become thinner due to the increase in temperature. When the temperature is higher than 150 degrees Celsius, the oil film will When the fracture occurs [2], the friction surface between the journal and the bearing pad will accelerate, causing a burnt shoe to hold the shaft failure; the temperature of the engine oil is too low will cause poor fluidity, and the amount of oil that enters the crankshaft bearing during startup is small, and an oil film cannot be formed, Intensified the degree of damage between the bearings.

　　3: Incomplete cleaning

　　Excessive lubricating oil impurities in the crankshaft and bearing bush will cause secondary pollution. Foreign substances will enter the working surface of the crankshaft journal. These foreign substances will cause friction between the contact surfaces of the journal. When the crankshaft rotates, it will cause the crankshaft. damage. After a lot of experimental results prove that one of the major causes of accidents is caused by too many impurities, these impurities will cause accelerated wear of machinery. Strengthen the cleanliness and filtering effect of parts to prevent component damage.

　　Maintenance strategy of NT855 diesel engine after burning bush and shaft

　　1: Preventive measures, standardizing technology

　　In the process of assembling the crankshaft or bearing bush, if the torque and clearance of the main bearing and the connecting bearing are not reached, it will cause the oil film between the bearing bushes to change, affecting the lubrication effect. Therefore, during the maintenance process, it must be carried out in accordance with the technical specifications to avoid other accidents.

　　2; Reasonable choice of engine oil

　　If the viscosity of the engine oil is low, the parts will be stuck; the resistance of the parts with too high viscosity will increase the difficulty of starting the engine and increase the power. When selecting engine oil, refer to the viscosity recommended by the manufacturer and use thickened lubricating oil; choose the engine oil reasonably according to the climate change; for example, choose a lower viscosity engine oil when the temperature is low, which is conducive to engine startup; choose a higher viscosity when the temperature is higher The engine oil is conducive to the lubrication of the oil film; the engine oil is selected reasonably according to the performance of the vehicle, and the engine with better vehicle quality chooses the engine oil with lower viscosity; the engine with the lower vehicle quality chooses the engine oil with higher viscosity; because the combustion pressure of the diesel engine is higher, Sulfurous acid will be produced and the engine oil will be diluted, so select a special diesel engine oil that can neutralize sulfurous acid.

　　3: Change oil regularly

　　The engine oil should be replaced regularly. It is best not to add oil directly after use, it will cause excessive accumulation of oil stains inside the engine, block the filter, and cause engine parts not to be lubricated, causing damage to the machine; for vehicles with organic oil maintenance lights , The maintenance light should be turned on; vehicles without maintenance lights should be replaced once every three months; for engines with spare oil pots, the oil should be replaced once every six months.

　　4: Develop good driving habits

　　Start the engine when the temperature is low, and wait for the parts to be fully lubricated before running the engine at high speed. Regularly check the oil level of the engine oil. If the liquid level is too high, it will increase the running resistance of the engine and cause oil leakage. If the liquid level is too low, the engine will be damaged due to poor lubrication. Check the engine for leaking oil. Observe the oil pressure gauge before starting the engine, and take prompt measures if the oil pressure is too low.

　　malfunction repair

　　1: Crankshaft repair

　　The most prone to cracks in the crankshaft are the journals and flexing arms. After the cracks occur, a magnetic flaw detector needs to be used for inspection. If there is no flaw detector, physical methods can be used. Put the crankshaft in kerosene for immersion, take it out about fifteen minutes later, sprinkle white powder on the journal, and then hit the work surface. If oil marks appear, it indicates that the journal is cracked. How to eliminate cracks? By using light grinding, the cracks can be continued to be used; if the cracks have lateral cracks or break, they can not be used. The straight length of the crankshaft should be maintained between 0.1 and 0.2. and the bending of the crankshaft exceeds the specified range needs to be corrected by the cold pressing method.

　　2: Bearing repair

　　Unqualified bearing bush material quality will lead to insufficient high temperature resistance and bearing capacity of the bearing bush, and cause burning bush holding shaft failure. Therefore, it is necessary to select bearing bushes of the same grade according to the degree of grinding of the journal, and to measure the thickness of the selected bearing bush. After the thickness is qualified, install the bearing bush in the seat hole, and the seat plane of the bearing bush should be kept at about 0.03mm. After the bearing bush is placed in the seat hole, the radius is larger than the seat hole, and it closely fits the bearing bush, which can play the role of heat dissipation. Tap on the back of the bearing pad. If there is no sound, the connection is firm. The space between the main journal and the bearing pad should be kept within 0.038~0.127mm. The center line of the main bearing should be in a balanced state. Check whether the connection between the various bearings is consistent. If they are not consistent, repair them immediately.

　　3: Reinstallation requirements

　　Clean all the parts, especially the crankshaft and lubricating oil channel; add proper amount of engine oil to the main bearing cap, and tighten the bolt thread in order, in three times, each time the torque is different. Check the gap between the crankshaft axial direction, specified between 0.127 ~ 0.335mm, can not exceed the specified range. If the crankshaft is turned, it will pass if there is no jamming. Add an appropriate amount of engine oil to the connecting rod group and bearing bush; check whether the gap between the connecting rod and the crank arm is maintained between 0.1 and 0.3 mm, and the rotating crankshaft cannot be stuck; when the engine is connected to the torque converter, check the crankshaft Use a crowbar to inspect the shock absorber. If the gap becomes smaller or there is no gap at all, add a pad between the engine and the torque converter, adjust the position of the torque converter, and ensure that the axial gap of the torque converter remains between 0.127~0.335mm.

　　Conclusion

　　Excessive expansion of the bearing bush will cause the engine to malfunction. In actual use, in addition to its own physical changes, it also has a great relationship with the engine speed and temperature. Insufficient lubricant viscosity and excessive load will cause the bearing bush. damage. The speed of the work schedule directly affects the reliability of the engine, so every part of the diesel engine must be carefully analyzed and researched to ensure that the diesel engine can work normally in the project.