Cause Analysis of 500KVA Diesel Generator Abnormal Bush Burning Faults

May. 12, 2022

Diesel engine is main part of 500KVA diesel generator, there are many reasons causing diesel engine bush burning. Lack of engine oil in diesel engine is one of the main reasons for diesel engine Bush burning. When the diesel engine is running without oil, it must burn the bush, but the bush may be burned when there is no lack of oil.

 

Today, Dingbo power, a diesel generators manufacturer, analyzed the causes of abnormal bush burning faults of 500KVA diesel generator. Hope this article is helpful to you.


1. Cause analysis

In the normal working process of diesel engine, there is a clearance between crankshaft journal and bearing bush and oil film exists to form liquid lubrication. In this way, the friction loss is small, the heat generated by friction is small, the heat is taken away by the oil, and the working temperature is normal. If the bearing bush is in direct contact with the journal to form a partial dry friction state, the friction power consumption will increase sharply, and a large amount of friction heat will be generated, which will be dissipated by the bearing bush, while the heat taken away by the oil is not much. The heat will accumulate in the bearing bush and the temperature will rise continuously. When the temperature exceeds the alloy melting point on the bearing bush surface, the bearing bush surface will begin to melt until burning loss occurs, resulting in the failure of the diesel engine. 


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2. Relevant factors causing failure


A. Oil temperature too high or too low

When the oil temperature is too low, the viscosity of the lubricating oil is too high and the fluidity is poor. Especially in the cold start-up stage, the amount of oil entering the crankshaft is less, which is easy to make the bearing bush in direct contact with the crankshaft journal and accelerate the wear and damage of the bearing. When the oil temperature is too high, the viscosity of the lubricating oil is too low and the strength of the oil film is weakened, resulting in the thinning of the oil film thickness, which is also easy to cause the early wear and damage of the bearing bush. It is generally believed that the maximum temperature of diesel engine lubricating oil is 130 ℃. However, in order to fully extend the service life of the bearing, the common temperature should be kept in the range of 95 ~ 105 ℃.


B. Thermal oxidation stability of lubricating oil

The thermal oxidation resistance of lubricating oil has a significant impact on the lubrication between crankshaft and bearing. If two different lubricating oils are used on the same prototype and work continuously under the same working condition, the measured results will be different.


C.Improper bearing assembly clearance

In order to improve the lubrication state of the main bearing of the existing diesel engine and prevent burning, the clearance between the bearing and the crankshaft journal should be controlled in strict accordance with the requirements of the diesel engine operation manual. When replacing the bearing bush, check the roundness and cylindricity of the crankshaft journal. If it exceeds the limit, it shall be polished to avoid reducing the contact area of the journal and bearing bush and increasing the pressure per unit area. In addition, the axial clearance of the crankshaft shall be controlled. If the wear exceeds the limit, it shall be repaired in time.


D. Lubricating oil deterioration

Generally speaking, during the use of lubricating oil, due to the wear of diesel engine cylinder liner and piston ring, as well as the change of piston ring opening clearance and opening position, the high-temperature and high-pressure combustible mixture flowing into the crankcase is increasing, which not only increases the temperature of lubricating oil, but also speeds up the oxidation and polymerization of lubricating oil. At the same time, due to the mixing of diesel engine combustion products, the mixing of external dust and metal wear debris, and the consumption of additives in lubricating oil, the deterioration and deterioration speed of lubricating oil is greatly accelerated. This not only increases the wear and corrosion of the friction pair of the lubricating part of the diesel engine, but also is the main reason for the burning loss of the bearing.


E. Poor quality of lubricating oil

The diesel engine uses inferior lubricating oil or fake high-quality lubricating oil in the process of use. If the quality grade of lubricating oil does not meet the requirements of diesel engine manufacturer, it will also lead to Bush burning failure of diesel engine.


F. Quality problem of bearing bush

If inferior materials are used, the high temperature resistance and bearing capacity of the bearing bush are insufficient. Even if the oil pressure is normal and the oil quantity is sufficient, the Bush burning fault will be caused.


G. The vibration of diesel engine is too large during operation

The vibration of the diesel engine during operation is too large due to shock absorption damage or other reasons; It may also be that the damping element of the diesel engine crankshaft itself is damaged, which makes the diesel engine crankshaft vibrate too much; After long-term operation, the bearing bush may become loose, resulting in Bush burning or sliding failure.


H. The temperature of diesel engine is too high

Due to the failure of the cooling system or other reasons, the overall temperature and oil temperature of the diesel engine are too high, resulting in the Bush burning failure of the diesel engine after long-time operation.


3. Precautions for use of 500kva diesel generator

a. Regular maintenance: clean the parts, dredge the oil passage, add or change the oil on time to prevent the oil from aging or getting too dirty and blocking the oil passage.

b. Select the lubricating oil that meets the requirements of the diesel engine manufacturer and carefully maintain it as required.

c. Before starting the diesel engine, carefully check the amount of lubricating oil. If it is insufficient, add it according to the regulations.

d. During cold start-up, first operate at idle speed under no-load for 3 ~ 5 minutes, and then gradually transition to high-speed or heavy load operation.

e. It is forbidden to operate the diesel generator for a long time under overload to avoid rapid acceleration; If it is found that the oil pressure alarm light is on, find out the cause and handle it properly before continuing operation.

f. During maintenance, pay attention to check all parts of the lubrication system. Important parts cannot be replaced (e.g. iron wire cannot replace cotter pin, etc.). When assembling, use clean lubricating oil. 

g. When replacing a new bearing bush, check the length of the bearing bush. The bearing bush is too short to ensure its reliable fit with the journal and good heat dissipation; When the bearing bush is too long, the interface will be deformed, which will lead to shaft gnawing.

h. Regularly check the cooling effect of the diesel engine cooling system, pay attention to supplement the coolant and tighten or replace the fan belt in time to ensure that the cooling system is always in good condition.

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