Analyze the cause of the burning and melting accident of the connecting rod large end bushing of the main engine of the "Jinrong" ship, point out that the main causes of the accident are the fracture of the fixed bolt and the displacement of the lower bushing of the crosshead pin, and propose handling and preventive measures.
1. Accident process
On January 31, 2005, the "Jinrong" ship with the main engine model 6L35MC returned to Yantai Port from Japan. When sailing to a certain port in Japan, the engine duty officer found that the main engine lubricating oil pressure had decreased and fluctuated, and the lubricating oil automatic flushing filter had been frequently flushed, mistakenly believing that it was caused by the shaking of the ship in strong winds and waves.
After inspection, it was found that there was abnormal vibration and sound at cylinder No.5/6 of the main engine. Immediate deceleration treatment was carried out, and obvious knocking and dry friction sounds were heard from cylinder No.6 of the main engine.
After the ship anchored, an immediate inspection of the main engine No. 6 cylinder revealed a large amount of alloy powder in the lower oil pan of the cylinder. The connecting rod large end bushing was rolled out from both sides of the bushing cover, confirming that the connecting rod large end bushing of the main engine No. 6 cylinder was melted.
Through hanging cylinder maintenance, dismantling and cleaning inspection of cylinder No.6, it was found that the thrust block and two fixing bolts on the stern side of the connecting rod small end were broken, and together with the locking plate, they fell off the oil pan. The large end bushing of the connecting rod was extensively melted.
The surface of the crank pin is coated with alloy, and the inner surface of the connecting rod big end bushing seat is pulled due to rolling of the bushing shell. The marine engineer has polished the surface of the big end upper bushing cover and the crank pin separately. The white alloy that was melted and pasted on the shaft neck was separated by prying with a screwdriver.
Replace the large end bushing of the connecting rod with a new thrust block, fixed bolt, and locking plate, and measure the temperature of the bushing, The No.6 cylinder is 4-5 ℃ higher than other cylinders.
The anchor lifting test run ran for 30 minutes, and during the operation, the No. 6 cylinder gradually made a slight dull knocking sound. After stopping for inspection, it was found that the connecting rod big end bushing had turned into a bushing again.
During the second inspection, it was found that the alloy layer on the connecting rod's large end bushing had fallen off, the locating tongue had deformed, and the bushing was scrapped; The No.6 crankshaft pin shaft has alloy attachment, with over 75% of the contact surface damaged and a roundness error of 1mm.
2. Cause of accident
Analysis shows that due to the fracture of the fixed bolt of the thrust block, the thrust block falls off, causing displacement of the lower bushing of the crosshead pin relative to the small end of the connecting rod during movement. The oil passage passing through the crosshead pin to the center hole of the connecting rod is blocked, and the amount of lubricating oil entering the large end bushing is reduced to the point where the oil film cannot be established. This results in dry friction between the shaft and the bushing, causing the bushing to melt.
As for the crew's self repair and installation of the machine, the re melting of the bushing shells is due to the surface roughness of the repaired crank pins not meeting the standard. Below is a specific analysis of the causes of the accident.
2.1 Cross head pin lower tile displacement
The height h of the cross head pin lower bushing installed in the small end hole of the connecting rod has the greatest impact on whether the steel back of the bushing can tightly adhere to the small end hole of the connecting rod, as shown in Figure 1.
Figure 1: The protrusion of the two ends of the bushing shell
D is the small end hole diameter of the connecting rod, P is the clamping force of the bushing shell, H is the excess, H is the amount of pressing in the bushing pad, H is the elastic deformation of the bushing during compression.
It is worth pointing out that flanges, locating lips, locating nail holes, etc. on the bushing shells are only auxiliary devices for installing and fixing the bushing shells, and do not play a tightening role during operation.
Figure 2 Analysis of the force on the lower tile of the crosshead
In Figure 2, D is the small unit of the bushing shell, P. Is the normal force, 8 is the stress, U is the friction coefficient, and △ p is the discrete value of pressure variation.
Due to the arc-shaped (curved) shape of the small units and the support of seat holes, a normal force P is generated under the action of force P; If the friction coefficient between the steel back and the aperture surface is u, then Pu is the frictional self-locking force between the steel back and the aperture surface.
The magnitude of the frictional self-locking force Pu is directly proportional to P.
P is generated by the small end aperture of the connecting rod in the locking of the upper and lower bushing shells of the connecting rod bolt pair (connecting rod screw).
The reasonable excess h is the most important factor to ensure the quality of the product.
Of course, the magnitude of the frictional self-locking force P.u depends not only on h, but also on the appropriate tightening force of the bushing bolt pair (screw), the stiffness of the connecting rod small end, the size of the working load, material, geometric accuracy, and surface roughness of the workpiece.
It should be pointed out that:
If h is too small, the self-locking force weakens, and the back of the tile steel cannot fit tightly with the small aperture surface. It is inevitable that the bushing will experience displacement in the aperture.
However, the small size of h is a congenital deficiency of this model (in fact, later models have increased their excess h), which is the main reason for the malfunction.
But if h is too large, the locked bushing will have an "internal arch", which means that the two ends of the bushing cannot be closely aligned with the connecting rod big end hole diameter, and even small gaps will occur.
During the operation of a diesel engine, under the action of pulse load and rotational force, the small end aperture of the connecting rod and the bushing will undergo bending elastic deformation.
In this way, a tangential force P is generated at the joint between the steel back of the bushing and the aperture surface.
In order to prevent circumferential slip between the steel back and the aperture surface due to P, it is necessary to ensure that the minimum frictional force between the tile back and the aperture is greater than the tangential slip force, i.e. P.u>P.
Otherwise, the aforementioned incidents of burning tiles or even holding the shaft may occur.
2.2 Fixed bolt fracture
Judging from the bolt cross-section observed on site, there are defects in the material used to fix the bolts. The material is hard, brittle, and not long enough, and the cross-section is neat and without torsional deformation. One of the bolts has been broken for a long time, and there are signs of polishing on the cross-section. However, it is difficult to detect them in a timely manner due to being wrapped by a stopper after the fracture.
Due to defects in the internal materials of spare parts, it is difficult for crew members to detect them during daily maintenance and upkeep. However, based on the results of on-site investigations and the entire process of fault occurrence and development, the supervisory personnel and the on duty personnel at the time of the fault have an inescapable responsibility.
According to the maintenance plan of the wheel, the chief engineer inspects the interior of the main engine crankshaft box on average every month. However, after on-site investigation, the position of the fixed bolt of the thrust block is relatively concealed and there is a chain anti loosening locking device. In routine maintenance inspections, the chief engineer ignored the inspection of the bolt. Therefore, when one fixed bolt had already broken, it was not discovered in a timely manner until all the bolts were broken, and the lower bushing of the connecting rod small end shifted, partially blocking the oil circuit (the lubrication of the connecting rod big end of this model is from the crosshead to the connecting rod big end), ultimately leading to the melting accident of the connecting rod big end bushing.
3. Handling and preventive measures
Specific measures have been taken to address the issues that have arisen.
The crankshaft was repaired, and the crankshaft diameter was machined from 360 mm to 356.13 mm using specialized equipment. Hardness testing, crack detection, journal polishing, measurement, and report were conducted. At the same time, thickened bushing shells were ordered from the supporting factory for installation.
The connecting rod was sent to the factory for processing, and the upper end face of the bushing seat was turned off by 1mm and re drilled.
No.6 cylinder crosshead, crosshead guide plate, piston head, piston rod, The No.6/7 main bushing was disassembled, inspected, measured, and recorded under the guidance of an engineer.
Clean the lubricating oil circulation cabinet of the main engine, clean the filters of the lubricating oil system, and continuously separate and filter the lubricating oil.
Replace the thrust block fixing bolts of ships such as "Jinrong" with EN61V1640 bolts labeled 8.8 instead of EN61V1635 bolts labeled 10.9, and thoroughly inspect the relevant equipment.
Further emphasize the sense of responsibility of crew members on duty to ensure continuous monitoring of equipment.
4. Conclusion
Based on the actual investigation and analysis of the burning and melting accident of the connecting rod large end bushing of the main engine No. 6 cylinder of the "Jinrong" ship, it is believed that the height h of the cross head bushing of the cylinder installed in the small end hole of the connecting rod is too small; The fracture of the fixing bolt of the thrust block and the detachment of the thrust block are the direct causes of the large end bushing of the connecting rod, while the reason for the expansion of the accident is the professional level and sense of responsibility of the management personnel.
After discovering a decrease in the main engine oil pressure, the on duty personnel did not report it in a timely manner, nor did they further investigate the cause of the pressure reduction. They simply used a bypass valve to maintain it, especially when the obvious signs of failure such as frequent flushing of the oil self-cleaning filter were ignored, and they mistakenly judged it to be caused by the shaking of the ship in strong winds and waves.
In fact, when the automatic flushing filter of the lubricating oil is frequently flushed, the surface alloy of the crankshaft pin bushing of the main engine has been damaged and detached, and the flowing lubricating oil flows through and blocks the automatic flushing filter, causing a large pressure difference between the inside and outside of the filter and causing excessive frequent flushing.
If the above fault phenomenon could attract the attention of the personnel on duty, identify the true cause of its occurrence, and take necessary rescue measures in a timely manner, such a serious machine damage accident would not have occurred.
2024 June 1st Week Marginal Product Recommendation:
Margianl LCB Plastic Clip Linear Bushings：
Made of high anti-wear material EPB13;
2.Maintenance-free, self lubricating;
3. Chemical resistant;
4. Dust resistant and quit running;
5. Installation by force fitting;
6. The E10 inner tolerance ensues only after the press fit.
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