Introduction
Diesel engines are widely used as power units in ships. As one of the key components of modern reinforced, high-speed, and more compact diesel engines, sliding bearings have become one of the limiting factors for increasing engine power, reducing fuel consumption, and improving operational reliability.
The report and statistical results of the 22nd Internal Combustion Engine Conference by Swedish Club Insurance Company (Sweden) are as follows:
The main faults of low-speed engines are turbocharger (47%), cylinder liner (19%), journal and bearing (8%), piston and piston rod (6%), and fuel pump (4%);
The main faults of medium speed engines are the turbocharger (20%), crankshaft and connecting rod (16%), exhaust valve and top rod (14%), journal and bearings (12%), and piston (9%).
From this, it can be seen that bearing failures account for a large proportion of both low-speed and medium speed engines. Proper maintenance of sliding bearings in marine diesel engines is an important measure to ensure the safety of ship power plants and safeguard ship and maritime safety.
1. Common damages and causes of sliding bearings
Bearings are installed between the relatively rotating components in diesel engines, with sliding bearings being the main type.
Diesel engine sliding bearings work under alternating loads, making it difficult to maintain a uniform and constant load-bearing oil film inside the bearings; The load on the bearing is very high; The relative motion speed between the journal and the bearing shell is high, sometimes even exceeding 10m/s. In addition, impurities in the lubricating oil and corrosion damage such as oil deterioration make the bearing prone to damage.
Diesel engines have complex operating conditions during use, with frequent starting and stopping, and many low-speed conditions. Therefore, bearings are prone to semi dry friction. In addition, mechanical deformation can cause local load concentration on the bearing surface.
Under these harsh working conditions, bearings will suffer various damages, mainly due to the damage of the wear-resistant alloy layer on the bearing shell.
1.1 Excessive wear and tear
After running the diesel engine for a period of time, excessive wear occurs on the lower bearing pads of the main bearing, the lower bearing pads of the crosshead bearing, and the upper bearing pads of the crank pin bearing. The main reasons for excessive wear are:
The diesel engine starts frequently, runs under overload for a long time, the working surface of the journal bearing is rough, the geometric shape error is too large, poor purification of the lubricating oil leads to a large amount of mechanical impurities, and improper daily maintenance and management.
Excessive wear of the bearing shell will increase the bearing clearance, causing impact and exacerbating wear.
1.2 Cracks and peeling
Cracks and peeling are common damages on thick walled white alloy bearing shells, which are fatigue failures of alloy materials under pulsating oil film pressure. Initially, small fatigue cracks appear on the working surface of the bearing shell due to various reasons. As the diesel engine continues to operate, the cracks on the bearing shell gradually expand and extend, causing the wear-resistant alloy on the bearing shell to peel off in the form of flakes. This is mainly due to the high explosive pressure of the diesel engine, excessive geometric shape errors of the shaft neck, excessive wear of the bearing shell, high local load peaks, or excessive clearance between the bearing shaft neck, resulting in excessive impact load on the bearing.
In addition, when the bonding performance or process quality of the bearing alloy and the back of the bearing is poor, cracks and peeling of the bearing alloy will occur faster. In addition, when the bearing is subjected to excessive bearing load or uneven axial load distribution, cracks will occur on the bearing, which will expand under the action of the oil wedge to form many closed cracks. In severe cases, the crack area may be large and even extend to the end face of the bearing or cause alloy peeling.
1.3 Bearing shell melting
Bearing alloy melting is a common and serious damage to sliding bearings, mainly due to small bearing clearance, insufficient lubricating oil pressure or loss of pressure causing the oil film to not be established, rough journal surface or excessive geometric shape error causing damage to the oil film. The inability to establish or damage the oil film results in direct contact between the metal of the shaft and the bearing, and dry friction generates high temperatures causing the alloy to melt.
1.4 Corrosion of bearing shells
The corrosion of bearing shells includes electrochemical corrosion and corrosion caused by leakage.
The presence of water or oil oxidation in lubricating oil, as well as the mixing of gas or fuel, can cause macroscopic or microscopic electrochemical corrosion pitting on the working surface of the bearing. The stray current caused by electrical leakage on board can also cause local electrostatic corrosion pitting on the inner and outer surfaces of the bearing.
2 Inspection of bearings
2.1 Detection of crankshaft journal wear
Long term operation of diesel engines causes uneven wear on the crankshaft main journal and crankshaft pin journal, resulting in reduced diameter and geometric accuracy, as well as errors in roundness and cylindricity. Excessive errors can cause changes in the fit clearance between the shaft and the bearing shell, damage the lubricating oil film, and reduce the bearing capacity.
Regularly measure the roundness and cylindricity errors of the main spindle neck and crankshaft pin neck. If they exceed the standards specified in the manual, they should be repaired.
2.2 The installation of sliding bearings must comply with the specified requirements
In order to ensure the safe and reliable operation of sliding bearings, the installation quality of bearings and their fit with the shaft are particularly important.
(1) The mating surface between the bearing shell and the bearing seat hole should fit well
When installing the bearing shell, the installation of the lower shell is crucial. The outer circular surface of the lower shell should be tightly and uniformly in contact with the inner circular surface of the bearing seat hole. A 0.05mm feeler gauge cannot be inserted, and the mating surface should be tightly fitted. The operation of the bearing shell is reliable, without deformation or cracks, which is conducive to heat dissipation. The fitting surface of the thick walled bearing shell when the lower shell is installed in the bearing seat can be checked by coating colored oil on the bearing seat surface, achieving at least 3 points of contact within a 25mm × 25mm area, that is, the back of the bearing shell of large and medium-sized diesel engines and? The contact area of the bearing seat shall not be less than 75%, and the tight fit between the thin-walled bearing shell and the bearing seat is achieved through the interference fit between the bearing shell and the bearing seat hole. GB/T3535-94 recommends an expansion amount of (0.3-1.0) mm for non flanged bearing shells and (0.1-0.4) mm for flanged bearing shells.
(2) Should the journal be at a certain angle to the bearing pad? uniform contact
The contact angle between the main journal of the diesel engine and the lower bearing of the main bearing should be uniformly in contact within the range of 40 ° to 60 ° on both sides of the centerline of the engine body; The contact angle between the crank pin neck and the bearing pad on the connecting rod big end should be uniformly in the range of 60 ° to 90 ° on both sides of the connecting rod centerline, and the contact angle should not be too large or too small.
Too small a contact angle can increase the pressure on the sliding bearing, and in severe cases, it can cause significant deformation, accelerate wear, and shorten the service life of the sliding bearing;
Excessive contact angle can affect the formation of oil film and prevent good liquid lubrication.
(3) The bearing clearance should meet the requirements
Appropriate bearing clearance is an important condition for forming a lubricating oil film to achieve liquid dynamic pressure lubrication.
The clearance between the bearings is too small, and the oil film cannot be established. The metal between the shaft and the tile comes into direct contact, generating a large amount of heat and causing the alloy to melt;
Excessive clearance can cause lubricant loss and impact, resulting in cracks and fractures in the alloy layer of the bearing shell. It is required that the bearing clearance be between the installation clearance and the limit limit.
For example, the bearing clearance of a cross head diesel engine with a journal diameter of 500mm should be within (0.40-0.90) mm to be considered normal.
The clearance between thick walled bearing shells is generally measured using lead pressure method, while the clearance between thin-walled bearing shells is measured using comparative method. The diameters of the corresponding parts of the shaft and hole are measured using inner and outer micrometers, and the difference between the two diameters is the bearing clearance.
2.3 Measurement of Bearing Wear
The wear amount of thick walled tiles is generally determined by measuring the settlement of the main spindle neck with a bridge gauge or directly comparing the thickness of the tile with the thickness of the new tile. The wear amount of the bearing on the connecting rod big end can be determined by direct measurement method. Thin walled bearing shells generally do not require measurement of wear, and if the bearing clearance does not meet the requirements, it needs to be replaced.
3. Strengthen operation, maintenance and management
From the analysis of common causes of damage to sliding bearings and the factors affecting the formation of liquid lubricating oil films, the working condition and service life of bearings mainly depend on maintenance, installation management, and quality. Therefore, the following aspects should be taken into account:
3.1 Lubrication system and lubricating oil management
(1) Adopting lubricating oil sample analysis technology
Constructing the lubricant oil sample analysis technology as a detection method for sliding bearings, comprehensive analysis such as lubricant physical and chemical analysis, spectral analysis, and iron spectrum analysis can effectively diagnose and predict faults in the sliding bearing system.
Conventional physical and chemical analysis mainly uses conventional physical and chemical performance analyzers to determine the viscosity, moisture content, total acid value (TAN), total base number (TBN), insoluble/mechanical impurities, flash point, pour point, ash content, residual carbon and other indicators of lubricating oil. Based on the changes in these performance indicators of lubricating oil, the state of lubricating oil and the wear state of machinery are judged.
The composition and content of various elements in the oil sample obtained by spectral analysis are important indicators for evaluating the wear during the running in process of diesel engines.
Iron spectrum analysis analyzes the morphology (shape, surface texture, edges, color, etc.) and concentration of wear particles in lubricating oil to further determine the wear morphology of mechanical friction pairs, providing a basis for early fault prediction, maintenance, and repair decisions.
(2) Selection of lubricating oil
When selecting lubricating oil for sliding bearings, it is necessary to comprehensively consider the influence of factors such as load, speed, clearance, temperature, and bearing structure.
During low-speed overload, impact or insufficient oil supply, as well as starting, stopping and shifting, bearings are often in a boundary lubrication state, and the oiliness and extreme pressure of lubricating oil play a significant role.
(3) The supply of lubricating oil should be ensured to be in good condition
The lubricating oil pressure is too low, and the lubrication system cannot achieve normal lubricating oil circulation and pressure lubrication, which cannot reliably supply sufficient oil to the bearing and shaft neck, resulting in insufficient lubrication between the bearing and shaft neck, less frictional heat dissipation, and abnormal temperature rise of the bearing, leading to bearing damage.
(4) Strengthen the maintenance and upkeep of the lubricating oil filter
Strengthening the maintenance and upkeep of the oil filter pedals can prevent mechanical impurities from entering between the bearings and the shaft neck, and is an important measure to extend the service life of diesel engines. The oil filter should be cleaned or replaced in a timely manner according to regulations to maintain its filtering performance and effectiveness, and prevent excessive oil contamination from causing bearing wear and tear.
(5) Strengthen navigation duty management
Strictly implement the duty system, always pay attention to changes in lubricating oil pressure, temperature, and cooling conditions. If there is no cooling water or insufficient water, the heat dissipation effect will deteriorate, causing the bearing working temperature to be too high and the viscosity of the lubricating oil to decrease. The lubricating oil film between the bearing and the shaft neck is not easy to form and maintain, and the lubrication conditions of the bearing will deteriorate, resulting in bearing damage.
3.2 Good break in
Failure to perform running in after replacing the bearing or repairing the journal can directly lead to or accelerate bearing failure.
Run in is the process of transitioning the surface of a mating component from its initial state to its operational state. After a new mating component is manufactured or replaced, a run in operation must be carried out before it is put into normal operation to ensure that the mating surface obtains initial effective wear and adapts to each other's morphology.
Good running in requires good lubrication, appropriate surface roughness, and running in time that matches the load.
During running in, the speed increases from low to high, the load increases from small to large, and the running time is allocated reasonably.
3.3 Minimize frequent stopping, starting, and overloading of diesel engines as much as possible
The amount of wear during a single start-up is often the wear after several hours of normal operation. If the fuel consumption is too high during start-up, the bearing will be severely impacted and prone to cracking.
Many defects in the bearing shell are related to excessive load on the diesel engine, especially high burst pressure, so long-term overload operation of the diesel engine should be avoided.
3.4 Regularly check and adjust the bearing clearance
When checking the crankcase, the connecting rod can be pried to determine the clearance between the connecting rod end and the crank pin based on the movement of the large end. Every 1200h-2000h of operation, the clearance between the main bearings should be checked and measured. The clearance should be adjusted or the bearing shell replaced before the clearance value reaches the wear limit.
During operation, if the lubricating oil filter, lubricating oil pump, etc. are working normally, but the pressure of the lubrication system drops significantly, it indirectly reflects the possibility of an increase in bearing clearance. At this time, it is necessary to check and adjust the bearing clearance as soon as possible.
2025 April 3rd Week Marginal Product Recommendation:
Material Specification Sheet–MG-6:
Material specification sheet – MG-6 is Ball and roller bearing steel according to EN ISO 683-17. Ball and roller bearing steel for balls and rollers of any dimension,rings and discs up to 30mm effective thickness.
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