Diesel engine is the most widely used power device in ships. As one of the key parts of modern enhanced, high-speed and compact diesel engine, sliding bearing has become one of the constraints to improve engine power, reduce fuel consumption and improve operation reliability.
The statistical results of the report of Swedish Club insurance company (Sweden) on the 22nd International internal combustion engine conference are as follows:
The main faults of low-speed engine are supercharger (47%), cylinder liner (19%), journal and bearing (8%), piston and piston rod (6%) and fuel pump (4%);
The main faults of medium speed engine are supercharger (20%), crankshaft and connecting rod (16%), exhaust valve and ejector rod (14%), journal and bearing (12%) and piston (9%).
It can be seen that bearing failures account for a large proportion of low-speed and medium speed engines. The maintenance of sliding bearings of marine diesel engines is an important measure to ensure the safety of marine power plants and the safety of ships and the sea.
1 Common damages and causes of sliding bearing
Bearings are arranged between the relatively rotating parts of the diesel engine, and sliding bearings are the main ones.
The sliding bearing of diesel engine works under alternating load, so it is difficult to maintain uniform and constant bearing oil film in the bearing; The load of the bearing is large; The relative movement speed between the journal and the bearing bush is high, and some even reach more than 10m / s. In addition, the impurities in the lubricating oil and the deterioration of the lubricating oil and other corrosion damage make the bearing easy to be damaged.
During the use of diesel engine, the working conditions are complex, the starting and stopping are frequent, and there are many low-speed working conditions, so the bearing is prone to semi dry friction. In addition, the deformation of parts will cause local load concentration on the bearing surface.
Under these bad working conditions, the bearing will be damaged. The bearing damage is mainly caused by the wear-resistant alloy layer on the bearing bush.
1.1 Excessive wear
After the diesel engine has been running for a period of time, the lower bush of the main bearing, the lower bush of the crosshead bearing and the upper bush of the crank pin bearing are excessively worn. The main reasons for the excessive wear are:
The diesel engine is frequently started and overloaded for a long time. The working surface of the journal bearing is rough and the geometric error is too large. The poor purification of the lubricating oil leads to more mechanical impurities and improper daily maintenance and management.
Excessive wear of Bearing Bush will increase the bearing clearance, cause impact and intensify wear.
1.2 Cracks and spalling
Crack and spalling is a kind of damage that often occurs on white alloy thick wall bearing shells. It is the fatigue damage of alloy materials under pulsating oil film pressure. At first, small fatigue cracks occur on the working surface of bearing shells due to various reasons. With the continuous operation of the diesel engine, the cracks on the bearing shells gradually expand and extend, so that the wear-resistant alloy on the bearing shells is flaked off, i.e. spalling. The main reason is that the explosion pressure of the diesel engine is too high, Or the bearing impact load is too heavy due to excessive geometric error of the journal, excessive wear of the bearing bush, excessive local load peak, or too large clearance of the bearing journal.
In addition, when the bonding performance or process quality of the bearing alloy and the back of the bearing pad is poor, the cracking and peeling of the bearing alloy will occur faster. In addition, when the bearing pad bears excessive bearing load or the axial load distribution is uneven, cracks will occur on the bearing pad, which will expand to form many closed cracks under the action of the oil wedge. In serious cases, the crack area is large, and even extends to the end face of the bearing pad or causes alloy peeling.
1.3 Bearing bush burning
The burning of bearing bush alloy is a common and serious damage of sliding bearing. It is mainly due to too small bearing clearance, insufficient lubricating oil pressure or loss of pressure, which makes the oil film unable to be established, too rough journal surface or too large geometric error, etc. the oil film can not be established or damaged, which makes the metal of the shaft and the Bush directly contact, and the high temperature caused by dry friction makes the alloy melt.
1.4 Bearing bush corrosion
Corrosion of bearing bush includes electrochemical corrosion and corrosion caused by electric leakage.
Water contained in lubricating oil, oxidation of lubricating oil, and deterioration of lubricating oil caused by mixing of gas or fuel oil will cause macro or micro electrochemical corrosion pitting on the working surface of bearing bush. Stray current caused by electrical leakage on board will also cause local pitting on the inner and outer surfaces of bearing bush.
2 Bearing inspection
2.1 Inspection of crankshaft journal wear
The long-term operation of the diesel engine causes uneven wear of the crankshaft main journal and crank pin journal, resulting in diameter reduction, geometric accuracy reduction, roundness and cylindricity error, which will change the fit clearance between the shaft and the bearing bush, damage the lubricating oil film, and reduce the bearing capacity of the bearing.
Regularly measure the roundness and cylindricity errors of the main journal and crank pin journal, and repair them if they exceed the standards specified in the manual.
2.2 The installation of sliding bearing shall meet the specified requirements
In order to ensure the safe and reliable operation of the sliding bearing, the installation quality of the bearing and the cooperation with the shaft are particularly important.
(1) The mating surface of bearing bush and bearing seat hole shall be well fitted
When installing the bearing bush, the installation of the lower bush is the most critical. The outer surface of the lower bush should be closely and evenly contacted with the inner surface of the bearing seat hole. The 0.05mm feeler gauge can not be inserted, and the fitting surface is closely fitted. The bearing bush works reliably during operation, without deformation and cracks, which is conducive to heat dissipation. When the lower bush of the thick wall bearing bush is installed in the bearing seat, the fitting surface of the lower bush can be inspected by applying colored oil on the bearing seat surface, and the fitting surface can be within 25mm × The adhesion point within 25mm area shall not be less than 3 points, i.e. the pad back of large and medium-sized diesel engines and? The contact area of the bearing seat shall not be less than 75%. The tight fit between the thin-wall bearing shell and the bearing seat is realized through the interference fit between the bearing shell and the bearing seat hole. The recommended value of the expansion of the non flanging bearing shell mouth in GB / t3535-94 is (0.3-1.0) mm, and the recommended value of the expansion of the flanging bearing shell mouth is (0.1-0.4) mm.
(2) The journal and bearing lower bush should be at a certain angle? Uniform contact
The contact angle between the main journal of the diesel engine and the lower bush of the main bearing shall be in uniform contact within the range of 40 ° ~ 60 ° on both sides of the center line of the engine body; The contact angle between the crank pin journal and the upper bearing pad of the connecting rod big end shall be in uniform contact within 60 ° ~ 90 ° on both sides of the connecting rod center line, and the contact angle shall not be too large or too small.
If the contact angle is too small, the pressure of the sliding bearing will increase, and if it is serious, the sliding bearing will produce large deformation, accelerate the wear and shorten the service life;
If the contact angle is too large, the formation of oil film will be affected and good liquid lubrication will not be obtained.
(3) The bearing clearance shall meet the requirements
Proper bearing clearance is an important condition to form lubricating oil film and realize hydrodynamic lubrication.
The bearing clearance is too small, the oil film cannot be established, and the metal of the shaft and the bearing pad is in direct contact, generating a large amount of heat, resulting in melting of the alloy;
If the clearance is too large, the lubricating oil will be lost and impact will occur, causing the alloy layer of bearing bush to crack and crack. The bearing clearance is required to be between the installation clearance and the limit.
For example, the bearing clearance of a crosshead diesel engine with a journal diameter of 500mm should be (0.40-0.90) mm.
The bearing clearance of thick wall bearing bush is generally measured by lead pressing method, while the bearing clearance of thin wall bearing bush is measured by comparison method. The diameters of the corresponding parts of the shaft and hole are measured by internal and external micrometers respectively. The difference between the two diameters is the bearing clearance.
2.3 Measurement of bearing bush wear
The wear amount of the lower pad of the thick wall pad is generally determined by measuring the sinking amount of the main journal with a bridge gauge or by directly measuring the thickness of the lower pad and comparing the thickness of the new pad. The wear amount of upper bearing bush of connecting rod big end can be determined by direct measurement. Generally, it is not necessary to measure the wear amount of Thin-walled bearing shells. If the bearing clearance does not meet the requirements, it needs to be replaced.
3 Strengthen operation and maintenance management
From the analysis of common damage causes of sliding bearings and the factors affecting the formation of liquid lubricating oil film, the working condition and service life of bearings mainly depend on the maintenance, installation management and quality. Therefore, the following aspects should be done well:
3.1 Lubricating system and lubricating oil management
(1) Adopt lubricating oil sample analysis technology
The lubricating oil sample analysis technology as shown in Fig. 1 is constructed as the detection means of sliding bearing. Through comprehensive analysis such as physical and chemical analysis, spectral analysis and ferrographic analysis of lubricating oil, fault diagnosis and prediction of sliding bearing system can be effectively carried out.
Fig. 1 sliding bearing analysis technology
The conventional physical and chemical analysis mainly uses the conventional physical and chemical performance analyzer to determine the viscosity, moisture, total acid value (tan), total alkali value (TBN), insoluble matter / mechanical impurities, flash point, freezing point, ash content, carbon residue and other indicators of the lubricating oil, and judge the state of the lubricating oil and the wear state of the machinery according to the changes of these performance indicators of the lubricating oil.
The composition and content of various elements in the oil sample obtained by spectral analysis is an important index to evaluate the wear of diesel engine during running in.
Ferrography analysis further judges the wear morphology of mechanical friction pairs by analyzing the morphology (shape, surface texture, edge, color, etc.) and concentration of wear particles in lubricating oil, so as to provide basis for early fault prediction and maintenance and repair decisions.
(2) Selection of lubricating oil
When selecting lubricating oil for sliding bearing, the influence of load, speed, clearance, temperature and bearing structure should be considered comprehensively.
Under low speed and heavy load, impact or insufficient oil supply, as well as startup, shutdown and speed change, the bearing is often in boundary lubrication state, and the oiliness and extreme pressure of lubricating oil will play a greater role.
(3) The supply of lubricating oil shall be in good condition
The lubricating oil pressure is too low, and the lubricating system can not realize normal lubricating oil circulation and pressure lubrication, and can not reliably supply sufficient oil to the bearing and the journal, so that the bearing and the journal can not be fully lubricated. The low friction heat dissipation makes the bearing temperature rise abnormally, resulting in bearing damage.
(4) Strengthen the maintenance of lubricating oil filter
Strengthening the maintenance of the lubricating oil filter pedal is an important measure to prevent mechanical impurities from entering between the bearing and the journal and prolong the service life of the diesel engine. The oil filter shall be cleaned or replaced in time as required to maintain its filtering performance and effect and prevent bearing wear and strain caused by excessive dirt of the lubricating oil.
(5) Strengthen the management of navigation duty
Strictly implement the duty system and pay attention to the change of lubricating oil pressure, temperature and cooling at all times. If there is no cooling water or insufficient water, the cooling effect will be poor, resulting in the high working temperature of the bearing, the viscosity of the lubricating oil will be reduced, the lubricating oil film between the bearing and the journal will not be easily formed and maintained, the lubrication condition of the bearing will deteriorate, and the bearing will be damaged.
3.2 Good running in
If the bearing bush is replaced or the journal is repaired, the bearing failure will be directly caused or accelerated without running in.
Running in is the process that the surface of mating parts transits from the initial state to the service state. After the new mating parts or one of them is replaced, the running in operation must be carried out before they are put into normal operation, so that the mating surfaces can obtain the initial effective wear and mutually compatible morphology.
Good running in requires good lubrication, proper roughness of mating surface and matching of running in time and load.
During running in, the rotation speed is from low to high, the load is from small to large, and the operation time is reasonably distributed.
3.3 Try to reduce frequent shutdown, starting and overload operation of diesel engine
Often, the wear amount during a start is the wear of several hours of normal operation. If the fuel amount is too large during the start, the bearing bush will be seriously impacted and cracks will easily occur.
Many defects of the bearing bush are related to the excessive load of the diesel engine, especially the high burst pressure. Therefore, long-term overload operation of the diesel engine should be avoided.
3.4 Regularly check and adjust the bearing clearance
When checking the crankcase, pry the connecting rod to judge the clearance between the big end of the connecting rod and the crank pin from the movement of the big end. Check and measure the clearance of the main bearing every 1200h-2000h. Adjust the clearance or replace the bearing bush before the clearance value reaches the wear limit.
During operation, if the lubricating oil filter, lubricating oil pump, etc. work normally, but the pressure of the lubricating system drops significantly, it indirectly reflects that the bearing clearance may become larger. At this time, check and adjust the bearing clearance as soon as possible.
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In order to enhance the anti-seizure property of the sliding bearing while making the content of Pb contained in the Sn-based overlay layer smaller or while using no Pb if possible, there is provided a multi-layer sliding bearing comprising a back metal layer, a bearing alloy layer, an intermediate plating layer and a Sn-based overlay layer, the Sn-based overlay layer containing reinforcing metal and/or inorganic particles, the content of the reinforcing metal and/or inorganic particles being varied step-wise or successively in a direction of a thickness of the overlay layer so that the content in a thickness-wise central portion of said overlay layer is relatively high and so that another content in a surface portion of the overlay layer is zero or smaller than the content in the central portion.