Introduction to the damage form and damage form of

Damage forms and damage forms of sliding bearings

damage forms

sliding bearings used in engines are usually divided into two types: one is liner type thin-walled bearings, which are shaped like tiles, commonly known as bearing pads; The other is bushing, also known as copper sleeve, which is a hollow cylinder. Lining thin-walled bearings are mainly used to support the crankshaft and connecting rod of the engine; The bushing is mainly used to support camshaft journal and piston pin. Next, we mainly understand the early damage forms and preventive measures of lining thin-walled bearings (bearing shells)

1. The form of early damage

during normal use, the bearing gradually wears out until it finally loses its working capacity and ends its service life. This natural damage is inevitable. However, if the bearing is prematurely worn or damaged due to improper engine assembly and adjustment, poor lubricating oil quality or poor service conditions, it is an early damage caused by man-made. Early damage not only greatly reduces the service life of the bearing, but also affects the normal operation of the engine

according to the long-term experience of diesel engine maintenance, the common forms of early damage of sliding bearings are mechanical damage, bearing cavitation, fatigue pitting, corrosion, etc

(1) mechanical damage mechanical damage of sliding bearing refers to the groove marks in varying degrees on the alloy surface of bearing bush, and in serious cases, metal peeling and large-area random scratches occur on the contact surface; Generally, contact surface damage and ablation exist at the same time. The main reason for bearing mechanical damage is that the bearing surface is difficult to form oil film or the oil film is seriously damaged

(2) bearing cavitation sliding bearings under the repeated action of cylinder pressure impact load), plastic deformation and cold work hardening occur on the surface layer, local loss of deformation ability, gradually form wrinkles and continue to expand, and then form cavities on the loaded surface layer with the falling off of wear debris. Generally, when the bearing bush is pitted, the pit first appears, and then this pit gradually expands and causes the crack of the alloy layer interface. The crack extends along the parallel direction of the interface until it peels off. The main reason for the cavitation of sliding bearings is that the sudden change of the cross section of structural elements such as oil grooves and oil holes causes a strong disorder of oil flow, forming bubbles in the vacuum area with disordered oil flow, and then cavitation occurs due to the collapse of bubbles due to the increase of pressure. Cavitation usually occurs in the high load area of the bearing, such as the lower bearing bush of the crankshaft main bearing

(3) fatigue pitting bearing fatigue pitting refers to the fatigue damage, fatigue pitting or fatigue falling off in the middle of the bearing due to the overload of the engine, which makes the bearing work overheated and the bearing clearance too large. This kind of damage is mostly caused by overload, excessive bearing clearance, or unclean lubricating oil, mixed with foreign matters. Therefore, when using, attention should be paid to avoid bearing overload and do not run at too low or too high speed; Adjust the engine to a stable state when idling; Ensure the normal bearing clearance and prevent the engine speed from being too high or too low; Check and adjust the working condition of the cooling system to ensure that the working temperature of the engine is appropriate

(4) bearing alloy corrosion bearing alloy corrosion is generally caused by the impure lubricating oil, and the chemical impurities (acid oxides, etc.) in the lubricating oil oxidize the bearing alloy to form acidic substances, causing part of the bearing alloy to fall off, forming irregular small cracks or pits. The main causes of bearing alloy corrosion are improper selection of lubricating oil and bearing failure 4: poor corrosion resistance of materials when increasing tensile load, or rough engine operation and high temperature

(5) the bearing is burnt. There is a small convex metal surface between the journal and the bearing friction pair, which forms a local high temperature. In the case of insufficient lubrication and poor cooling, the bearing alloy is blackened or partially burnt. This fault is often caused by the tight fit between the journal and the bearing; Insufficient lubricating oil pressure is also easy to burn the bearing

(6) the bearing moves to the outer circle, which means that the bearing rotates relatively in the seat hole. After the bearing goes round, it will not only affect the heat dissipation of the bearing, which is easy to ablate the alloy on the inner surface of the bearing, but also damage the back of the bearing and burn the bearing in serious cases. The main reasons are that the bearing is too short, the tenon is damaged, and the processing or installation does not meet the specifications

2. Preventive measures

early damage of sliding bearings is much more common than bearing burnout, so it is very important to prevent early damage of sliding bearings. The correct maintenance of sliding bearings is an effective way to reduce the early damage of bearings and a reliable guarantee to prolong the service life of bearings. Therefore, in the daily maintenance and repair of the engine, we must pay attention to the appearance shape of the alloy surface, back, end and edge corners of the bearing. If there are abnormal or signs of excessive wear, we must carefully find out the causes, take corresponding measures, improve the working conditions of the bearing, and pay attention to the prevention of early damage of the sliding bearing

(1) when improving the design and manufacturing process design of bearings or selecting bearings, the thermal balance of bearings should be considered to control the temperature rise. Because the bearing works under the friction state, the internal friction (viscosity) of the lubricating oil liquid causes the successful consumption, which will cause the bearing temperature to rise after being converted into heat, the viscosity of the lubricating oil will decrease, and the clearance will change, which will soften the Babbitt alloy of the bearing, and in serious cases, "Bush burning and shaft holding" accidents will occur. Therefore, in the structural design, the oil hole should be opened from the top of the upper bearing bush (non bearing area) of the bearing, so that the lubricating oil can be introduced from the non bearing area; An oil groove is opened longitudinally or horizontally on the inner surface of the bearing shell with the oil inlet hole as the center, which is conducive to the uniform distribution of lubricating oil on the journal to control the temperature rise. According to the working condition of the bearing, the bearing material must have the following properties: low friction coefficient; Good thermal conductivity and small coefficient of thermal expansion; Strong wear resistance, corrosion resistance and gluing resistance; It should have sufficient mechanical strength and plasticity. Therefore, Babbitt alloy is selected as the bearing material. Babbitt alloy can work well under stable load, but it is prone to cavitation under unstable load, so it should not be used in high-power engine. High tin lead base alloy and low tin lead base alloy have high strength and hardness, strong anti fatigue and anti cavitation ability, and have a good effect in high-power engines. In recent years, the sputtering method of physical vapor deposition has been used abroad to coat the nickel grid on the surface of copper lead bearing with aluminum alloy containing 20% tin or pure tin, and the effect is very good. In addition, changing the whole round oil groove bearing bush to the half round oil groove or part of the oil groove bearing bush can not only improve the lubrication state of the engine sliding bearing, but also improve its bearing capacity

(2) improve the maintenance and assembly quality of the bearing, improve the hinge quality of the bearing, and ensure that the back of the bearing is smooth and free of spots, and the positioning bumps are intact; Its ejection amount is 0 5mm, which can ensure that the bearing bush fits closely with the bearing seat hole with the help of its own elasticity after assembly; Each end of the upper and lower bearing shells installed in the bearing seat shall be higher than the plane of the bearing seat μ m. The excess can ensure that the bearing fits closely with the bearing seat after tightening the bearing cover bolts according to the specified torque, generating sufficient friction self-locking force, preventing the bearing from loosening, with good heat dissipation effect, and preventing the bearing from ablation and wear; The working surface of the bearing cannot be measured by 75%-85% contact marks with the scraping method. The fit clearance between the bearing and the journal should meet the requirements without scraping. In addition, during assembly, pay attention to check the processing quality of crankshaft journal and bearing. Strictly 9. Repeat 7 and 8 grids after changing the sample to implement the repair process specification, so as to prevent incorrect installation and uneven or non-conforming torque of bearing bolts due to improper installation method, resulting in bending deformation and stress concentration, resulting in early damage to the bearing

(3) reasonably select and fill lubricating oil. In the process of use, choose lubricating oil with low surface tension of oil film, so that the impact of oil flow will be reduced correspondingly when the formed bubble collapses, which can effectively prevent bearing cavitation; The viscosity grade of lubricating oil shall not be increased at will to avoid increasing the coking tendency of bearings; The lubricating oil level of the engine must be within the standard range, and the lubricating oil and refueling tools must be clean to prevent any dirt and water from entering, while ensuring the sealing effect of all parts of the engine. Pay attention to regular inspection and lubricating oil replacement because it is a 1-dimensional quantum material perfectly connected by a hexagonal structure; The place where lubricating oil is filled shall be free of pollution and sand, so as to prevent the invasion of all pollutants; It is forbidden to mix lubricating oils of different quality, viscosity grades and types of use. The sedimentation time before lubricating oil filling should generally not be less than 48h

(4) when correctly using and maintaining the engine and installing bearings, the moving surfaces of the shaft and bearings should be coated with clean engine oil of the specified brand. After the engine bearing is reinstalled, turn off the fuel switch before the initial start, drive the engine to idle for several times with the starter, turn on and turn on the fuel switch when the engine oil pressure gauge shows, set the throttle to the medium and low speed position, and start the engine for operation observation. Idling time shall not exceed 5min. Do a good job in the running in period of the new engine and the engine after overhaul. During the running in period, it is forbidden to work under the condition of sudden increase and decrease of load and high speed for a long time; The engine cannot be shut down immediately after long-time full load operation. The engine must be operated at no-load medium and low speed for 15min before it can be shut down, otherwise the internal heat cannot be dissipated. Strengthen the cleaning and maintenance of oil filter and crankcase ventilation device, and replace the filter element in time according to the requirements of the manual; Ensure the normal operation of the engine cooling system, control the normal temperature of the engine, prevent the radiator from "boiling", and it is strictly forbidden to drive without cooling water; Correctly select fuel, accurately adjust the valve timing and ignition timing, etc., to prevent abnormal combustion of the engine; Check and adjust the technical condition of crankshaft and bearing in time

installation procedure

basic requirements

not only make the journal and sliding bearing contact evenly and closely, but also have a certain fit clearance

contact angle

refers to the center angle of the contact surface between the journal and the sliding bearing. The contact angle should not be too large or too small. Too small contact angle will increase the pressure of sliding bearing, and in serious cases, it will cause greater deformation of sliding bearing, accelerate wear and shorten 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. The experimental study shows that the limit of contact angle of sliding bearing is 120 °. When the sliding bearing is worn to this contact angle, the liquid lubrication will be destroyed. Therefore, the smaller the contact angle is, the better without affecting the pressure condition of the sliding bearing. From the theoretical analysis of friction distance, when the contact angle is 60 °, the friction torque is the smallest. Therefore, it is suggested that for sliding bearings with a speed higher than 500r/min, the contact angle should be 60 °, and for sliding bearings with a speed lower than 500r/min, the contact angle can be 90 ° or 60 °

contact point

the actual contact between the journal and the sliding bearing surface can be expressed by the actual contact points per unit area. The more, thinner and more uniform the contact points are, the better the sliding bearing scraping is. On the contrary, the sliding bearing scraping is not good. Generally speaking, the finer the contact points, the more difficult it is to scrape and grind. During production, the contact point shall be determined according to the performance and working conditions of the sliding bearing. The data listed in the following table can be used for reference: sliding bearing speed (r/min) contact point (every 25 × 25 mm