Bearings are the parts that support the shaft. According to the friction properties of their work, bearings can be divided into two categories: sliding friction bearings (referred to as sliding bearings) and rolling friction bearings (referred to as rolling bearings). And each type of bearing can be divided into radial bearing, thrust bearing and radial thrust bearing according to the direction of the load it is subjected to. So what is a sliding bearing?
Sliding bearing, a bearing that works under sliding friction. Sliding bearings work smoothly, reliably and without noise. Under liquid lubrication conditions, the sliding surfaces are separated by lubricating oil without direct contact, which can greatly reduce friction loss and surface wear, and the oil film also has a certain ability to absorb vibration.
But the starting friction resistance is large. The part of the shaft supported by the bearing is called the journal, and the part that matches the journal is called the bearing bush. In order to improve the friction properties of the bearing surface, the layer of anti-friction material cast on its inner surface is called a bearing lining.
The materials of the bearing shell and bearing bush are collectively referred to as sliding bearing materials. Sliding bearing applications are generally under low speed and heavy load conditions, or operating parts where maintenance and lubricating oil are difficult.
1. What are the characteristics and applications of sliding bearings:
1. Characteristics of sliding bearings
Sliding bearings work smoothly, with lower noise than rolling bearings and reliable work. Friction losses and surface wear can be greatly reduced if it is ensured that the sliding surfaces are separated by a lubricating oil film without contact. However, the starting friction resistance of ordinary sliding bearings is large.
2. Application of sliding bearing
① Bearings with a particularly high working speed, such as the main shaft of a grinder;
② Bearings subjected to extreme shock and vibration loads, such as rolling mill rolls;
③ Special precision bearings are required;
④ The assembly process requires the bearing to be split, such as the bearing of the crankshaft;
⑤ Bearings with small radial dimensions are required.
Second, what is the structure of the sliding bearing:
Sliding bearings are generally composed of bearing shells and bearing seats. Sliding bearings can be divided into radial sliding bearings (mainly bearing radial loads) and thrust sliding bearings (mainly bearing axial loads) according to the direction of the load it bears. The commonly used structural forms of radial sliding bearings are integral and split.
1. Integral sliding bearing
Integral radial sliding bearing, connected with the frame by bolts. A bearing bush (or bushing) made of anti-friction material is pressed into the bearing seat hole, an oil cup is installed on the top of the bearing seat, there is an oil inlet hole on the bearing seat, and an axial oil groove is opened on the inner surface to distribute lubricating oil for lubrication .
The biggest advantage of the integral sliding bearing is the simple structure, but the bearing clearance cannot be adjusted when the working surface of the bearing is too worn; the journal can only be loaded from the end, which is inconvenient for heavy shafts or shafts with intermediate journals, even Unable to install. To overcome these two shortcomings, split sliding bearings can be used.
2. Split sliding bearing
The split sliding bearing consists of a bearing seat, a bearing cover, a split bearing shell (divided into upper and lower shells) and connecting bolts.
The split surface of the bearing should be nearly vertical to the load direction. Most bearing split planes are horizontal and some are inclined. The split surface of the bearing cap and the bearing seat is often made into a stepped shape to locate and prevent misalignment during work. The bearing clearance after the bearing bush is worn can be adjusted by reducing the metal shim at the split surface or scraping the metal of the bearing bush.
The split sliding bearing is easy to assemble and disassemble, and the gap between the bearing bush and the shaft can be adjusted, so it is widely used.
Third, what is the classification of sliding bearings:
①According to the direction that can bear the load, it can be divided into two categories: radial (radial) sliding bearing and thrust (axial) sliding bearing.
②According to the type of lubricant, it can be divided into seven categories: oil-lubricated bearings, grease-lubricated bearings, water-lubricated bearings, gas bearings, solid-lubricated bearings, magnetic fluid bearings and electromagnetic bearings.
③ According to the thickness of the lubricating film, it can be divided into two types: thin film lubricated bearings and thick film lubricated bearings.
④According to the bearing material, it can be divided into bronze bearings, cast iron bearings, plastic bearings, gemstone bearings, powder metallurgy bearings, self-lubricating bearings and oil-impregnated bearings.
⑤ According to the bearing pad structure, it can be divided into round bearings, elliptical bearings, three-oil vane bearings, stepped surface bearings, tilting pad bearings and foil bearings.
Fourth, what is the installation and maintenance of sliding bearings:
① The installation of the sliding bearing should ensure that the journal rotates flexibly, accurately and smoothly in the bearing hole.
② The bearing bush and bearing seat hole should be repaired and scraped, and the split surface of the bearing bush should be 0.05-0.1 mm higher for compaction. When the integral bearing bush is pressed in
To prevent deflection, and secure with set screws.
③ Note that the oil circuit is unblocked, and the oil circuit and the oil tank are connected. When scraping and grinding, the dots on both sides of the oil tank should be soft to form an oil film, and the dots at both ends should be even to prevent oil leakage.
④ Pay attention to cleanliness. All parts that may have oil stains during the repairing and debugging process must be cleaned and oiled after repairing and scraping.
⑤ During the use of the bearing, always check the lubrication, heat generation and vibration problems. In case of fever (generally below 60°C is normal), smoke, stuck, abnormal vibration, sound, etc., you should check, analyze and take measures in time.
During the operation of the sliding bearing, friction occurs due to the contact between the journal and the bearing bush, which causes the surface to heat up, wear and even “seize”. Therefore, when designing the bearing, the sliding bearing material with good friction reduction should be used to manufacture the bearing bush, and appropriate lubrication should be selected. agent and use suitable supply methods, improve the structure of the bearing to obtain thick film lubrication, etc.
5. What are the 8 main failure forms and treatment measures of sliding bearings:
Bearing is an important part of rotating machinery, how its working state will directly affect the performance of the whole machine.
The detection of the working state of the bearing and the research of the fault diagnosis technology must be paid attention to. The sliding bearing is generally composed of the bearing body and the softening system. During operation, the journal rotates in the bearing to drive the lubricating oil to form a dynamic pressure oil film, which depends on the pressure of the oil film. Support and ensure the flexible rotation of the journal by offsetting the external load, which determines the particularity and complexity of the Huadong bearing failure, and the bearing failure is the main form of failure.
(1) Normal wear and tear
The working surface of the bearing is smooth and flat, and the wear rate of the bearing is usually close to a constant value. When the accumulated wear amount is greater than the allowable wear value of the design, the bearing will fail and die into a wear and tear. The main reason for the wear is the overload or overspeed operation of the unit. ; The lubricating oil contains too many magazines, poor lubrication, poor bearing running-in, etc.
Methods and measures to reduce normal wear and tear failures:
1. Ensure the lubrication of the normal operation of the bearing, maintain and maintain the lubrication system and sealing system of the unit in time, replace the lubricating oil, ensure that the particle content and size in the lubricating oil are within the allowable range, and operate in strict accordance with the operating procedures;
2. Avoid running the bearing under overload and overspeed conditions.
(2) scratches
The direct metal contact between the bearing and the journal surface produces fine and shallow plough marks. The main reason is that the oil film pressure of the sliding bearing is not enough, the lubricating oil is too little, or the oil inlet pipeline is broken or the lubricating oil is not enough during the start or stop of the unit. mixed with other impurities.
Methods and measures to prevent chafing: the same methods and measures to reduce normal wear and tear.
(3) Gluing phenomenon
Occurs when the unit is overloaded, the local bearing load is too high, overheated, lack of lubricating oil, bearing seat vibration, etc., the bearing seat temperature rises rapidly, so that the bearing and the shaft mating surface are in direct contact and partially fused together.
Methods and measures to prevent gluing:
1. Avoid overload operation of the unit;
2. Improve bearing assembly quality and lubricating oil cleanliness to avoid excessive local load on bearings;
3. Improve the oil supply system of lubricating oil to avoid running without oil;
4. Strengthen the connection rigidity of the bearing seat to avoid vibration of the machine seat.
(4) Burning tile phenomenon
Burning pad is the malignant damage of East China bearing. The bearing pad and the journal material thermally expand, the bearing gap disappears, and the metals are in direct contact, causing the lubricating oil to burn. The bearing bush and the shaft instrument are rotated or seized. At this time, the bearing anti-friction material is seriously deformed and torn. The reason is that the bearing has been rotated for a long time without lubricating oil, and the temperature of the bearing bush has risen rapidly.
Methods and measures to prevent burning of bearing bushes:
1. Start the lubricating oil system first to ensure that the oil pressure of the lubricating system meets the requirements, and then start the unit. The unit should not be accelerated immediately after it is started;
2. Avoid abnormal overload or overspeed (load rejection) operation of the unit;
3. Improve the assembly quality, ensure that the bearing clearance is not too small but not too large, and the oil seal quality is better and does not leak;
4. Avoid running without oil.
(5) Fatigue damage
The surface of the sliding bearing is subjected to alternating loads, which causes the reciprocating tensile stress, compressive stress and shear stress on the bearing surface, resulting in micro-cracks. Later, with the continuous repetition of the stress, especially when the lubricating oil enters the crack gap , Due to the sharp cracking effect of the lubricating oil, the cracks continue to expand in the bearing, and finally fatigue damage is formed. In addition, in lead-copper alloy bearings, if the lubricating oil is mixed with water and heavy oil, the lead phase will be corroded and exuded, leaving only copper dendrites, reducing the strength and forming a fatigue source; thermal stress can also cause fatigue failure.
The characteristics of fatigue failure: the working surface of the bearing load-bearing area is a crack that expands in a network, and the crack develops in the depth direction of the anti-friction layer. Finally, the material of the anti-friction layer is granular, flake or block peeled off, with metallic luster.
(6) Corrosion damage
Due to the chemical reaction between the surrounding environment medium and the bearing working surface, the sliding bearing causes the bearing working surface to be black or dull discoloration, isolated and discontinuous fluff, or randomly distributed pits, honeycombs or irregularities of connected pits.
Causes of abrasion: the lubricating oil is oxidized or polluted; the bearing has poor anti-wear; the bearing anti-friction material contains harmful impurity elements; the working surface of the bearing has parasitic current passing through it.
(7) Cavitation failure
When the sliding bearing of the generator set operates under heavy load and high speed, small oil vapor bubbles will be formed in the lubricating oil. When the bubbles move to the high-pressure area or the pressure of the lubricating oil increases, the bubbles will burst, and the surrounding lubricating oil will quickly replenish the original oil. The space occupied by the air bubbles forms a strong pressure shock wave one by one, which makes the bearing surface strongly impacted, and the surface plastic deformation occurs, forming a large stress, and finally causing the bearing surface to partially peel off.
Methods and measures to prevent cavitation:
1. Strengthen the support of the bearing, improve the stiffness matching between the bearing and the bearing seat, and avoid local high-frequency vibration of the bearing;
2. Improve the supply and quality of lubricating oil;
3. Avoid the formation of a strong eddy current area.
(8) Oil film oscillation
The hazards of oil film oscillation:
1. Oil film oscillation is a harmful vibration, which makes the rotor more deviated from the bearing center and increases the instability factor of the rotor;
2. The oil film oscillation causes alternating stress, and the alternating stress will lead to fatigue failure of the lubricated moving bearing.
Methods and measures to prevent oil film oscillation:
1. Increase the stiffness of the rotor system, that is, to increase the natural frequency of the rotor itself, and also to increase the destabilizing speed of the oil film oscillation. Generally, the destabilizing speed of the system should be more than 25% of the working speed;
2. Select the best bearing form and bearing parameters, the cylinder Zhuocheng is simple to manufacture, but the vibration resistance is the worst; elliptical bearings, three oil wedges, and multiple oil wedge bearings can significantly improve the stability of the bearing and the rigidity of the oil film, which are ideal An option; the inclination of the pivot pad of the pivotable pad bearing can be automatically adjusted with the position of the journal to adapt to different loads, rotational speeds, elastic deformation and deflection of the shaft, etc., and establish oil-liquid lubrication, which is the most a good choice;
3. Increase the bearing specific pressure and change the inlet oil temperature and viscosity; the greater the bearing specific pressure, the less likely the journal to float;
4. Reduce the bearing clearance, change the oil inlet pressure, increase the external resistance, etc.
