The reason for bearing runs hot
Rolling bearing “hot” is often encountered in production. If the cause is not clear, the treatment is not timely, and the measures are not appropriate, it will often get twice the result with half the effort, and may even cause vicious accidents such as bearing burnout and gear breaking. The following is an analysis and summary of specific problems in design, installation, lubrication, assembly and other links based on several typical cases encountered by the author in the production and debugging process, for your reference.
1. Bearing fever caused by improper lubrication
1.1 There are inherent deficiencies in the design of bearing lubrication parts
1.2 The lubricating oil pipe is blocked by foreign matter
Because the lubrication management is not strict, the container is dirty or the pipeline welding and pickling is not standardized during the refueling and oil changing process, causing welding slag, rust and other sundries to enter the oil pipe and block the oil hole, which is more common in the early stage of the project.
1.3 Lubricating oil (grease) has deteriorated or is not refueled in time
The lubricating oil (or grease) used in rolling bearings has a certain working temperature. When the temperature is too high, water or dust enters the bearing seat, it will be seriously oxidized and emulsified, which will lose its lubrication effect and cause the bearing to suffer from high temperature. And burned. In addition, it is a common phenomenon that the lubricating oil (or grease) itself is of poor quality or the oil (grease) is not added in time during operation. Material powder will cause the temperature of the bearing to rise or produce abnormal noise.
1.4 The cooler of the lubrication pipeline is fouled and blocked, resulting in poor cooling effect
Especially in summer production, this problem is especially common, and individual manufacturers do not hesitate to increase or parallel series coolers to enhance the cooling effect. Because of the serious scaling of the high-temperature fan cooler, the frequent alarms for excessively high bearing temperature have been encountered in all branches. A more effective treatment is to pickling and descaling the cooler before the beginning of summer each year.
2. Bearing heating caused by improper installation
2.1 Bearing fever caused by insufficient design expansion
This situation is more likely to occur on long-axis equipment such as large fans and crushers. This is also a link that is easier to ignore by equipment design, manufacturing, installation and maintenance personnel. In a typical case, the rear exhaust fan of the rotary kiln of CX Company experienced severe heat generation at the free end bearing of the fan during the initial stage of production. Because of the severe friction between the rotor main shaft and the bearing end cover after thermal expansion under working conditions, the high temperature generated in a short period of time was The end cover of the bearing seat and the end face of the rotor main shaft are welded together. Therefore, when the new fan is installed and accepted, it is necessary to check whether the axial clearance of the bearing at the free end can meet the expansion requirements of the working conditions.
The method of calculating the expansion of the shaft is very simple:
△L=L×(t-t0)×0.000012
In the formula: △L—the expansion of the shaft; mm;
L—The length of the shaft between the bearing seats; mm;
t—working condition temperature; ℃;
t0—Ambient temperature when the equipment is installed; °C.
It should be noted that the shrinkage of the equipment in winter should also be considered when installing in summer. This is especially important for northern regions. Generally speaking, the maximum environmental temperature difference between winter and summer in the cold northern region can reach 80℃. If a shaft with a length of 3m is installed in summer, the maximum shrinkage in winter (when it is stopped) can be close to 3mm.
2.2 Bearing fever caused by skew installation
The skew of the bearing installation will cause the balls to not roll in the correct position of the bearing raceway, and even cause larger axial forces between the end faces of the balls and the inner race and outer race stop of the bearing housing, causing the bearing to overheat. It takes a lot of effort to troubleshoot the situation of incorrect bearing. This is the case when the bearing of LY’s limestone crusher is heated. The solution is to suck the dial indicator on the shaft and hit the needle on the end face of the bearing outer ring to check the end face bounce around one week. The end-face runout value is controlled within 0.05. You can also use a feeler gauge to directly check the clearance on the left and right sides of the bearing.
2.3 Fever caused by improper assembly of the bearing itself
(1) Improper cooperation
The matching of the inner hole of the bearing and the shaft adopts the basic hole system, and the matching of the outer circle of the bearing and the bearing seat hole adopts the basic shaft system. Generally, under normal load conditions, the shaft and the bearing inner race are matched with j5, js5, js6, k5, k6, and m6, and the bearing seat hole and the bearing outer race are matched with j6, j7. The rotating seat ring usually adopts an interference fit, which can avoid rolling and sliding of the seat ring on the mating surface of the shaft diameter and the bearing seat hole under load.
But sometimes due to the inaccurate measurement of the shaft diameter and the size of the bearing seat hole or the roughness of the mating surface does not meet the standard requirements, excessive interference fit is caused, and the bearing seat ring is greatly squeezed, resulting in the radial clearance of the bearing itself. The reduction will make the bearing difficult to rotate, heat up, wear intensified or stuck, and in severe cases, it will cause the inner and outer races of the bearing to crack during installation. The non-rotating seat ring often adopts a fit with little clearance or interference. In this way, the non-rotating seat ring may produce a small creep, and the contact surface of the seat ring and the rolling element is constantly replaced, and the raceway of the seat ring wears evenly. At the same time, it can also eliminate the phenomenon of axial jamming of the rolling elements in the bearing due to thermal elongation of the shaft. However, an excessive clearance fit will cause the non-rotating race to rotate with the rolling elements, causing serious wear on the shaft (or bearing seat hole) and the inner race (or outer race), and friction will cause the bearing to heat and vibrate.
(2) Improper assembly method
When the interference between the bearing and the shaft diameter or the bearing seat hole is small, the press-fit assembly is often used. The easiest way is to use a copper rod and a hand hammer to strike the bearing races with interference fit symmetrically in a certain order to make the bearing press in smoothly. In addition, a soft metal casing can be used to drive in with a hammer or press in. If the operation is improper, it will deform and crack the seat ring, or hit the seat ring with a non-interference fit with a hand hammer, which will cause indentation of the raceway and rolling elements or indirectly damage the bearing.
(3) Improper temperature control during assembly
When the rolling bearing is assembled, if its interference with the shaft diameter is large, the hot assembly method is generally used. That is to say, the bearing is put into an oil drum containing organic oil, and the outside of the oil drum is heated with hot water or flame. The process requires that the heating oil temperature is controlled at 80℃～90℃, generally not exceeding 100℃, and not exceeding 120℃ at most. After the bearing is heated, it is quickly taken out and sleeved on the journal. If the heating temperature is too high due to improper temperature control, the bearing will be tempered and the hardness will be reduced, and the bearing will be easily worn, peeled off, and the temperature rises too fast or even cracks during operation.
It should be noted that when using oil to “boil” the bearing, the bearing should be placed flat in the oil drum, and a piece of wood or steel support with a height of about 50mm should be placed between the bearing and the bottom of the cylinder. When heating, use infrared temperature measurement. An instrument or thermometer controls the oil temperature. Since the oil bath heating method is inconvenient for the measurement of bearing expansion, temperature control and installation, it is more appropriate to use electromagnetic induction heaters.
(4) Improper adjustment of bearing clearance during assembly
The clearance of rolling bearings is divided into radial clearance and axial clearance, and its function is to ensure the normal operation and lubrication of the rolling elements and compensate for thermal elongation.
For bearings with adjustable clearance, because there is a proportional relationship between the axial clearance and the radial clearance, the installation is as long as the axial clearance is adjusted to obtain the required radial clearance, and they are generally into For the one used (that is, installed on the two ends or one end of the shaft), therefore, only the axial clearance of one bearing needs to be adjusted. But for the reducer, pay attention to the tooth surface meshing when adjusting the bearing clearance. Generally, the axial gap is adjusted with a gasket (pressing lead wire method), and some can also be adjusted with a screw or a thrust ring. If the gap adjustment is too large or the end cover of the reducer is loose during operation, the bearing gap is too large, which will not only cause the bearing itself to vibrate, noise, and the cage is easy to damage, but also cause the gear surface to mesh incorrectly and cause the reducer to hit Tooth accident. For example, the gearing accident of the gear reducer of DY Company’s No. 2 kiln and SS Company’s No. 1 kiln coal mill reducer was mainly caused by excessive bearing axial clearance and incorrect tooth surface meshing.
For rolling bearings with non-adjustable clearance, the radial clearance has been determined according to the standard during manufacture and cannot be adjusted. After this type of bearing is installed on the shaft diameter or in the bearing housing hole, the actual radial clearance is called assembly Radial clearance, when assembling, the size of the assembling radial clearance should be just enough to create the necessary working radial clearance during operation to ensure the flexible rotation of the bearing. When this type of bearing is working, the inner race is relatively displaced due to the heat elongation of the shaft when the temperature rises, so that the radial clearance of the bearing is reduced, and even the rolling elements are stuck between the inner and outer races. If an axial gap is left between one bearing (the other bearing is fixed on the shaft and in the bearing seat) and the side cover in the double-support rolling bearing, the above phenomenon can be avoided.
A typical example is PY’s No. 2 grate cooler No. 9 fan. I replaced 8 disc bearings within a month, and finally suspected that the quality of the bearings was not good, so I changed them to imported bearings, but it still didn’t help.
The real reason is that the maintenance personnel lost the gasket added between the upper and lower bearing seats when replacing the bearing. After pressing the upper cover, the outer ring of the bearing is deformed and flattened, and the radial clearance becomes smaller or almost zero. Once the bearing runs When heat is generated and expansion is blocked, it will burn out due to the rapid high temperature. In the worst case, the bearing will be burned out after almost one or two hours of operation.
After inspecting the pressure lead wire of the fan bearing, it was found that the amount of over gain of each part of the bearing seat mating surface was very different. The most part was added with a 0.23mm gasket, and the least part was hardly added with a gasket. It was only compensated by applying sealant. . After the bearing clearance is proper, the operating temperature is about 40℃, everything is normal.
2.4 Poor alignment of the coupling can also cause bearing heating or failure
The input shaft of most operating equipment is connected to the power shaft through a coupling. Therefore, the coupling must be aligned during assembly so that the driving shaft and the driven shaft are on the same axis. 80% of the heating of the motor tile is caused by poor alignment. Such as PY company’s raw material mill reducer motor tile. The machining error of the coupling of the cement mill reducer of XC company is too large.
Large, resulting in poor alignment, high motor bearing temperature, and large equipment vibration. In addition, pay attention to the relative position marks of the coupling halves of the driving end and the driven end during the installation of the coupling to avoid too large deviation of the pin hole, which may cause the pin to be installed too tightly.
3. The rotor is unbalanced
Some rotors are corroded by the medium or worn by solid impurities during operation, or the shaft is bent, which will cause unbalanced centrifugal force, which will cause the bearing to heat and vibrate, and the raceway will be severely worn until it is destroyed. This is particularly important for the circulating fan in the mill system. Because the impeller wears severely, the rotor balance is poor after wear, and the fan vibrates greatly, which often causes the bearing to fail prematurely.
4. Check and replace not in time
If the bearing is found to be severely fatigued, peeling, oxidized, rusted, worn pits, cracks, hardness reduced to HRC<60, or if there is too much noise that cannot be adjusted, it should be replaced in time. If the inspection and replacement are not timely, it will cause heating, abnormal noise, vibration, etc. of the bearing, and even serious damage to the rotor, which will affect normal production. In addition, the vibration caused by improper disassembly of the bearing and the loosening of equipment anchor bolts can also cause indentations on the bearing raceways and rolling elements, and cracks in the inner and outer races of the bearing. During the operation of the bearing, the inspection should be carried out according to the specified period.
5. Poor bearing quality
Rolling bearing parts work for a long time under high alternating contact stress in the form of point contact or line contact. The accuracy, life and reliability of the main engine are largely determined by the bearings. Whether it is ordinary bearings or special special bearings, the main engine puts forward high requirements on its life, performance and reliability. Therefore, it is necessary to pay attention to the inspection during the purchase and acceptance of the bearing. First, use the products of the regular famous brand manufacturers. The key parts even use the original imported bearings, such as SKF bearings, NSK bearings, TIMKEN bearings and FAG bearings. Special parts should be specially designed, such as the bearings of roller presses and vertical grinding rollers.
Although the rolling mill bearings of PY and JY have been specially ordered, due to the unstable quality of domestic large-scale bearing manufacturing, pitting and spalling of the inner surface of the rolling ring still occurred, which caused the bearing to fever and the roller press frequently tripped and stopped accidents.
6. Improper bearing selection
When selecting the bearing, pay attention to the limit speed and load capacity of the bearing, and it cannot be used with excessive speed or overload.
You can check more details of bearing types as below
7. Bearing heating caused by comprehensive factors
AQ’s first-stage rear exhaust fan (model: Y6-2×40-14NO30F) has affected production due to frequent bearing heating problems. The final resolution of this matter took more than two years and the direct cost was more than 300,000 yuan.
At first, it was because of the fever of the free end bearing of the impeller that the inner ring of the bearing was overheated and the shaft was welded together. The production was maintained after the journal was trimmed with a grinding wheel on site, because there was a concern that the poor fit (large fan vibration) in this part would cause long-term hidden troubles in production. , The rotor main shaft and bearing seat were replaced during the overhaul. After replacement, the free end bearing heats up first, and then the coupling side bearing heats up, and the temperature rise sharply reaches more than 80°C in about half an hour after starting the machine. Repeated many times, causing the kiln system to fail to operate normally. The main reasons for the problems are as follows:
(1) The main reason for the initial free end bearing heating and burning is that the outer race is tightly pressed, and the bearing cannot produce axial displacement along with the expansion of the main shaft. Another reason is that the actual fit between the inner race and the shaft is insufficient (or has become a transition fit). When the system gas temperature rise is small, the bearing only slightly heats up, and it does not affect the operation. However, if the process system operates abnormally and the gas temperature is too high, it will cause insufficient expansion. Slippage occurs between the inner ring and the shaft, and the adhesion is burnt out.
(2) After replacing the rotor main shaft and the free end bearing seat, the free end bearing heats up because the sharp corners of the joint surface are not blunt during the processing of the bearing seat, and the “clamping” phenomenon occurs after the bearing is installed. For the new bearing seat, attention should be paid to the inspection during assembly, and if necessary, it should be handled in accordance with the requirements of “scraping pads”.
(3) The bearing on the coupling side heats up because the verticality of the bearing and the main shaft is not good, the outer race is skewed, and the ball running position is not correct. Because it is an old bearing, once the shaft temperature is abnormal, the bearing position will change greatly, and it will deviate from the original raceway position, which will cause heat. In addition, during the operation of the fan, the top wire seat of the motor base is crooked and the motor is displaced, resulting in poor coupling accuracy, which is also one of the important reasons.
(4) The oil return pipe of the bearing seat is thin, the temperature is lower in winter, and the oil return slows down. Because of fear of oil leakage in the axial direction of the bearing seat, the post personnel reduce the oil supply of the bearing. Objectively reducing the amount of oil supply and circulation of bearing lubricating oil is also an important reason for excessively rapid bearing temperature rise. In response to this situation, we increased the fuel supply pressure on site. (Reduce the opening of the return pipe valve at the safety valve of the oil station)
In conclusion
Central control operation and on-site inspection personnel should be sufficiently alert to abnormal conditions of equipment bearing temperature rise. It is the early signal of bearing and equipment failure. If the processing is delayed and the method is improper, the state will be further deteriorated and cause major accidents. Therefore, equipment operators and maintenance personnel should pay attention to observe the temperature curve and alarm information of the bearing, and combine vibration monitoring and other means, especially to find the real cause of the bearing fever, in order to completely eliminate the hidden danger of the equipment.
