In production, it is common to encounter the situation where rolling bearings "overheat". If the cause analysis is not clear, the handling is not timely, and the measures are inappropriate, it often leads to twice the effort for half the result, and may even cause serious accidents such as bearing burnout and gear breakage of the reducer. The following, based on several typical cases encountered by the author during the production and debugging process, analyzes and summarizes the specific problems existing in the design, installation, lubrication, assembly and other links respectively, for your reference.

In 2003 (the early stage of production), the excess fan at the kiln head of No. 1 kiln of ZB Company experienced multiple bearing burnout accidents. Later, after on-site investigation, it was found that there were problems with the oil gauge design of the bearing housing. When the oil level is at the lower mark of the oil gauge, in actual measurement, the lowest row of balls of the bearing just touches the oil surface. Only when the oil level is displayed at the upper mark can it meet the operational requirements. When the oil level approaches the lower marking line, the lubrication condition deteriorates. At first, the bearing temperature rises slowly. The on-site personnel did not pay enough attention to the bearing overheating alarm. Once the temperature rise reaches a certain level, it will rise sharply in a very short time and burn out the bearing. Therefore, incorrect oil gauge marking can mislead inspection and maintenance personnel. After identifying the cause, the refueling standard line was redefined.

Another example is the high-speed shaft bearing of the rotary kiln reducer of CL Company. Just 2 to 3 hours after startup, the bearing suddenly started smoking. The post-event analysis showed that the cause of this accident was that the return oil hole in the bearing housing of the high-speed shaft was too low, and most of the oil coming from the oil pipe directly flowed into the reducer through the return oil hole. After adjusting the Angle of the return oil hole to maintain a certain oil level in the bearing housing, the operation was normal.

Due to lax lubrication management, dirty containers or non-standard acid washing of pipeline welding during the oiling and oil change process, welding slag, rust and other impurities enter the oil pipes and then block the oil holes. This situation was quite common in the early stage of the project.

For instance, the raw material mill reducer of Company ZB had its bearings burned due to the oil passage being blocked, and the cement mill reducer of Company TH also had its bearings exposed to high temperatures for a long time because the oil passage was blocked by cotton yarn. It is particularly worth mentioning that when checking the oil level of the high-speed shaft bearing of the TH company's reducer, it was always impossible to see exactly how much oil could be added to the high-speed bearing. It was not until the oil pipe was completely removed and blown clean with high-pressure air that it was found that two thin cotton ropes were blocked inside! In addition, during the process of replacing the bearings of the reducer, TH Company also found that the oil inlet hole of the eccentric sleeve of the bearing was too thin (the oil hole of the eccentric sleeve can be one specification larger in diameter than the oil hole of the bearing itself), so the oil hole was expanded.

Due to the fact that the bearings of various parts of the hard-toothed surface reducer are generally lubricated by a thin oil station, and the oil pipes at each lubrication point are in a parallel relationship, it is difficult to detect the blockage of individual oil pipes when observing the total oil supply and return volume. Therefore, experienced maintenance personnel will carefully check the temperature of each oil pipe and determine whether the oil passage is unobstructed based on the temperature difference of the oil pipes. Generally, the temperature of the oil pipe that does not pass oil is lower than that of other pipelines.

The lubricating oil (or grease) used in rolling bearings all have a certain working temperature. When the temperature is too high, water or dust enters the bearing housing, severe oxidation, emulsification and other deterioration will occur, thereby losing its lubricating effect and causing the bearing to burn out due to high temperature. In addition, poor quality of the lubricating oil (or grease) itself or failure to add oil (grease) in time during operation are also common phenomena. For instance, rainwater entering the idler bearings of belt conveyors, coal powder or raw material powder entering the bearings of rotary feeders and full pump, can all cause the bearing temperature to rise or produce abnormal sounds.

Especially in summer production, this problem is particularly common. Some manufacturers do not hesitate to increase the size or connect the coolers in series to enhance the cooling effect. Due to severe scaling in the high-temperature fan cooler, frequent alarms due to excessively high bearing temperatures have been encountered in all branch companies. A relatively effective treatment method is to acid wash and remove scale from the cooler before the beginning of summer every year.

This situation is more likely to occur on long-shaft equipment such as large fans and crushers, and it is also a link that is often overlooked by equipment design, manufacturing, installation and maintenance personnel. A typical case is the rear exhaust fan of the rotary kiln of CX Company. During the initial stage of production, there was a severe heating phenomenon of the free end bearing of the fan. This was because under the working conditions, the rotor main shaft expanded thermally and caused intense friction with the bearing end cover. Within a very short period of time, the high temperature generated welded the bearing housing end cover and the end face of the rotor main shaft together. Therefore, when installing and accepting the fresh air machine, it is necessary to pay attention to calculating whether the axial clearance of the free end bearing can meet the expansion requirements of the working conditions.

The method for calculating the expansion amount of shafts is very simple:

△L=L× (t-t0) ×0.000012

In the formula: △L - the expansion amount of the axis; mm;

L - Shaft length between bearing housings; mm;

t - Operating temperature ℃;

t0 - Ambient temperature during equipment installation; ℃.

It should be noted that when installing in summer, the contraction of the equipment in winter should also be taken into account. This is especially important in northern regions. Generally speaking, in the cold northern regions, the maximum temperature difference between winter and summer can reach 80℃. If a 3-meter-long shaft is installed in summer, the maximum contraction in winter (when the machine is shut down) can approach 3mm.

If the bearing is installed incorrectly, it will cause the balls to not roll in the correct position on the bearing raceway, and even lead to a large axial force between the end face of the balls and the inner and outer retaining rings of the bearing housing, resulting in overheating of the bearing. It is rather laborious to check for misaligned bearings. The overheating of the limestone crusher bearing of LY Company is a case in point. The solution is to attach a dial indicator to the shaft and place the pointer on the outer end face of the bearing. Then, check the end face runout for one full circle. Generally, the end face runout value should be controlled within 0.05. The clearance on the left and right sides of the bearing can also be directly checked with a feeler gauge.

1. Improper cooperation

The fit between the inner hole of the bearing and the shaft adopts the base hole system, while the fit between the outer circle of the bearing and the bearing housing hole adopts the base shaft system. Under normal load conditions, the shaft and the inner ring of the bearing are generally fitted with j5, js5, js6, k5, k6, and m6, while the bearing housing hole and the outer ring of the bearing are fitted with j6 and j7. The rotating seat ring usually adopts an interference fit, which can prevent the seat ring from rolling and sliding on the mating surface between the shaft diameter and the bearing housing hole under the action of load.

However, sometimes due to inaccurate measurement of the shaft diameter and the bearing housing hole dimensions or the roughness of the mating surface not meeting the standard requirements, excessive interference fit occurs, causing the bearing housing ring to be subjected to significant compression. This leads to a reduction in the radial clearance of the bearing itself, making it difficult for the bearing to rotate, causing overheating, accelerated wear or jamming. In severe cases, it may result in cracking of the inner and outer rings of the bearing during installation. The non-rotating seat ring often adopts a fit with a small clearance or interference. As a result, the non-rotating seat ring may experience slight crawling, causing the contact surface between the seat ring and the rolling elements to constantly change and resulting in uniform wear of the seat ring raceway. At the same time, it can also eliminate the phenomenon that the rolling elements in the bearing get axially stuck due to the thermal elongation of the shaft. However, excessive clearance fit will cause the non-rotating seat ring to rotate along with the rolling elements, resulting in severe wear between the shaft (or bearing housing hole) and the inner seat ring (or outer seat ring), leading to friction and causing the bearing to heat up and vibrate.

The cement mill reducer of TH Company was burned out again after only running for more than two hours after the new bearings were replaced. At first, it was because the gap between the eccentric sleeve used to fix the bearing and the outer seat ring of the bearing was too large, causing the reducer to vibrate. During the maintenance, many pits were hit on the inner surface of the eccentric sleeve, and at the same time, a tooth-fixing adhesive was applied, which prevented the outer seat ring of the bearing from generating axial displacement. Thus, it could not meet the expansion requirements of the shaft and was burned out.

2. Improper assembly method

When the interference between the bearing and the shaft diameter or the bearing housing hole is relatively small, the press-in assembly method is mostly adopted. The simplest method is to use a copper rod and a hand hammer to strike the interference fit rings of the bearing symmetrically in a certain order, so that the bearing can be smoothly pressed in. In addition, soft metal sleeves can also be driven in with a hand hammer or pressed in with a press. If the operation is improper, it will cause the seat ring to deform and crack. Or if the hand hammer hits the seat ring with a non-interference fit, it will cause indentations on the raceway and rolling elements or indirectly damage the bearing.

3. Improper temperature control during assembly

When assembling rolling bearings, if the interference between them and the shaft diameter is relatively large, the hot mounting method is generally adopted for assembly. The bearings are placed in an oil drum filled with engine oil. The outside of the oil drum is heated with hot water or a flame. The process requires that the oil temperature for heating be controlled at 80℃ to 90℃, generally not exceeding 100℃, and at most not exceeding 120℃. The bearing was quickly taken out after heating and fitted onto the journal. If the temperature is not properly controlled, causing the heating temperature to be too high, it will lead to tempering of the bearing, resulting in a decrease in hardness. During operation, the bearing is prone to wear, spalling, rapid temperature rise and even cracking.It should be noted that when "boiling" bearings with oil, the bearings should be placed flat in the oil drum, and a wooden block or steel bar support about 50mm high should be placed between the bearing and the bottom of the drum. During heating, an infrared thermometer or thermometer should be used to control the oil temperature. Since the oil bath heating method is inconvenient for measuring the expansion of bearings, temperature control and installation, it is more appropriate to use an electromagnetic induction heater.

4. Improper adjustment of the bearing clearance during assembly

The clearance of rolling bearings is divided into radial clearance and axial clearance. Their functions are to ensure the normal operation and lubrication of the rolling elements and to compensate for thermal expansion.

For bearings with adjustable clearance, since there is a direct proportional relationship between the axial clearance and the radial clearance, the installation only requires adjusting the axial clearance to obtain the required radial clearance. Moreover, they are generally used in pairs (i.e., installed at both ends or one end of the shaft), so only the axial clearance of one bearing needs to be adjusted. However, for reducers, when adjusting the bearing clearance, attention should be paid to the meshing condition of the tooth surfaces. Generally, the axial clearance is adjusted by gaskets (pressed lead wire method), and in some cases, screws or thrust rings can also be used for adjustment. If the clearance is adjusted too much or the end cover of the reducer becomes loose during operation, causing excessive bearing clearance, it will not only lead to problems such as large vibration and noise of the bearing itself and easy damage to the cage, but also further cause incorrect meshing of the tooth surface, resulting in a tooth breakage accident of the reducer. For instance, the gear breakage accidents of the reducer in the No. 2 kiln of DY Company and the coal mill reducer in the No. 1 kiln of SS Company were mainly caused by excessive axial clearance of the bearings and improper meshing of the tooth surfaces.

For rolling bearings with non-adjustable clearances, since their radial clearances have been determined according to standards during manufacturing and cannot be adjusted, the actual radial clearance after such bearings are installed on the shaft diameter or in the bearing housing hole is called the assembly radial clearance. During assembly, the size of the assembly radial clearance should be exactly enough to create the necessary working radial clearance during operation to ensure the flexible rotation of the bearing. When this type of bearing is in operation, due to the thermal elongation of the shaft as the temperature rises, the inner ring of the bearing undergoes relative displacement, thereby reducing the radial clearance of the bearing and even causing the rolling elements to get stuck between the inner and outer rings. If an axial clearance is left between one bearing of the double-supported rolling bearing (the other bearing is fixed on the shaft and in the bearing housing) and the side cover, the above phenomenon can be avoided.

A typical example is the No. 9 fan of the No. 2 grate cooler of PY Company. Within a month, I replaced eight sets of bearings. Eventually, I suspected that the quality of the bearings was poor and switched to imported ones, but it still didn't help.

The real cause is that when the maintenance personnel replaced the bearing, they lost the gasket placed between the upper and lower bearing housings. As a result, after pressing the upper cover tightly, the outer ring of the bearing was deformed and flattened, and the radial clearance became smaller or almost zero. Once the bearing generates heat during operation and its expansion is blocked, it will be burned out due to the sudden high temperature. In the most severe cases, the bearing will be burned out almost after running for one or two hours.

After inspecting the lead wire of the bearing of the fan, it was found that the interference fit at different parts of the bearing housing mating surface varied greatly. The part with the largest interference fit added a 0.23mm gasket, while the part with the smallest interference fit hardly added any gasket and only compensated by applying sealant. After the bearing clearance is appropriate, the operating temperature is around 40℃, and everything is normal.

Most of the input shafts of moving equipment are connected to the power shaft through couplings. Therefore, during assembly, the couplings must be aligned to ensure that the driving shaft and the driven shaft are on the same axis.

Eighty percent of the heating of the motor bearing is caused by poor centering. For example, the motor bearing of the raw material mill reducer of PY Company. The processing error of the coupling of the cement mill reducer of XC Company itself is too large, resulting in poor alignment, high temperature of the motor bearing and large equipment vibration. In addition, during the installation of the coupling, it is essential to pay attention to the relative position marking of the two halves of the coupling at the driving end and the driven end to avoid excessive deviation in the pin holes, which may cause the pins to be installed too tightly.

Some rotors, during operation, due to corrosion by the medium or wear by solid impurities, or when the shaft bends, will generate unbalanced centrifugal force, causing the bearings to heat up, vibrate, and the raceways to wear severely until they are damaged. This is particularly important for the circulating fan within the grinding mill system. Due to severe wear of the impeller, the rotor balance is poor after wear, and the fan vibrates significantly, which often leads to premature failure of the bearings.

If the bearing is found to have severe fatigue spalling, oxidation and rust, worn pits, cracks, hardness reduced to HRC<60, or excessive noise that cannot be adjusted, it should be replaced in time. If inspection and replacement are not carried out in a timely manner, it will cause the bearings to overheat, make abnormal sounds, vibrate and even cause serious damage to the rotor, thereby affecting normal production. In addition, improper disassembly of bearings and vibration caused by loose anchor bolts of equipment can also lead to indentations on the raceways and rolling elements of the bearings, as well as cracking of the inner and outer rings of the bearings. During the operation of the bearing, it should be inspected at the prescribed intervals.

Rolling bearing parts work for a long time in the form of point contact or line contact under high alternating contact stress. The accuracy, lifespan and reliability of the mainframe are largely determined by the bearings. Whether it is ordinary bearings or special-purpose bearings, the mainframe has very high requirements for their service life, performance and reliability. Therefore, during the procurement and acceptance of bearings, it is essential to pay close attention to inspection. Firstly, products from well-known and reputable manufacturers should be adopted. For key parts, even original imported bearings such as SKF bearings, NSK bearings, TIMKEN bearings and FAG bearings should be used. Special parts should be specially designed, such as the bearings of roller presses and vertical grinding rollers.

Although the rolling mill bearings of PY Company and JY Company were specially ordered, due to the unstable manufacturing quality of large domestic bearings, the inner surface pitting and spalling of the roller rings still occurred, causing the bearings to overheat and the rolling press to frequently trip and stop.

When selecting bearings, attention should be paid to their maximum rotational speed and load capacity. They must not be used beyond their rotational speed or load capacity.

Bearing heating caused by a combination of factors

The first phase of the rear exhaust fan of AQ Company (model: Y6-2x40-14NO30F) was once affected in production due to frequent overheating of the bearings. The final resolution of this matter took more than two years and the direct cost was over 300,000 yuan.

At first, due to the overheating of the free end bearing of the impeller, the inner ring of the bearing overheated and welded to the shaft. After dressing the journal with a grinding wheel on site, production was maintained. As there was concern that poor fit at this part (excessive fan vibration) would cause long-term production risks, the rotor main shaft and bearing housing were replaced during the major overhaul. After replacement, the free end bearing heated up first, followed by the side bearing of the coupling. About half an hour after startup, the temperature rose sharply to over 80 ℃. Repeated many times, causing the kiln system to fail to operate normally.

The main reasons for the problems that occurred are analyzed as follows:

（1）The main reason for the initial heating and burnout of the free-end bearing is that the outer ring is pressed too tightly, preventing the bearing from undergoing axial displacement along with the expansion of the main shaft. Another reason is that the actual fit interference between the inner seat ring and the shaft is insufficient (or has become a transitional fit). When the temperature rise of the system gas varies slightly, the bearing only gets slightly hot and does not affect its operation. However, if the operation of the process system is abnormal and the gas temperature is too high, it will result in insufficient expansion. Sliding occurs between the inner ring and the shaft, and the adhesion is burned out.

(2) After replacing the rotor main shaft and the free end bearing housing, the free end bearing heated up because the sharp corner of the mating surface of the bearing housing was not blunted during processing, and the "clamping" phenomenon occurred after the bearing was installed. For new bearing housings, attention should be paid to the inspection during assembly. If necessary, they should be handled in accordance with the requirements of "scraping the bearing shells".

(3) The overheating of the side bearing of the coupling is due to the poor perpendicularity between the bearing and the main shaft, the deviation of the outer seat ring, and the incorrect running position of the balls. Because it is an old bearing, once the shaft temperature is abnormal and the bearing position changes significantly, deviating from the original raceway position, it will cause overheating. In addition, during the operation of the fan, if the top screw seat of the motor base is tilted and the motor moves, resulting in poor alignment accuracy of the coupling, it is also one of the important reasons.

(4) The return oil pipe of the bearing housing is relatively thin. In winter, when the temperature is low, the oil return slows down. To prevent axial oil leakage from the bearing housing, the on-duty personnel have reduced the oil supply to the bearing. Objectively, it reduces the supply and circulation volume of lubricating oil for bearings, which is also an important reason for the rapid temperature rise of bearings. In response to this situation, we increased the fuel supply pressure on site. (Reduced the opening degree of the return oil pipe valve at the safety valve of the gas station)

Conclusion

The central control operation and on-site inspection personnel should be highly vigilant about the abnormal temperature rise of the equipment bearings. It is an early signal of faults in the bearings and equipment. If the handling is delayed or the methods are improper, the condition will deteriorate further, causing major accidents. Therefore, equipment operators and maintenance personnel should pay close attention to observing the temperature curve and alarm information of the bearings, and combine it with means such as vibration monitoring. In particular, they should identify the true cause of the bearing overheating to completely eliminate equipment hazards.