What are the causes of bearings “overheating”?

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What are the causes of bearings “overheating”? In production, it is common for rolling bearings to overheat. If the causes are not properly analyzed, if corrective actions are not taken in a timely manner or if the measures taken are inadequate, it can result in less effective outcomes; in severe cases, it may even lead to damaged bearings or gear damage in the reducer. Below are several typical cases encountered by Jinfen during the production commissioning process. Analyses and summaries of the specific problems arising in various stages such as design, installation, lubrication, and assembly are provided for your reference. 1. Bearing overheating caused by improper lubrication
1.1 Inherent flaws in the design of bearing lubrication points
At ZB Company’s Kiln No. 1, the excess air fan at the kiln head experienced multiple instances of bearing failure in 2003 (during the initial stage of operation). Subsequent on-site investigations revealed that there were problems with the design of the oil level indicator on the bearing housing. When the oil level is below the lower mark on the oil gauge, measurements show that the lowest row of balls in the bearing just barely touches the oil surface. Only when the oil level reaches the upper mark can the operating requirements be met. When the oil level approaches the lower mark, the lubrication conditions deteriorate. Initially, the bearing temperature rises slowly, and the personnel on site do not pay sufficient attention to the alarms indicating elevated bearing temperatures. Once the temperature rises to a certain level, it increases rapidly in a very short time, resulting in the bearing being damaged. Therefore, incorrect oil level markings can mislead inspection and maintenance personnel. After determining the cause, the refueling standard line was redrawn. Another example is the high-speed shaft bearing of the rotary kiln reducer at CL Company; just 2–3 hours after startup, the bearing suddenly started smoking. A post-incident analysis of the cause of this accident revealed that the return oil hole in the high-speed shaft bearing housing was too low, causing most of the oil coming from the oil pipe to flow directly into the gearbox through that return hole. After adjusting the angle of the oil return hole to maintain a certain oil level in the bearing housing, it operated normally. 1.2 Lubricant pipes become clogged by foreign objects. Due to inadequate lubrication management, dirty containers during refilling or oil changes, or improper acid washing of pipes during welding, debris such as weld slag and rust can enter the oil pipes and thus block the oil holes – a situation that was common in earlier projects. For example, the raw material mill reducers of ZB Company suffered bearing damage due to blocked oil passages, while the cement mill reducers of TH Company also experienced prolonged high temperatures in their bearings as a result of cotton yarns blocking the oil passages. Notably, when TH Company was checking the oil level in the bearing of the high-speed shaft of its reducer, it was impossible to tell exactly how much oil could be added to the high-speed bearing. It wasn’t until the oil pipe was completely removed and the interior was blown out with high-pressure air that they discovered two thin cotton ropes blocking it! Additionally, during the process of replacing the reducer bearings, TH Company also noticed that the oil hole in the bearing eccentric sleeve was too small (the diameter of the oil hole in the eccentric sleeve can be one size larger than that of the bearing itself). Therefore, the oil hole was enlarged. Since the bearings in various parts of hardened-tooth-surface reducers are generally lubricated by a forced lubrication system using an oil mist lubricator, the oil pipes leading to each lubrication point are connected in parallel. As a result, it is difficult to detect any blockages in individual pipes based on the total oil supply and return volumes. Therefore, experienced maintenance personnel carefully monitor the temperature of each oil pipe; they determine whether the oil flow is unobstructed by observing any temperature differences among the pipes. Generally, the temperature of oil-free oil pipes is lower than that of other pipelines. The inspection staff at PY Company once detected that the temperature of the oil pipe in the high-speed shaft of the reducer inside the kiln was low, and ultimately identified the serious risk of blockage in the oil pipe, thereby preventing a major accident. 1.3 Degradation of lubricating oil (grease) or failure to replenish it in a timely manner: The lubricating oil (or grease) used for rolling bearings has a certain operating temperature range. When the temperature becomes too high, or when water or dust enters the bearing housing, serious degradation such as oxidation and emulsification occurs, causing the lubricant to lose its effectiveness. This, in turn, leads to overheating and damage of the bearings. Additionally, poor quality of the lubricating oil (or grease) itself, or failure to replenish it in a timely manner during operation, are also common occurrences. For instance, rainwater getting into the bearings of conveyor idlers, or coal powder or raw meal getting into the bearings of rotary feeders and Fuller pumps can both lead to an increase in bearing temperature or the generation of abnormal noises. 1.4 Scaling and blockage in the coolers of the lubrication pipelines result in a reduced cooling effect. This problem is particularly common during summer production, and some manufacturers go so far as to use more coolers or connect them in series to enhance the cooling performance. Due to severe scaling in the coolers of high-temperature fans, the problem of excessive bearing temperatures triggering frequent alarms has been encountered at all branch companies. A relatively effective solution is to carry out acid cleaning to remove scale from the cooler before the onset of summer each year. 2. Bearing overheating caused by improper installation
2.1 Bearing overheating due to insufficient designed expansion allowance
This situation tends to occur more frequently in equipment with long shafts, such as large fans and crushers. It is also a aspect that designers, manufacturers, installers, and maintenance personnel often overlook. A typical example is the rear exhaust fan of the rotary kiln at CX Company. In the initial stage after commissioning, the bearing at the free end of the fan became extremely hot. Under operating conditions, the thermal expansion of the rotor’s main shaft caused intense friction with the bearing cover; the resulting high temperature melted and welded the bearing housing cover to the end face of the rotor’s main shaft within a very short period of time. Therefore, during the installation and acceptance of fresh air units, it is necessary to check whether the axial clearance of the bearings at the free end meets the expansion requirements under the operating conditions. The method for calculating the expansion of shafts is quite simple: △L = L × (t – t0) × 0.000012. In this formula, △L represents the amount of expansion of the shaft ； mm ； 　　　　　　L—Shaft length between bearing housings ； mm ； 　　　　　　t—Operating temperature ； ℃ ； 　　　　　　t0—Environmental temperature at the time of equipment installation ； ℃. It should be noted that when installing in summer, one must also consider the amount of contraction the equipment undergoes in winter; this is particularly important in northern regions. Generally, in the cold regions of the north, the temperature difference between winter and summer can reach up to 80°C. If a shaft with a length of 3 meters is installed in summer, its maximum contraction amount (when the machine is shut down) can approach 3 mm. 2.2 Bearing overheating caused by misaligned installation – When bearings are installed incorrectly, the balls do not roll in the correct positions within the bearing races, which can result in significant axial forces between the ends of the balls and the inner and outer raceways of the bearing housing, leading to overheating of the bearing. Identifying issues related to misaligned bearings can be quite time-consuming. The overheating of the bearings in LY Company’s limestone crushers is an example of such a problem. The solution involves attaching a dial indicator to the shaft, with its pointer pointing at the outer surface of the bearing, in order to check the runout of that surface; generally, the runout value should be kept within 0.05. A feeler gauge can also be used to directly check the clearance on the left and right sides of the bearing. 2.3 Overheating caused by improper assembly of the bearing itself (1) Improper fit: The fit between the bearing bore and the shaft follows the hole-based system, while the fit between the bearing outer diameter and the bearing housing bore follows the shaft-based system. Under normal load conditions, for the shaft and the inner ring of the bearing, fits such as j5, js5, js6, k5, k6, and m6 are typically used; for the bearing housing bore and the outer ring of the bearing, fits of j6 and j7 are employed. The rotating ring, which typically uses an interference fit, prevents rolling and sliding of the ring on the mating surfaces of the shaft diameter and the bearing housing bore under load. However, sometimes due to inaccurate measurements of the shaft diameter and the bearing housing bore, or because the roughness of the mating surfaces fails to meet standard requirements, an excessive interference fit occurs. This causes significant compression on the bearing rings, thereby reducing the radial clearance within the bearing. As a result, the bearing becomes difficult to rotate, heats up, experiences increased wear, or even seizes up. In severe cases, the inner and outer bearing rings may crack during installation. Non-rotating rings typically use a fit with little clearance or interference; this allows the non-rotating ring to experience slight creeping, thereby continuously changing the contact surface between the ring and the rolling elements, resulting in even wear of the ring’s raceway. It can also prevent the rolling elements in the bearings from getting axially stuck due to thermal expansion of the shaft. However, an excessively large clearance fit causes the non-rotating ring to rotate together with the rolling elements, resulting in severe wear between the shaft (or bearing housing bore) and the inner ring (or outer ring). This friction leads to overheating and vibration of the bearing. After new bearings were installed, the cement mill reducer at TH Company malfunctioned after operating for just over two hours. Initially, excessive clearance between the eccentric sleeve used to secure the bearings and the outer race of the bearings caused vibrations in the reducer. During repairs, numerous dimples were created on the inner surface of the eccentric sleeve, and gear adhesive was also applied. This prevented any axial movement of the bearing outer race, thereby making it impossible for the shaft to expand as required; consequently, the bearings overheated and failed. (2) Improper assembly method: When the interference between the bearing and the shaft diameter, or between the bearing and the bearing seat hole is small, the press-fitting method is commonly used for assembly. The simplest method is to use a copper rod and a hammer to symmetrically strike, in a certain order, the raceways of the bearing with an interference fit, thereby allowing the bearing to be pressed in smoothly. Additionally, a sleeve made of soft metal can also be driven in using a hammer or pressed in using a press. Improper operation may cause the raceway to deform or crack; moreover, striking a raceway that is not in an interference fit with a hammer can result in indentations on the raceway and rolling elements, or indirect damage to the bearing. (3) Improper temperature control during assembly: When assembling rolling bearings, if the interference between them and the shaft diameter is large, the hot mounting method is generally used. The bearings are then placed into a oil drum filled with lubricating oil. The exterior of the drum is heated using hot water or a flame. According to the process requirements, the temperature of the heated oil should be maintained between 80°C and 90°C; generally it does not exceed 100°C, and at most it can reach 120°C. After heating, the bearing is quickly removed and fitted onto the shaft journal. If the temperature is not controlled properly and the heating temperature becomes too high, it will cause the bearings to undergo tempering, resulting in a decrease in their hardness. As a result, the bearings are prone to wear and stripping during operation, experience rapid temperature rises, and may even crack. It should be noted that when using oil to \"cook\" the bearings, they must be placed flat inside the oil tank; a wooden block or steel support with a height of about 50 mm should be placed between the bearings and the bottom of the tank. During heating, an infrared thermometer or regular thermometer should be used to control the temperature of the oil. Since the oil bath heating method is inconvenient for measuring bearing expansion, temperature control, and installation, it is recommended to use an electromagnetic induction heater instead. (4) Improper adjustment of the bearing clearance during assembly. The clearance of rolling bearings is divided into radial clearance and axial clearance; its purpose is to ensure the proper operation of the rolling elements, facilitate lubrication, and compensate for thermal expansion. For bearings with adjustable clearances, since there is a direct proportional relationship between their axial clearance and radial clearance, adjusting the axial clearance is sufficient to achieve the desired radial clearance during installation. Moreover, they are generally used in pairs (that is, installed at both ends or one end of the shaft), so it is only necessary to adjust the axial clearance of one bearing. However, for reducers, when adjusting the bearing clearance, attention should be paid to the tooth surface meshing condition. Axial clearance is generally adjusted using shims (the lead compression method), but screws or thrust rings can also be used for this purpose. If the clearance is adjusted too much, or if the end cover of the reducer becomes loose during operation, resulting in an excessive bearing clearance, this will not only cause problems such as excessive vibration of the bearings, high noise levels, and easy damage to the retainers, but it can also lead to improper tooth engagement and thus accidents involving tooth damage in the reducer. The tooth damage incidents in the reducer of DY Company’s No. 2 kiln, and in the coal grinding reducer of SS Company’s No. 1 kiln, were mainly caused by excessive axial clearance of the bearings and improper tooth meshing. For rolling bearings with non-adjustable clearances, since their radial clearance is determined according to standards at the time of manufacturing and cannot be adjusted, the actual radial clearance after such bearings are installed on the shaft or in the bearing housing is referred to as the assembly radial clearance. During installation, this clearance must be set such that it results in the necessary operating radial clearance, thereby ensuring smooth rotation of the bearing. When such bearings are in operation, the shaft expands due to heat when its temperature rises, causing relative displacement of the inner ring; this reduces the radial clearance of the bearing and may even result in the rolling elements getting stuck between the inner and outer rings. The above phenomenon can be avoided by leaving an axial clearance between one of the bearings in a double-support rolling bearing (the other bearing being fixed to the shaft and the bearing housing) and the side cover. A typical example is Fan No. 9 of Cooler No. 2 at PY Company. Eight bearing sets were replaced within a month; eventually, it was suspected that the quality of the bearings was poor, so imported bearings were used, but that didn’t help either. The real reason is that the maintenance personnel lost the shims placed between the upper and lower bearing housings while replacing the bearings; as a result, when the upper cover was tightened, the outer ring of the bearing deformed and was compressed, causing the radial clearance to decrease or even become zero. Once the bearing starts to generate heat during operation, the restricted expansion leads to overheating, which can damage the bearing. In the worst cases, the bearing gets damaged after just one or two hours of operation. After inspecting the lead wires used to compress the bearings of this fan, it was found that there were significant variations in the amount of shimming across different areas of the bearing housing surfaces; the highest amount of shimming used was 0.23 mm in some areas, while in other areas almost no shimming was used, and sealing compound was relied on instead to compensate for the gap. With the proper bearing clearance, the operating temperature is around 40°C, and everything is normal. 2.4 Poor alignment of couplings can also cause bearings to overheat or fail. In most operating equipment, the input shaft is connected to the drive shaft via a coupling; therefore, during assembly, it is necessary to align the coupling so that the driving shaft and the driven shaft are on the same axis. 80% of the heating in motor windings is caused by poor alignment. Such as the motor bearings for the raw material mill reducer in PY Company. The coupling of the cement mill reducer in XC Company has excessive manufacturing errors, which leads to poor alignment, high temperatures in the motor bearings, and significant equipment vibration. Additionally, during the coupling installation process, it is essential to pay attention to the relative position markings of the two halves of the coupling at the driving and driven ends, in order to avoid excessive deviation in the pin holes, which could lead to the pins being installed too tightly. 3. Rotor imbalance: In some rotors, during operation, corrosion caused by the medium or wear from solid impurities, or shaft bending can lead to the generation of unbalanced centrifugal forces. This, in turn, causes the bearings to heat up and vibrate, results in severe wear on the raceways, and may ultimately lead to bearing failure. This is particularly important for the circulation fans in the mill system. Due to severe wear of the impeller, the rotor loses its balance after wear, resulting in significant vibration of the fan, which often leads to premature failure of the bearings. 4. Failure to inspect and replace in a timely manner: If bearings show severe fatigue spalling, oxidation and corrosion, worn-out pits, cracks, or a hardness reduction to HRC