Overheating of bearings, resembling an abnormal "fever," is a common and serious malfunction in rotating equipment. Improper handling often leads to wasted effort, reduced bearing lifespan, increased maintenance costs, and even bearing burnout.
Therefore, quickly identifying the cause of the malfunction and taking appropriate measures to resolve it is crucial for ensuring the continuous and safe operation of equipment.
01
Overheating of Bearings-The Culprit is Here
There are many reasons for overheating of bearings. Here are some common problems:
►Poor Lubrication
Lubrication has a significant impact on bearing lifespan, friction, wear, and vibration. Good lubrication is essential for the normal operation of bearings. Statistics show that approximately 40% of bearing failures are related to poor lubrication.
The main functions of lubrication for bearings include:
1) Preventing metal corrosion;
2) Preventing foreign object intrusion, acting as a seal;
3) Dissipating frictional heat, preventing excessive bearing temperature rise;
4) Reducing friction and wear, extending bearing life.
Common factors causing poor bearing lubrication include:
1) Insufficient lubricating oil (grease);
2) Blockage of lubrication lines by foreign objects;
3) Problems with the quality of lubricating oil (grease);
4) Failure to add lubricating oil (grease) on time;
5) Impurities in the lubricating oil (grease).
►Bearing Wear
Bearings are crucial components used in various large and small machines. Some machines (such as crushers) operate in dusty environments. When fine dust enters the high-speed rotating bearing housing, it deteriorates the lubricating oil or grease, leading to poor lubrication and subsequent bearing wear.
If a bearing continues to operate under worn conditions, increased friction and heat will cause the bearing temperature to rise.
►Improper Installation
Improper installation is another important cause of bearing overheating. Correct bearing installation directly affects its lifespan and the precision of the host machine; therefore, the center lines of the shaft and bearing bore must be aligned during installation.
If bearings are improperly installed or have low precision, they will deflect, generating torque during rotation, leading to overheating or wear. Additionally, bearings will vibrate, experience increased noise, and experience a gradual increase in temperature.
►Insufficient Cooling
Insufficient cooling typically manifests as: blocked pipes, unsuitable coolers, and poor cooling performance.
Scale buildup and blockage in the lubrication pipe cooler will reduce cooling efficiency, especially prevalent during summer production. Some manufacturers resort to increasing the size of coolers or connecting them in parallel to improve cooling.
Severe scale buildup in coolers, resulting in frequent high bearing temperature alarms, is a common problem in many production sites. A more effective solution is to perform acid cleaning and descaling of the coolers before each summer.
►Excessive Vibration
For example, poor coupling alignment, rotor dynamic and static imbalance, poor foundation rigidity, loose foundation feet, and rotational stall and surge.
Some rotors, during operation, may experience uneven centrifugal force due to corrosion from the medium, wear from solid impurities, or shaft bending. This can lead to bearing overheating, vibration, severe raceway wear, and even failure.
►Delayed Inspection and Replacement
If bearings exhibit severe fatigue spalling, oxidation corrosion, wear pits, cracks, or excessive, unadjustable noise, failure to replace them promptly will cause overheating, abnormal noise, and vibration, affecting normal production.
Additionally, improper bearing disassembly and vibration caused by loose equipment foundation bolts can also lead to indentations on the bearing raceways and rolling elements, and cracking of the inner and outer bearing races. Bearings should be inspected according to the prescribed schedule during operation.
►Poor Bearing Quality
Rolling bearings operate under high alternating contact stress for extended periods, with point or line contact components. The precision, lifespan, and reliability of the main unit largely depend on the bearings. Therefore, careful inspection is crucial during the bearing procurement and acceptance process, ensuring the use of qualified, high-quality products from reputable manufacturers.
►Improper Bearing Selection
When selecting bearings, pay attention to their limiting speed and load capacity. Do not use them at excessive speeds or under excessive loads, as this will only shorten their lifespan and is counterproductive.
02
Tips for Cooling Bearings
When the bearing temperature is high, address the following issues first:
►Improper Lubrication: Too Much or Too Little Lubricating Grease
The bearing housing should be lubricated regularly according to operating requirements. Sometimes, even after lubrication, the bearing temperature may still be high, mainly due to over-lubrication.
In this case, the temperature will continuously rise, reach a certain point (generally about 10℃~15℃ higher than the normal operating temperature), then remain constant and gradually decrease.
►Inappropriate or Contaminated Grease:
Unsuitable lubricating grease will not form a uniform lubricating film, failing to reduce internal friction and wear, resulting in insufficient lubrication and increased bearing temperature.
When different types of grease are mixed, a chemical reaction may occur, causing the grease to deteriorate, clump, and reduce lubrication effectiveness.
Contaminated grease can also cause bearing temperature to rise. Dust falling into the grease during the grease-adding process can cause grease contamination, leading to grease deterioration inside the bearing housing, damaging bearing lubrication, and causing temperature increases.
Therefore, appropriate grease should be selected. During maintenance, the bearing housing and bearings should be cleaned, and the grease lines should be checked and cleared. Different types of grease must not be mixed. If changing to a different type of grease, the original grease should be thoroughly cleaned first. Grease should be added regularly during operation and maintenance, and the grease should be properly stored with moisture-proof and dust-proof measures.
►Insufficient cooling:
Check for blockages in the pipelines and whether the inlet and return water temperatures exceed the standards.
If the cooler is unsuitable and the cooling effect is poor, failing to meet the usage requirements, it should be replaced promptly or a new cooler should be installed in parallel. For axial flow fans, the insulation and sealing of the central core cylinder should also be checked.
►If the above aspects are not problematic, check the coupling:
The alignment of the coupling must meet the process standards. When aligning axial flow fans, hydraulic couplings, etc., the thermal expansion of the equipment during operation should also be considered.
Due to thermal expansion, the impeller side of the induced draft fan rises, causing the bearing housing to rise. Similarly, during operation, the hydraulic coupling's temperature rises, causing the bearing housing to expand and the bearing to rise. Therefore, the motor should be positioned higher during alignment, and the allowance should be determined based on the equipment's characteristics and operating temperature parameters.
03
What should be considered during bearing inspection?
►Bearing Quality
First, check the lubricating grease for deterioration, clumping, and impurities. This is crucial for determining the cause of bearing damage.
Second, check for seizing and wear on the bearing; inspect the surface finish of the inner and outer rings, rolling elements, and cage for cracks, rust, peeling, pitting, overheating discoloration, and other defects. Measure the bearing clearance to ensure it is within specifications. Check the bushing for wear, pitting, and peeling. If any of these are found, replace the bearing.
►Bearing Fit
The fit between the bearing's inner diameter and the shaft, and between its outer diameter and the housing, is crucial during bearing installation. When the fit is too loose, relative sliding will occur between the mating surfaces, a phenomenon known as creep.
Creep, once it occurs, will wear down the mating surfaces, damaging the shaft or housing. Furthermore, wear particles can penetrate the bearing interior, causing heat, vibration, and eventual failure.
Excessive interference fit will cause the outer ring diameter to decrease or the inner ring diameter to increase, reducing the bearing's internal clearance.
To select a bearing suitable for its application, various factors must be considered, including the nature and magnitude of the bearing load, temperature conditions, and the rotational state of the inner and outer rings.
►Adjustment of Bearing Clearance
If the bearing clearance is too small, excessive shear friction loss of the grease within the clearance will also cause the bearing to overheat. Simultaneously, a smaller clearance will reduce the amount of grease, which cannot effectively dissipate the heat generated by friction, further increasing the bearing's temperature rise.
However, excessive clearance will alter the bearing's dynamic characteristics, causing rotor instability. Therefore, it is necessary to select an appropriate bearing clearance based on different equipment and operating conditions.
