Five steps to reduce motor bearing failures

Experts from Dalian Motor Group have given the following detailed answers on the five steps to reduce motor bearing failures. We hope it will be helpful to you.

1. Control the quality of supply and do a good job in bearing selection
For important load motor bearings, famous brand bearings are often used. When purchasing bearings, the purchasing department starts from the source of supply, conducts comparisons in terms of quality and price, and selects suppliers with good reputations, qualifications, and scale. Once selected, they will not easily change agents and start from the source. Prevent the entry of inferior and counterfeit bearings.
Selection is very important, and many selection problems have occurred. Some motors have just been installed and put into operation, and the drive side bearings overheat. The inspection found that it was not a problem with the grease, installation, load, and bearing quality, but that the bearing radial clearance was selected too small. For example, a Y200L2-2 type, 37kW motor, bearing SKF6312 bearing. The original bearing radial clearance was C2, which was smaller than the ordinary type. The load side bearing temperature of the motor was as high as 96°C. The bearing clearance was changed to C3. After the motor was put into operation, the load side bearing temperature dropped by 58°C. The rated speed of the motor is 2890r/min. The temperature of the inner ring of the bearing rises rapidly, the inner ring expands, and the friction of the ball increases, causing the bearing to overheat. The bearing clearance codes include CN, C2, C3, C4, and C5. The appropriate clearance must be selected based on the mechanical load characteristics. The characteristics of the driven machine cannot be ignored.
For a certain motor, the bearing type selection has been taken into consideration based on the load characteristics. However, in actual applications, factors such as cages and bearing roller types should also be considered, and attention should be paid to the selection of bearing cages. For light loads, low noise requirements, and low operating temperatures (<120°C), enhanced nylon 66 cages can be selected. However, nylon cages have weaknesses such as aging, brittleness, and temperature resistance. Nylon cages are basically no longer used in on-site motors. For heavy impact and high-temperature loads, steel cages can be selected. For loads with large temperature differences and large vibrations, copper cages can be used. The rolling elements of ordinary motor bearings are selected from roller type or ball type according to the load conditions. For example, in 315M4, 132kW, and 232A motors, the bearing temperature reaches 76°C during operation, and the motor often suffers from oil shortage and abnormal noise. After checking the load-side bearing of the motor, it was found that it was designed as a roller-type bearing (NU318C3) and the mechanical load on the motor was unbalanced. After changing it to a ball-type (6318C3) bearing, the motor temperature dropped to 56°C.
2. Strengthen oil management
The basic indicators of grease are penetration, dropping point, oxidation stability, and low-temperature performance. When selecting a grease, environmental, temperature, speed, and load factors need to be considered. If the ambient temperature is high, grease with strong water resistance should be used. For high speeds, a grease with high penetration should be selected. For large loads, grease with low penetration should be selected. Deep groove ball bearings are mostly used in electric motors in power plants, and No. 2 and No. 3 lithium-based greases are generally used. For some high-speed (>1500r/min) and heavy-duty bearings, lithium molybdenum disulfide-based grease is still used, and calcium-based grease is generally not used. In many years of practice, we have found that people who frequently replenish lubricants can easily add different lubricants by mistake. To avoid this, all solid greases were replaced with high-quality No. 3 Cogon-based grease from the same manufacturer. It sets a record for the use of molybdenum disulfide (MoS2) potassium-based grease in one of the few high-speed motor bearings. When installing bearings, it is generally stipulated that bearings with an inner diameter of less than 80mm can be installed by cold pressing. Bearings with an inner diameter >80mm should be heated before installation. However, if the double-sided sealed bearing needs to be heated during installation, the temperature should not exceed 70°C to prevent the hot melt of grease from flowing out and affecting the lubrication effect of the bearing.
3. Install bearings according to installation process standards
1. Use special tools for bearing installation and disassembly
Advanced installation tools can avoid bearing damage caused by improper tools and operations during installation. For example, when installing bearings, workers used the tapping method of copper rods, which could easily cause uneven axial stress on the bearings, deformation of the cage, damage to the rolling elements, and an increase in clearance. In addition, during the tapping process, the copper rods of Copper powder flying into the bearing cage can easily cause bearing failure.
2. Check the bearings before installation
For old bearings, check whether there are burrs, scratches, or cracks on the ball (column) surface. Whether the radial clearance and axial clearance of the old bearing are qualified, generally only the radial clearance is measured. For new bearings, check that the bearing model is correct. 3. Check the matching dimensions of the bearing and rotor shaft
When assembling the motor, it is also necessary to carefully check the matching dimensions of the bearing and the rotor shaft when the bearing is installed, and the matching dimensions of the bearing outer ring and the end cover hole.
4. Control the amount of oil in bearings and bearing chambers
Too much oil in the bearing and box will cause the bearing rolling elements to slip, causing the rolling friction to change from rolling friction to sliding friction, damaging the bearing rolling elements. Due to the excessive amount of oil in the bearing, the free space in the bearing box will be small, and the operating temperature of the bearing will increase. As the temperature rises, the density of the grease decreases, the lubricating oil film on the rolling elements becomes thinner, and the lubrication conditions are poor, which can easily cause bearing noise, and surface slippage, and shorten the life of the bearing. Generally, there is a bearing oil chamber on the side of the motor end cover (except for motors designed with double-sealed bearings). According to the motor speed, the amount of oil that can be filled into the bearing chamber can refer to the following standards: When the motor speed is <1500r/min, the amount of oil added to the bearing chamber is 2/3 of the volume. When the rotation speed is between 1500 and 3000r/min, it is 1/2 of the bearing chamber volume. When the rotation speed is >3000r/min, it should be less than or equal to 1/3 of the bearing volume. In the actual working process, for bearings operating at high temperatures and high speeds, bearings with sealing surfaces should be used as little as possible, the amount of oil stored in the motor oil cap should be increased, and a grease nozzle should be installed to extend the operating life of the motor bearings.
5. Pay attention to ensure that the insulation is intact
For bearings with an insulation design on the outer ring, care must be taken to ensure that the insulation is intact. If the bearing insulation is destroyed during installation, the very thin bearing oil film will be broken down by the shaft voltage. After the oil film is broken down, not only will the rolling element lubrication conditions be poor, but the generated sparks will cause electric corrosion of the bearing rolling elements, causing the rolling elements to malfunction. The surface is not shiny and accelerates bearing wear.
4. Strengthen bearing operating status monitoring and tracking management
Under normal circumstances, the service life of the bearing follows the typical bathtub curve law, but during operation, it should still be tracked and monitored according to the normal monitoring cycle. There are two types of tracking and monitoring:
(1) Operators conduct inspections during each shift to determine whether the motor and its bearings have overheating, vibration, or abnormal noise. To judge the temperature, the operator uses a point thermometer to measure the temperature of the motor at fixed points, and the vibration is measured by hand touch, and hearing. Use spot thermometers, vibrators, listening sticks, and other tools for precise inspections every week.
(2) Professional testers use bearing diagnostic instruments to conduct diagnostic and precise inspections of bearings of important electrical equipment. Whether it is the inspection results of the operating personnel or the diagnostic reports of the testers, the results are summarized at the inspection office through network software. The inspection office then analyzes the deterioration trend and determines the maintenance time of the motor bearings.
5. Improve the operating conditions of bearings
(1) Control bearing temperature. For some important motor bearings that directly affect the load and safety of the unit, such as blower motors, primary fan motors, induced draft fan motors, etc., modify the bearing temperature settings and lower the temperature alarm value from 90°C to 70°C so that when temperature abnormalities occur When rising, there is enough time to take measures to ensure the safety of the unit. When the weather is hot, a temporary fan is put into operation to reduce the bearing temperature, ensure that the lubricating oil has sufficient a degree, and improve the lubrication conditions of the bearing.
(2) Strict workmanship to eliminate center misalignment, uneven motor feet, loose foundation, unbalanced rotor, abnormal load, and other faults. The results of these faults are mostly reflected in abnormal vibration of the motor. Motor bearings under ideal working conditions can theoretically run for more than 100,000 hours, but in actual life, running for 10,000 hours is not bad.