Bearing overheating failure phenomenon and solution

1. Bearing selection

In addition to the manufacturing process and testing level of the bearing manufacturer, the quality of the bearing is also closely related to the material of the bearing steel, the heat treatment process and the forging process. At present, the vacuum degassing bearing steel and vacuum remelting bearing steel adopted abroad are not popular in China, and they are superior to the bearing steels chrome steel GCr15 and GCr15SiMn which are widely used in China. Therefore, it is advisable to use high-quality imported bearings for bearings, such as SKF in Sweden, NSK and KOYO in Japan, and FAG in Germany. However, if carefully tested and screened before use, Wafangdian, Harbin and Luoyang are also good choices. In short, the miscellaneous and three no products should not be used as much as possible, otherwise the motor (especially the medium and large three-phase asynchronous motor) will have endless troubles.

2. Bearing is squeezed

Balls or rollers can be easily rotated in the raceway because of the play between them and the inner jacket of the bearing. If the inner or outer ring of the bearing sleeve is squeezed, the play will become smaller. Because the value of the bearing clearance is small, it is sensitive to changes in the clearance. The clearance becomes smaller, and the bearing rolling body and the ferrule generate a large contact stress, which increases the friction and heat of the bearing. There are three countermeasures to prevent extrusion: (1) The outer ring of the bearing and the bearing sleeve are mostly made of H7. In actual machining, the difference should be as high as possible because the bearing and the shaft are interference fit, and the bearing table tolerance is k6 or m5. When k6 and m5 appear to be poor and the radial clearance of the selected bearing is small and the inner ring tolerance is also small, the radial fit is relatively tight, and the radial clearance will be affected after assembly. In hot state operation, the large expansion of the inner ring may result in a negative play. A small amount of negative clearance is beneficial to the normal operation of the bearing, but the larger negative clearance is unfavorable for the bearing operation, the temperature is higher, and the degree of negative clearance is reduced by the expansion of the outer ring. In addition, when the bearing is running, the outer ring of the bearing and the bearing sleeve must also have an interference, and the rolling element is squeezed, causing the bearing to be damaged prematurely due to heat generation. (2) In order to avoid the circumferential sliding of the bearing casing in the bearing chamber, the cumulative tolerance of the end cover (or bearing sleeve) and the inner and outer cover depths of the bearing in the axial direction is “+”, and the accumulation is inside and outside. The flat joint surface of the cover is to ensure that the inner and outer covers clamp the bearing outer casing (in the design of the inner and outer cover of the bearing, the tolerance level of the length of the opening can be H11 instead of h11). For flameproof motors, this clearance should not be greater than 0.2mm. This tolerance can be guaranteed by CNC or digital display machines. (3) In the bearing clearance selection of three-phase asynchronous motor, according to the production experience, in low-voltage motors, 2P motors generally use C3 group clearance, 4P motor can use C3 group clearance or standard clearance, 6P and above. Then you can use the standard clearance. In high-voltage motors, 2P and 4P motors generally use C3 group clearance, 6P and 8P motors use CM clearance, and 10P and above motors use C0 clearance. Of course, the choice of this clearance is only for the standard design of the motor. If the customer has special requirements, it is another matter.

3. Insufficient machining accuracy of parts and improper assembly of the motor leads to heat generation of the bearing

There are two points in the bearing process that cause the bearing to heat up: (1) The ends of the motor frame are not concentric, causing the center lines of the two end covers or bearing sleeves to be disjointed after the two ends of the cover or the bearing sleeve are installed. Affects the inner and outer rings of the bearing are not parallel and the bearings at both ends are not concentric, causing additional wear and heat. (2) The end cover or the bearing sleeve is eccentrically processed, causing the end caps or bearing sleeves at both ends to be different from each other to cause the bearing to heat up. It is particularly important to point out that the bearing sleeve is a part that can be used without guaranteeing the coaxiality. Because of it, the bearing is first installed in the bearing sleeve, the bearing sleeve is then installed in the end cover, and the error of the dimensional chain of one radial fit is obviously better than the direct loading of the bearing into the end cap for ensuring the coaxiality. . The above two points require the mechanical operator to ensure the coaxiality in strict accordance with the processing technology when processing the parts.

There are two points in the improper assembly of the motor that cause the bearing to heat up: (1) The end cover and the base are improperly assembled. If the paint is not clean, the end cap bolts are unevenly loaded, and the end cover is biased to make the bearing heat. (2) The bearing cover, especially the bearing inner cover, is unevenly loaded when the bearing is installed, so that the bearing cover and the rotating shaft are rubbed to make the bearing heat. In order to ensure the quality of the assembly, firstly wipe the oil on the mating surface thoroughly before assembly. The bolts should be symmetrically tightened so that each bolt is evenly stressed and the rotor is flexible. Otherwise, the bolts should be re-adjusted and tightened. In addition, it is better to use the hot-sleeve method when assembling the bearing, because the inner or outer ring of the bearing is tapped by the copper sleeve with the boring head (some assemblers even hit the bearing directly with the hoe), the bearing will be unevenly stressed and the bearing will be damaged. . The heating method should be heated on the oven or the induction core bearing heater. It should not be heated by oil. Because the oil will penetrate into the bearing and mix with the grease, the grease will deteriorate. The temperature is controlled within 100 °C when heating, because the grease must not exceed 100 °C, otherwise it will flow out. In addition, the bearing will change at high temperature (such as over 200 °C), and the retained austenite will change to martensite in the bearing steel, which will lead to the inside and outside of the bearing. The circumferential length of the ring increases to cause vibration and heat generation, and such a high temperature is equivalent to annealing the bearing to lower the bearing hardness.

4. Bearing lubrication

Firstly, the bearing grease grade should be selected correctly. Under normal circumstances, the low-voltage three-phase asynchronous motor uses more ZL-3 lithium base grease, and the high-pressure three-phase asynchronous motor uses BHP high-temperature pressure-resistant No. 2 lithium base grease. When assembling the bearing grease, be sure to insert the grease into the bearing raceway with the oil gun, and the amount of grease must be packed, because the grease that is squeezed in is the source of the oil film generated when the bearing works. When the assembler greased the bearing, the bearing was coated on both sides, but the inside of the raceway was neglected and not squeezed in. The motor was assembled and tested, and the motor bearing was found to have an empty sound with fever. The reason is that the grease does not squeeze into the bearing raceway and the bearing is dry ground. The second is the amount of oil added, the test reflects that too much or too little oil is not suitable. Too much will cause the bearing to heat up, too little, the bearing will ring, it will not be lubricated, and it will also heat up, so it can only be just right. Generally, when the amount of grease is 2P, it is 1/2 of the bearing chamber, and when it is 4P or more, it is 2/3. The grease must be kept clean and free from deterioration or contamination. There is a YJL450-12 380V IMV2 installation motor. When the grease is applied to the cylindrical roller bearing, a small and small sand particle is accidentally mixed. When the motor test is found, the motor bearing vibration and heat are generated. There is a clear groove mark on every circumference of the roller on the inner ring of the bearing. The original bearing can only be scrapped and replaced with a new bearing. It is a few hundred dollars lost due to accidental mixing of a small sand during assembly. The plane was crashed by a bird that crashed into the impeller of the engine.

5. Mechanical load

(1) The mechanical load is not connected to the motor coupling, and the centering work is not performed. Generally, the rigid coupling allows a radial deviation of 0.03 to 0.05 mm and an allowable axial deviation of 0.02 to 0.04 mm. A larger allowable deviation value is used for the larger diameter of the coupling. When the elastic coupling is used, when the diameter of the coupling is less than 200mm, the allowable radial and axial deviation is 0.05mm; when the diameter is larger than 200mm, the allowable radial and axial deviation are calculated as follows:

Where is the outer diameter of the coupling, mm.

(2) The tension of the pulley is too large and the bearing is pulled in the radial direction to make the bearing heat. A Shanghai paint company ordered a batch of low-voltage flameproof three-phase asynchronous motors, which were driven by belt pulleys. During operation, most of the motor bearings were found to have noise and heat, and the pulleys could not be adjusted for various reasons. In desperation, we can only use the flange end cover with the tolerance of the bearing chamber to be replaced. As a result, all the above phenomena disappeared, and the motor has been running normally. The emergence of this problem can be thought and resolved in a variety of different ways.

Reprinted from the network