Abstract：This article talks briefly methods to extend the service life of motor rolling bearings. Key words：motor；rolling bearings；grease；motor assembly

The motor is a kind of equipment that provides power in the mining industry and plays an extremely important role. The bearing is an important part of the motor. If it is not selected properly, installed improperly, or maintained, its service life will be affected, and normal production will be affected.

1 Type of motor rolling bearing

1.1 Classification by rolling element type

① The rolling element of a ball bearing (ball bearing) is a sphere;

② The rolling element of roller bearing (roller bearing) is cylinder.

1.2 According to the direction of load

① Radial bearing;

② Radial thrust bearing;

③ Thrust radial bearing;

④ Thrust bearing.

1.3 Classification according to the load direction and rolling element type

① Radial ball bearing;

② Radial spherical ball bearing;

③ Radial short cylindrical roller bearing;

④ Radial spherical roller bearing;

⑤ Radial long cylindrical roller bearings and radial needle roller bearings;

⑥ Radial spiral bearing;

⑦ Radial thrust ball bearing;

⑧ Radial thrust tapered roller bearing;

⑨ Thrust ball bearing and thrust radial ball bearing;

⑩ Thrust roller bearings and thrust radial roller bearings.

2 Assembly of motor bearing

2.1 Bearing inspection

2.1.1 Appearance inspection

Ball and roller surfaces and raceways shall be smooth, clean, free of cracks and discoloration pits, and the ball rack shall not shake and rotate flexibly.

2.1.2 Radial clearance inspection

Use ordinary feeler gauge to measure the radial clearance of ball rack simply and quickly. Set up the bearing and measure 3~5 points evenly, with the deviation not exceeding 10%. Different specifications of bearings have different allowable clearance ranges, as shown in Table 1.

Table 1 Radial clearance of rolling bearing

Bearings with radial clearance smaller than normal are better. Bearings with large clearance close to the maximum allowable value can not be used, which is easy to generate noise and increase vibration. But the gap is too small and easy to heat.

2.2 Matching dimension of shaft and bearing

Before installing the bearing, check the fit size of the shaft and bearing. If the shaft is slightly worn, it can be repaired by electroplating or spraying to ensure that the tightening force between the shaft and the bearing is 0.02 mm~0.03 mm and belongs to positive deviation. Beyond this range, bearing installation is difficult; However, the tightening force should not be too small, otherwise the sliding between the inner ring of the bearing and the shaft will cause wear of the shaft.

2.3 Bearing installation

After passing the inspection, the bearing can be heated. The control of heating temperature is very important. The temperature is too low, the bearing expansion is small, and it is difficult to install the bearing, even if it is half installed; If the temperature is too high, it is easy to cause discoloration of the bearing, change of the material, and accelerate the aging of the bearing. The heating temperature is controlled within the range of 90 ℃~100 ℃, and the bearing installation and durability are relatively reliable.

2.4 Bearing lubrication

When the temperature of the installed bearing drops to room temperature, it can be cleaned with clean gasoline. All parts should be cleaned to ensure that the bearing is clean. After that, the lubricating grease can be added. Apply lubricating grease to the bearing, squeeze each gap of the rotating part of the bearing with hand pressure, and rotate the bearing from time to time to let the oil enter each part evenly, so that the lubrication effect is the best; However, the oil volume shall not exceed 2/3 of the bearing chamber cavity. Here is the selection of lubricating grease.

2.4.1 Common lubricating grease for rolling bearings

2.4.1.1 Lithium base grease

① Features: After adding antioxidant, antirust agent and extreme pressure agent to lithium base grease, it becomes a multi effect long-life universal grease, which can replace calcium base, sodium base and calcium sodium base grease; Lithium has low solubility in water and good water resistance.

② Application: Lithium base grease is used for the lubrication of sealed bearings of Y series motors and derived series motors, which can reduce the maintenance workload, increase the service life of bearings and reduce the maintenance cost.

2.4.1.2 Calcium base grease

① Features: strong water resistance, good stability, short fiber, good pumping performance, and high temperature resistance; If it is used in high temperature occasions, when it runs above 100 ℃, it will gradually become soft or even lose, and lubrication cannot be guaranteed, resulting in bearing damage.

② Application: It is generally used for the lubrication of high speed and light load motor bearings that are not in contact with water at more than 60 ℃, and for the lubrication of medium speed and medium load closed motor bearings.

2.4.1.3 Sodium base grease

① Features: non water resistance, good stability, long fiber, high temperature resistance, good protection, strong adhesion, vibration resistance; When the lubricating grease is diluted and lost after being hydrolyzed in contact with water, the bearing will be damaged prematurely due to lack of oil.

② Applicable occasions: lubrication of medium speed, medium load, low speed, high load open or closed motor bearings under the premise of high working temperature, cleanness and moisture free.

2.4.1.4 Calcium sodium base grease

① Features: It has both the water resistance of calcium based grease and the high temperature resistance of sodium based grease, and has good transportation and mechanical stability.

② Application: It is used under the condition of high working temperature and steam.

Lithium based grease is widely used in motor bearings due to its good performance and wide temperature range.

3 Bearing maintenance

3.1 The bearing assembly is too tight

If the inner ring of the bearing is forced into the journal position, after a period of operation, the bottom of the raceway has a serious spherical wear track in the light cases, and there are cracks and peeling on the inner, outer ring and ball in the heavy cases. This is because the clearance between the ball and the raceway is pressed too small, which increases the torque, friction, and bearing operating temperature, thus increasing wear and fatigue.

Prevention method: recover the radial clearance of the bearing and reduce the interference. It is appropriate that the journal diameter is 0.02 mm larger than the diameter of the bearing inner race, and the best fit size is that the diameter of the end cap bearing mounting hole is 0.02 mm smaller than the diameter of the bearing outer race.

3.2 Bearing installation is not aligned

Bearing installation is not aligned, not only there are concave bead marks on the bearing slideway, but also uneven wear marks of the ball, which also causes deflection; The radial clearance between the rings on both sides of the bearing is not equal, and the difference amplitude is large. The deflection greater than 1/1000 causes the bearing operating temperature to rise, accompanied by severe wear of slideways and balls; The shaft is bent.

Prevention method: Use a steel pipe sleeve with flat and smooth end face and nearly the same wall thickness as the inner ring of the bearing to press the bearing into the correct position of the clean journal; Then, check the allowable radial runout of the bearing according to different speeds of the motor.

3.3 Motor end cover installation is not aligned

The motor end cover was not installed flat, and the bearing force was deflected, resulting in overheating of the bearing.

Treatment method: reinstall the motor end cover or adjust the end cover screws to make the end cover installed in place and centered. Similarly, the installation deflection of the small outer cover of the bearing will also cause bearing heating. Therefore, during major, medium and minor maintenance of the motor, it is important to tighten the screws of the small cover of the motor in place to avoid the occurrence of bearing heating caused by the deflection of the small cover because the screws are not tightened in place.

3.4 Failure of bearing lubricating grease

When the bearing end cover gets hot and the noise increases, it is found that the ball, retainer, inner race and its journal become brown or blue after inspection, which is caused by long-term use without adding grease.

Prevention method: Determine the time for supplementing and changing the lubricating grease during motor operation according to the factors such as motor operation time, ambient temperature, load condition and the quality of lubricating grease. Generally, the grease shall be replenished once in 6000 h~10000 h and replaced once in 10000 h~20000 h. For motors with 2 or more poles, it is advisable to replace 2/3 of the bearing chamber cavity with grease. When replacing grease, clean it carefully to ensure that the interior is clean. You can also use your fingers to grind old grease. If there is iron filings, you must carefully check the fit of the inner sleeve of the bearing and the clearance between the ball and the ball rack.

For many years, our station has always adhered to the oil injection system of No. 1, No. 11 and No. 21 for mechanical and electrical maintenance, and insisted on cleaning the motor grease every year, so that the equipment maintenance is planned and recorded. During a normal patrol inspection, the chief shift found that the rear end cover of the loading belt motor was hot. After inspection, it was caused by excessive oil injection at work. The end cover was immediately removed to reduce the amount of grease to 2/3 of the bearing chamber cavity. After a period of observation, the operation returned to normal and the accident was avoided. As long as the quality of bearings, installation quality and daily maintenance are well controlled, the service life of motor bearings will be extended.

More about GQZ Tapered Roller Bearing：

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Tapered roller bearings are rolling element bearings that can support axial forces (i.e., they are good thrust bearings) as well as radial forces.

The inner and outer ring raceways are segments of cones and the rollers are tapered so that the conical surfaces of the raceways, and the roller axes, if projected, would all meet at a common point on the main axis of the bearing. This geometry makes the motion of the cones remain coaxial, with no sliding motion between the raceways and the outside diameter of the rollers. This conical geometry creates a linear contact patch which permits greater loads to be carried than with spherical bearings.