According to the maintenance and operation conditions for many years, the following problems often occur after the motor, especially the high-voltage motor, is repaired:
1. After the motor is overhauled, the motor bearing temperature is high during the trial operation;
2. After the motor is overhauled, the motor vibrates greatly during the trial operation;
3. After the motor is overhauled, the single unit test run is normal, and the vibration increases after the mechanical coupling;
4. After the motor runs for a period of time, the bearing has abnormal noise or even damage;
5. Supplementing grease during the normal operation of the motor causes the temperature of the motor bearing to soar.
The above problems are often caused by ignoring quality control during the motor maintenance process. Therefore, we often say that the quality of the manufacturer's motor is better. In fact, the production and assembly process of the motor factory is the same as the maintenance process we use on site. , Only by paying attention to the quality control in the motor maintenance process can the motor after the maintenance be restored to the good state of the motor when it leaves the factory.
1. Selection of motor bearings:
It is well known that the quality and suitability of the running bearing of the motor is the first key to the operating life and quality of the motor.
First of all, you need to check the bearing model, manufacturer and other information on the motor nameplate, and check whether the technical parameters of the bearing to be used can meet the use of the motor. Take our coal mill motor as an example: the motor nameplate indicates that the front end bearing used is 23044CC/C3W33 (SKF) Description The motor bearings are: inner diameter of 220 mm, outer diameter of 340 mm, thickness of 90 mm, bearing radial clearance of 220-290um, steel cage, and self-aligning cylindrical roller bearings produced by SKF in Sweden. Coal mill motor model YTM710-6 power 1600KW rated speed 993r/min.
The material of the cage of the motor bearing is generally divided into a steel cage, a copper cage and a nylon cage. The bearing of the copper cage is large, but the load bearing capacity of the steel cage bearing is slightly stronger than that of the copper cage bearing. Nylon cage bearings are generally used for equipment with special requirements for light loads. For example, the motor bearing of the main transformer submersible oil pump is a nylon cage. The use of this type of cage prevents metal chips from entering the main transformer oil system when the cage is worn or broken.
When we choose bearings, take the three brands of SKF, NSK and FAG as examples:
As can be seen:
A. The limit speed of NSK bearing cannot meet the use of this motor. If it is used forcibly, the cage of the bearing will be easily fatigued and fractured, causing failure.
B. The load bearing capacity of steel cage bearings is slightly stronger than that of copper cage bearings.
Therefore, the coal mill motor can choose bearings from two brands of SKF or FAG.
2. Inspection of motor bearings:
Not all purchased bearings can be used at will without inspection. The purchased bearings may have damaged packaging and cause bearing corrosion. Suppliers supply fake bearings for profit; supply process There are deviations in the bearing, resulting in the deviation of the bearing specifications and models. Therefore, after the new bearing is used, it must be tested accordingly. When in doubt, it must be clarified before installation.
2.1 The steel code of the model, specification, manufacturer, etc. on the bearing is clear and correct, and the bearing selection meets the requirements of the motor nameplate.
2.2 Clean and inspect the bearing, the bearing must be smooth; there is no scar, hole and rust spot, the cage is not loose, the inner and outer diameters meet the requirements, the sound is well-proportioned when rotating, there is no noise and no vibration and swing.
2.3 Inspection of bearing cage: The cage is allowed to shake slightly, the steel cage is not allowed to contact the inner and outer rings, and the copper cage is allowed to rub against the inner ring.
2.4 The old bearing wear and the new bearing clearance are within the following specifications:
Clearance inspection method: Check the radial clearance wear degree of the bearing. First wash the lubricating oil of the bearing, mount it on the rotor journal with a magnetic dial indicator, put the dial indicator on the outer ring of the bearing, and apply 150N pressure or thrust on the outer ring of the bearing opposite to the dial indicator, then Read the radial clearance of the bearing on the dial indicator.
3. Inspection of motor journal and bushing parameters
After the motor is disassembled, remove the motor bearing and measure the motor journal and end cover bushing to detect whether the inner and outer rings are running. At the same time, check and measure the surface finish and ovality of the journal and bushing. When the surface finish is insufficient Or when the ovality exceeds the standard, the bearing will cause deformation, resulting in the change of bearing clearance.
The tolerances of journals and bushings are within the range of the following table:
If the shaft sleeve and journal are too loose or too tight, it will affect the assembled motor. If it is too loose, it is easy to run the inner and outer rings. If it is too tight, the bearing clearance will be compressed, and the change of bearing clearance will cause the bearing to be too tight. If it is too large, the temperature during operation is too high, which will affect the life of the bearing.
4. Precautions for bearing installation:
4.1 Bearing selection and measurement of each component are generally installed by heating or cold punching. The installation of medium and large motor bearings is generally heated and installed. At present, bearing heaters are mostly used for heating. Bearing heaters need to pay attention to the heating temperature when heating the bearing, not the higher the heating temperature, the better. According to the bearing manufacturer's data, the bearing heating temperature is generally 80-105 degrees Celsius and can be installed. Therefore, it is necessary to strictly control the temperature during heating. If the bearing heating temperature is too high, the material of the bearing steel will change, which will affect the bearing life.
4.2 The bearing heater adopts the principle of magnetic field eddy current for heating, so the heater magnetizes the steel of the bearing. When selecting the heater, the bearing heater with demagnetization function should be used for heating. After the bearing is magnetized, it is easy to absorb iron such as iron filings. Magnetic substances are difficult to clean when the bearing is cleaned, which affects the service life of the bearing.
4.3 The bearing installation must be in place. When cleaning the bearing, it is necessary to check whether it is clean. There is no jamming and jumping phenomenon in the disc bearing.
4.4 After the bearing is installed, it is necessary to review the radial clearance of the bearing. The clearance at this time is called the installation clearance. The clearance measured during the inspection of the new bearing is the factory clearance. When the compression of the clearance after installation is too large, it means that There is a problem with the journal or sleeve fit of the motor bearing.
5. Grease selection and refueling amount:
There are many kinds of bearing greases, and different types of greases are required for the running speed of the bearing, the size of the load force and whether the load is even. Generally, the power plant adds one or two kinds of grease to the motor with uniform grease, which may easily cause the shaft temperature to be high or the lubrication effect to be unsatisfactory when the motor is running.
Generally speaking, extreme pressure type high-viscosity grease should be used in motors with high speed and high load, and extreme pressure type grease with lower viscosity should be used in low-speed and high-load motors. When high-viscosity grease is used for a low-speed motor, it is easy to cause insufficient grease lubrication (that is, the grease cannot be thrown away), resulting in a high temperature of the motor bearing and affecting the life of the motor bearing. When low-viscosity grease is used in a high-speed motor, the grease is easily thrown out, resulting in a lack of oil in the bearing chamber and affecting the life of the motor bearing. Therefore, attention should be paid to the selection of suitable grease during the bearing maintenance and installation of the motor.
The more oil is added to the motor bearing, the better. When there is too much grease, the temperature of the motor bearing will rise due to the friction between the grease and the bearing, oil retaining ring and other components of the high-speed motor. Large fluctuations will affect the correct judgment of maintenance and operation personnel. Therefore, the refueling amount of the bearing should be 1/3 to 2/3 of the bearing oil cavity volume, (3000 rpm/min 1/3; 1500 rpm/min 1/2; 1000 rpm and below 2/3) .
After the motor runs normally, add grease reasonably according to the running time of the motor. Generally, the amount of refueling is larger for the first time. The reason is that the oil filler pipe, oil cap, etc. are not completely filled with oil. In the early stage of refueling, the oil first replenishes the missing part of the oil. Therefore, when refueling You should listen to the lubricating sound of the grease in the bearing while refueling.
6. Check other parts:
During the motor replacement and bearing maintenance, the end cover, oil cover, stop ring, cap nut, and oil deflector are often cleaned and put aside for installation. the quality of the machine installation. The following inspection measurements must therefore be carried out after cleaning:
6.1 Whether the motor end cover and oil cover stop are smooth and complete, without wear or deformation, and repair if necessary.
6.2 When the oil cap of the motor is worn out, it needs to be processed or replaced on a lathe. During turning, it is necessary to turn out the joint and the joint surface of the oil cap according to the height of the original end cap, otherwise the assembly clearance will be affected.
6.3 Before the old bearing is disassembled, it is necessary to check the bearing ball and ballistic wear, so as to analyze whether the bearing has been installed eccentrically in the last operating cycle, and if so, it needs to be adjusted according to the actual situation when the new bearing is installed.
6.4 The installation of the oil baffle ring should be installed on the bearing side with the inclined surface of the oil baffle ring. If the installation is reversed, it is easy to cause the waste oil to be unable to be discharged and the bearing to rub the oil baffle ring, resulting in the shortening of the life of the motor bearing;
6.5 The positioning piece of the stop ring has no wear. If there is wear, the stop ring needs to be replaced. Otherwise, the displacement of the stop ring will cause the oil baffle ring to loosen and rub against the motor bearing.
6.6 The deformation of the cap nut is within the allowable range. During normal installation, the cap nut is not installed by striking, but by a hook wrench. The cap nut must be tightened and the snap ring should be reversed firmly, otherwise it will cause friction between the cap nut and the oil cover or the friction between the oil deflector, the snap ring and the bearing during the operation of the motor.
7. Adjustment of the motor magnetic field center:
Under ideal conditions, that is, under the condition that the grease of the bearing will not be fouled and thrown out during the operation of the motor, the position of the motor running rotor is fixed by the magnetic field center of the motor designed by the motor, or the motor has no front and rear inner and outer oil caps. The motor can still run normally after the motor is balanced in the center of the magnetic field. In fact, our common bearing motor is this type of operation. We can notice that the bearing motor after overhaul sometimes has a large shaft movement at the moment of starting and then returns to a stable state. The movement at this time is that the motor is searching for the magnetic field of the motor by itself. center.
Most of the cases encountered are that in the three-way vibration of the unit test run after the motor is overhauled, only the axial vibration is large or the unit test run is in good condition, but even the mechanical operation is the case where the axial vibration increases significantly. The above situations are mostly caused by the deviation of the center position of the magnetic field of the motor during the assembly or centering process of the motor. All are installation issues.
Still taking the coal mill motor as an example, the thickness of the front bearing bushing of the coal mill motor is 108mm, the thickness of the front bearing of the motor is 90mm, and the height of the oil caps at both ends of the front bearing is 10mm. From this calculation, it can be concluded that the motor is assembled after completion. The rear two oil caps cannot be sewn together with the shaft sleeve, and the error is as high as 2mm. The general answer to asking the motor factory is to save materials. When processing the shaft sleeve, 110mm steel plate is used, so there is a machining allowance of 2mm. That's it. I often think that how thick the rubber pad is after the motor is disassembled and then install it, or it doesn't matter if it is thicker, or is it just a 2mm gap? The problem of incompatibility. But is this the case? If so, the oil cover can be processed to 9mm. In fact, the gap of two millimeters is used by the motor factory to adjust the magnetic field center of the motor during the motor assembly process.
So is there any deviation in the center of the magnetic field, and how to measure the deviation? Medium and large motors, especially high-voltage motors, are designed with air guide air ducts on the stator and rotor core. This design is to enhance the cooling effect of the stator and rotor. In the design of the motor, the air guide air ducts of the stator and rotor are one by one. Correspondingly, that is to say, if the magnetic center line of the motor is aligned, the air guide air duct should be completely aligned.
After the motor is assembled, check whether the magnetic center line of the motor is aligned. If the magnetic center line moves forward, loosen the rear bearing oil cap and remove the pad of the front outer oil cap and thicken the pad of the front inner oil cap. Reinstall the oil. cover, slowly tighten the oil cap bolts diagonally, use the oil cap stop to press the rotor backward until the magnetic center line is aligned, remove the oil cap after alignment, and measure the distance from the bearing end face to the shaft sleeve. Thickness deviation re-create a new pad according to the thickness, and at the same time replace the pad of the inner oil cover according to the calculated deviation value. Then remove the rear end oil cover and measure the distance from the bearing end face to the shaft sleeve. According to the thickness deviation from the oil cap stop, make a new pad according to the thickness, and at the same time replace the pad of the rear inner oil cap according to the calculated deviation value. If the center of magnetic force first moves backward, you can press in the opposite direction. Note that the four pads of the two sets of oil caps must be replaced.
Why do you need to replace 4 pads? In theory, it is enough to replace one. Isn't the motor positioned by the magnetic centerline? In fact, it is not the case. The purpose of replacing the two pads at the rear is to ensure that the rollers of the rear cylindrical roller bearing and the inner ring of the bearing are not offset, and the bearing can run better while avoiding friction caused by the offset of the rear bearing and the insufficient distance between the slinger ring. , causing bearing damage. The front inner oil cap is replaced with a suitable pad to prevent the motor from vibrating too much in the axial direction caused by the deviation of the magnetic center line again during transportation or centering of the motor after maintenance.
Therefore, I personally think that the five questions raised in the opening paragraph should be answered as follows:
1. The reason for the high temperature of the motor bearing during the test run after the motor is overhauled is analyzed as A. The selection of grease is wrong or the oil is too much; B. The journal or bushing is too tight, causing the bearing installation creep to be large, and the bearing clearance is insufficient; C. Shaft The journal or bushing is too loose, causing the bearing to run; D is the wrong choice of bearing and the wrong clearance series.
2. Analysis of the reasons for the large vibration of the motor during the test run after the motor is overhauled A. The bearing is not cleaned properly, and there are foreign objects in the bearing; B. The foot of the motor is not flat and lame; C. The magnetic center line is not aligned; D. When the motor is assembled The joint surface of the end cover and the joint surface of the oil cover are deformed, resulting in not being installed in place.
3. After the motor is repaired, the single unit test run is normal. The reason for the increase in vibration after the mechanical coupling is that A. The cushion of the oil cover is not suitable, which is caused by the change of the magnetic center line after coupling, and the general manifestation is that the axial vibration is large. The amplitude increases, and the horizontal and vertical vibrations are normal; the center of B is not correctly caused, and the vibration is generally increased on three sides.
4. After the motor runs for a period of time, the reason for abnormal noise or even damage to the bearing is as follows: A. The bearing is not cleaned properly, and there are foreign objects in the bearing; B. The magnetic center line is not completely aligned, although the axial vibration is not large, but the motor is unilateral wear, speed up Bearing damage; C, bearing quality is not good.
5. The reasons for the soaring temperature of the motor bearing caused by the replenishment of grease during the normal operation of the motor are as follows: A. The selection of supplementary grease is incorrect or too much oil is added; B. The motor oiling channel or grease has foreign matter entering the bearing, and the bearing should have abnormal noise at this time. .
Generally speaking, the maintenance item that is often carried out on the motor site is to replace the bearing. From the selection of the bearing to the assembly process, it is easy to have such omissions and negligence, which will cause frequent failures after the maintenance is completed. Therefore, the personnel involved in the maintenance should be based on the operation. The process of the instruction book and the quality standard of the maintenance process are carried out step by step, and it should not be misled by the rush of the construction period or the idea that the installation is not wrong, resulting in the equipment not being put into operation normally or the equipment life being greatly reduced after the maintenance.
