Failure form and cause analysis of centrifugal pump rolling bearing and technical measures for improving the service life of centrifugal pump rolling bearings.

Abstract: This paper expounds the failure mechanism of the rolling bearing, analyzes various forms and influencing factors of the failure of the rolling bearing of the centrifugal pump in combination with the operation practice and maintenance experience of the pump station for many years, and puts forward the technical measures to prevent the premature failure of the rolling bearing of the centrifugal pump from the four aspects of the operating environment, lubrication, installation, operation and monitoring.

Key words: rolling bearing; spalling; clearance; lubrication

Rolling bearing is a general standard part and one of the wearing parts of centrifugal pump. According to the statistics of a pump station, in the daily maintenance of centrifugal pump, the failure of rolling bearing accounts for about 70% of the failure of the whole pump station unit, and the damage caused by cleaning factors is about 40%; The damage caused by lubrication is about 30%; The damage caused by installation factors is about 15%; The damage caused by bending of pump shaft and other factors is about 15%. For the maintenance of large-scale water pump units, replacing the rolling bearings will bring a large workload and maintenance costs. Therefore, it is of practical significance for the normal operation of the water pump and the safety of the pump station to correctly analyze the various forms and influencing factors of the failure of the rolling bearings of the water pump, and to explore the technical ways to improve the service life of the rolling bearings of the centrifugal pump.

1. Failure mechanism of centrifugal pump rolling bearing

1.1 Contact fatigue failure

Contact fatigue failure refers to the failure caused by the alternating stress on the working surface of rolling bearing. It is the main form of rolling bearing failure. Contact stress on raceway surface or rolling surface of inner and outer rings of rolling bearings σ H can be approximately regarded as a pulsating cycle, which changes periodically with the running time of the pump. Such periodic stress changes are easy to cause contact fatigue on the contact surface. When the cyclic stress and the number of cycles reach a certain value, fatigue cracks will first occur at a certain depth under the surface layer. When the rolling parts pass through these cracks, the range of the crack surface will gradually expand and continue to develop to the contact surface, making the metal flake off, forming contact fatigue pitting corrosion, and eventually leading to the failure of the rolling bearing.

1.2 Wear failure

Wear failure refers to the failure caused by the continuous wear of the metal on the working surface caused by the relative sliding friction between the surfaces of rolling bearings. The long-term and continuous wear causes the rolling bearing parts to be damaged gradually, and eventually leads to the loss of dimensional accuracy, shape change, clearance increase and working surface morphology change of the rolling bearing. At the same time, the wear of parts also affects the efficacy of the lubricant. When the pollution impurities accumulate to a certain degree, the lubrication function will be completely lost, and the rolling bearing will lose its rotation accuracy, so that it cannot operate normally. Wear failure is one of the common failure forms of rolling bearings. Its wear source comes from foreign hard foreign bodies or wear debris on the metal surface.

1.3 Fracture failure

Fracture failure refers to the fracture of bearing parts caused by the external load exceeding the material strength limit. The main reasons are the quality defects of the bearing itself and the overload of the external load. Sudden failure or improper installation of the water pump will overload the rolling bearing. At this time, the raceway, rolling element and cage will heat rapidly during rotation until they become discolored, softened, deposited and damaged. The micro crack, shrinkage cavity, overheated structure and local burn of inferior rolling bearing parts will also cause fracture failure in case of impact overload or severe vibration.

1.4 Corrosion failure

Corrosion failure refers to pitted corrosion, full-scale rust and corrosion on the surface of the raceway or rolling element in the environment with corrosive media. Most of them are caused by the invasion of water and corrosive media, the presence of acidic substances in the lubricant, and the formation of condensate at high temperature and humidity. Rust and corrosion change the original fit clearance of the bearing, reduce the accuracy, and even cause corrosion failure.

2. Failure form and cause analysis of centrifugal pump rolling bearing

2.1 failure forms of centrifugal pump rolling bearings

Failure forms of centrifugal pump rolling bearings include: ① cracks and fragments in inner and outer rings, rolling elements and cages; ② Peeling occurs in inner and outer rings and rolling elements; ③ There are obvious stuck marks on the raceway surface, retaining edge and rolling element; ④ Serious wear of cage; ⑤ The raceway surface and rolling element are rusted or damaged; ⑥ There are obvious indentations or marks on the rolling surface and rolling element; ⑦ There is creep or overheating on the inner diameter surface of the inner ring or the outer diameter surface of the outer ring.

2.2 Failure cause analysis

2.2.1 Environmental factors

The shaft seal of centrifugal pump is generally sealed with soft packing. The packing box is very close to the rolling bearing box. If the rolling bearing box is not sealed tightly or the packing leakage is serious, the sewage will directly splash into the bearing box and mix with the lubricating oil in the bearing box, so as to corrode the working surface of the bearing and produce rust oxides. In addition, during the operation of the water pump, foreign particles or small dust enter the rolling bearing, or the bearing and pump shaft are not thoroughly cleaned during the maintenance of the water pump, and pollutants such as sediment enter the rolling body, accelerating the wear of the bearing, making the surface peel off and generating noise and vibration.

2.2.2 Lubrication factors

The contact area of each element of rolling bearing is small, and the pressure is often greater than 20MPa. The lubricating oil forms an oil film between the two rolling surfaces in contact with each other to separate the two surfaces and reduce the friction and wear of the contact surface. However, if the lubricating oil is improperly selected or inferior lubricating oil is used, the oil film will be damaged, and the lubrication effect and heat dissipation effect will not be achieved. The components are easy to be glued, or the rolling element will be backfired due to high friction temperature, resulting in premature bearing damage.

In addition, when the centrifugal pump is running, the lubricating oil is not added at the specified time, so that the moving surface completely loses the protection of the oil film. The direct contact between the rolling element and the raceway produces dry friction, and the temperature rise leads to the damage and damage of the moving surface.

2.2.3 Installation factors

The installation method is incorrect. For example, if the rolling bearing is directly knocked with a hammer, the installation force acts directly on the raceway through the ball, and the pressure is transmitted through the rolling body, causing the contact surface of the bearing to sag or peel off. When the hot sleeve method is used for assembly, the temperature control is improper, or the quality of the used bearing is unqualified, resulting in poor tight fit between the pump shaft and the bearing, resulting in too small working clearance. The improper clearance of rolling bearings will affect the temperature rise, noise and vibration of bearings during operation, thus shortening the fatigue life of bearings.

2.2.4 Operation factors

During the operation of the centrifugal pump, if the concentricity of the pump shaft and the motor shaft is poor, or the pump shaft and the impeller rotor are unbalanced, the deflection of the pump shaft is too large, causing additional overload impact to the rolling bearing, resulting in the fracture and failure of the rolling surface of the parts.

In addition, the operating condition point changes during the operation of the pump, and the actual operating state exceeds the allowable performance range of the centrifugal pump. The change of head is not considered, and the change of impeller inlet speed triangle causes cavitation vibration. This vibration due to hydraulic characteristics will cause premature failure of the bearing.

3. Technical measures for improving the service life of centrifugal pump rolling bearings

3.1 Correct use and maintenance

Keep the rolling bearings and their surroundings clean. The bearing chamber shall be well sealed. It is strictly forbidden for moisture or other foreign matters to enter the lubricant to prevent the corrosion of rolling bearings. Anti-rust measures shall be taken when the pump is stopped. The rolling bearings shall be regularly maintained according to the pump maintenance standards. Generally, the rolling bearings shall be cleaned every 1500h of operation, and the oil shall be changed or cleaned at least every half a year. During disassembly and maintenance of the water pump, sediment, moisture and rust shall be prevented from entering the rolling bearing.

3.2 Maintain good lubrication

Lubrication quality is the key to the normal operation of rolling bearings. It has a variety of effects on the friction, wear, heat dissipation, rust prevention, sealing and impact mitigation of bearings. The cause analysis of rolling bearing damage shows that about 30% of rolling bearing damage is related to poor lubrication. Therefore, maintaining good lubrication is an effective measure to reduce the friction and wear failure of rolling bearings.

(1) The type of lubricating oil shall be correctly selected according to the speed and load of the centrifugal pump. Viscosity is an important characteristic of lubricating oil. The viscosity directly affects the fluidity of lubricating oil and the thickness of oil film formed between friction surfaces. The kinematic viscosity of lubricating oil under the working temperature of rolling bearing is generally 12 ~ 15mm²/s. The lubricating oil with lower viscosity shall be selected for higher rotating speed, and the lubricating oil with higher viscosity shall be selected for larger load. Moreover, the lubricating oil selected for large and small centrifugal pumps cannot be generalized.

(2) Before starting the water pump every time, be sure to check the oil level and quality of the lubricating oil. The normal lubricating oil shall be clean and clean. It shall be kept clean during operation. If the lubricant has become dirty, it will be mixed with worn particles or pollutants. The replacement cycle of lubricating oil depends on the service conditions and oil quantity of centrifugal pump. A scientific and reasonable replacement cycle of lubricating oil shall be formulated and strictly implemented according to the operation conditions of each pump station.

3.3 Improve the installation quality of rolling bearings

The installation quality of rolling bearings directly affects the accuracy, service life and performance of rolling bearings. During installation, special tools shall be used as much as possible to transmit installation force or torque through corresponding rings. It is not allowed to punch the rolling bearing with force or directly knock the rolling bearing with a hammer. It is not allowed to transmit pressure through the rolling element. The inner ring and shaft of the rolling bearing of the centrifugal pump are generally closely matched, and the outer ring and the hole of the rolling bearing seat are loosely matched. For small and medium-sized centrifugal pumps, press the rolling bearing onto the pump shaft with a press, and then install the pump shaft together with the rolling bearing into the bearing hole seat. When pressing, pad the end face of the inner ring of the rolling bearing with an assembly sleeve made of soft metal materials such as copper. The inner diameter of the assembly sleeve should be slightly larger than the diameter of the journal, The outer diameter shall be slightly smaller than the retaining edge of the inner ring of the rolling bearing to avoid being pressed on the cage; For the installation of large-scale centrifugal pump, due to the large amount of interference, it can be assembled by hot assembly method to avoid scratches on the mating surface. Before hot installation, put the rolling bearing into the oil tank and heat it evenly to 80 ~ 100 ℃, then take it out of the oil and install it on the shaft as soon as possible. In order to prevent the end face of the inner ring and the shaft shoulder from not fitting tightly after cooling, axial tightening shall be carried out after the rolling bearing is cooled.

After the installation of rolling bearings, first check whether the running parts collide with the fixed parts, whether the lubricating oil can flow smoothly into the rolling bearings, and whether the sealing device and axial fastening device are installed correctly. Secondly, radial and axial clearances are detected. The size of the clearance has a great impact on the smooth operation, bearing life, friction resistance and temperature rise of the centrifugal pump. The rolling bearing can be inspected by hand rotation, and it is better to be stable and flexible without vibration and left-right shaking. If necessary, insert the feeler gauge between the roller and the rolling bearing ring to measure whether the working clearance reaches the standard value. If it does not meet the standard, it shall be adjusted.

3.4 Operation and monitoring

3.4.1 Operation control

The operation control referred to here is mainly for centrifugal pumps. During the operation of the pump, the start-up and discharge water level and pumping capacity shall be strictly controlled to make the pump operate in the high-efficiency area within the range of design working conditions and minimize the vibration caused by the change of hydraulic characteristics such as cavitation of the pump. Meanwhile, the technical management regulations for pump stations (SL255-2000) and relevant standards shall be carefully implemented to strictly control the maintenance quality of centrifugal pumps. If the pump shaft is bent or the motor is not concentric with the pump shaft, it shall be adjusted or replaced in time to reduce the impact of additional centrifugal force on the bearing.

3.4.2 Condition monitoring

Condition monitoring is an important part to prevent the early failure of rolling bearings. Through early condition monitoring, potential problems of rolling bearings and pumps can be found to avoid the shutdown of water pump units during unplanned maintenance caused by the damage of rolling bearings. In addition to listening, touching and observing, the common monitoring methods can also monitor the abnormal vibration and temperature of the bearing with the help of advanced detection instruments such as ultrasonic detector (sounder), vibration detector or spectrum analyzer. The inspection items during the operation of the water pump rolling bearing include rolling sound, vibration, temperature, etc. when one of them is abnormal, it is a sign of failure.

(1) Rolling sound monitoring of rolling bearings. The ultrasonic detector (sound detector) is used to check the rolling sound and sound quality of the rolling bearing in operation, find out abnormal conditions in time, and compare with the normal sound. During normal operation, it should be a steady buzzing sound, while the abnormal condition is a sharp squeak or hiss. Even if the rolling bearing has slight peeling damage and cracks, it will make abnormal and irregular noise.

(2) Rolling bearing vibration monitoring. Vibration is very sensitive to the damage of rolling bearings, such as spalling, indentation, corrosion, crack, wear, etc. will be reflected in the vibration measurement. The magnitude and frequency of the vibration can be measured by the vibration measuring instrument, and the vibration source and failure point can be accurately determined by spectrum analysis.

(3) Rolling bearing temperature monitoring. The temperature of the rolling bearing can generally be inferred from the temperature outside the rolling bearing chamber. It is more appropriate to use the oil hole to directly measure the outer ring temperature of the rolling bearing or use the temperature detector. Generally, the temperature of the rolling bearing rises gradually with the operation, and reaches a stable state after 1 ~ 2H. The normal operating temperature of the rolling bearing varies with the speed and load of the water pump. If the lubrication or installation is not appropriate, the temperature of the rolling bearing will rise sharply, resulting in abnormal high temperature. At this time, the operation of the centrifugal pump must be stopped and necessary measures must be taken.

4. Conclusion

There are many factors that cause the premature failure of the rolling bearing of the centrifugal pump, such as unclean operating environment, poor lubrication, incorrect installation method, unstable operating conditions of the centrifugal pump, and the quality problems of the bearing itself. Therefore, we should focus on improving the internal and external working environment and operating factors of the rolling bearing from these aspects to extend its service life, so as to ensure the normal operation of the centrifugal pump.

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