The Development Status of Vertical Motor Thrust Bearings

Abstract: The development status of vertical motor bearings in foreign countries is introduced, and the characteristics of vertical motor thrust bearings are discussed. On the basis of analyzing the working characteristics of bearings, supporting structures, and geometric shapes of bearing shells, this paper compares the effects of different forms of bearing support structures and geometric shapes of bearing shells on thrust bearings. Based on the actual situation of engineering applications, a more reasonable selection scheme is proposed. Finally, feasible solutions are proposed for common faults in the application of thrust bearings in vertical motors.

Keywords: Vertical motor; Thrust bearing; Elastic support; Circular thrust pad

Introduction

The bearing of a vertical motor is one of the important components of a large water pump unit, which bears axial loads such as the weight of the pump unit rotor and axial water thrust. It not only affects the output and efficiency of the unit, but also directly relates to the safe operation of the pump unit. Therefore, studying the current development status of vertical motor thrust bearings is of great guiding significance for future bearing product design and development work.

1. Products of foreign bearing companies

At present, Kingsbury and Waukesha in the United States, as well as Michell in the United Kingdom, have mature research and development technologies and product lines for vertical motor bearings, which can serve as typical representatives in the field of vertical motor bearings.

Kingsbury thrust bearings are suitable for many types of vertical water pumps, including deep well pumps, condensate circulation pumps, hot well pumps, municipal hydraulic engineering systems, and mining pumps, and can meet many requirements for high speed, heavy load, and high reliability. Products can be divided into two types based on their lubrication forms: direct lubrication and immersion lubrication. The VK model product of oil immersed lubrication bearings is a thrust bearing for water and electricity use. The characteristics of this type of bearing are low vibration, high reliability, simple maintenance, and convenient installation. Its load-bearing capacity ranges from 29 to 226 tons, and the speed range is from 100 to 4000 r/min. The thrust bearing is equipped with an oil tank, and the bearing can be arranged separately from the oil tank or completely immersed in the oil tank. The motor drives the water pump to rotate, and the heat generated by friction at the thrust pad oil film is either naturally cooled or carried away in the form of heat exchange through the circulating water cooling pipeline arranged in the oil tank.

The bearing part of Michell thrust bearing is composed of a set of automatically adjustable thrust pads, which use dynamic pressure oil film to meet load requirements and are widely used in thrust bearings and radial bearings. Waukesha vertical bearings have unique technical characteristics, including excellent forward and reverse load characteristics, up to five cooling methods (water cooling coil, water tank circulating cooling, forced air cooling, air cooling, and oil cooling coil), convenient installation and maintenance, reliable sealing, and the ability to withstand reverse thrust. They also have excellent corrosion resistance.

2. Characteristics of vertical motor thrust bearings

2.1 Working characteristics

Vertical motor thrust bearings (hereinafter referred to as thrust bearings) are mainly used on vertically installed rotating motors and other equipment. In addition to bearing the axial and radial loads on the rotating parts of the motor rotor, it also needs to bear the loads and axial water thrust from the rotating parts of the water pump, which is the most critical supporting component of the entire pump set. According to the different operating conditions and load capacity of the bearings, self-lubricating or pressure oil circulation lubrication can be selected as the lubrication method.

The commonly used thrust bearings nowadays generally adopt a self-lubricating form, and their cooling usually involves natural cooling or cooling by passing water through the cooler installed in the upper frame, with lubricating oil circulating internally. Specifically, lubricating oil flows into the groove designed by the support plate between the thrust head and the inner cylinder, and enters the working surfaces of the thrust pad and guide pad respectively. It then flows out from the oil outlet holes of the guide bearing seat and upper frame, and is cooled by an oil cooler before being recycled.

The thrust bearing bears a significant weight and axial water thrust on the rotating part of the pump unit. The bearing should be able to ensure that the motor can be started directly without the need to top the rotor, while also ensuring that the pump unit can operate safely at low speed during shutdown without the need for a braking device; The guide bearing bears the radial mechanical and electromagnetic unbalanced forces of the rotating part of the unit. The thrust and guide bearings are required to be easy to install, maintain and replace, have a long service life, and structurally meet the overall requirements of a withdrawable core water pump, as well as the daily cold and hot start times of the pump unit. Therefore, the rationality of the thrust bearing structure plays a crucial role in the long-term safe and stable operation of the unit.

2.2 Supporting Structure

The structural form of thrust bearings can generally be divided into two categories based on the different bearing support methods: rigid support thrust bearings and elastic support. At present, small capacity motor units in domestic pump stations, hydropower and other related industries mostly use rigid support and elastic pad support, while large and medium capacity units mostly use elastic oil tank support and balance block support. The supporting structure is an important component of thrust bearings, which has a significant impact on the distribution of load between pads. In addition to meeting the requirements of strength and stiffness, it should also ensure that the load is evenly distributed on each tile, easy to manufacture, and easy to install and maintain.

In recent years, with the emergence of different types of units and the development of technology, elastic support thrust bearings have been applied, such as the Linhongdong pumping station in Jiangsu Province and the cooling water circulation pumping station in some power plants. Below, a comparison and analysis of the structure and performance of elastic supported thrust bearings and rigid supported thrust bearings will be conducted as an important basis for the selection of thrust bearing structures.

2.2.1 Rigid support thrust bearings

The structure of the rigid supported thrust bearing is shown in Figure 1, mainly composed of thrust head, mirror plate, fan-shaped thrust pad, anti weight bolt, thrust pad frame and other components. The material for the thrust head and mirror plate is generally 45 steel. The surface of the mirror plate undergoes multiple processes such as rough and fine turning, grinding, etc. to achieve strict surface smoothness requirements. The fan-shaped thrust pad adopts a structural form of casting Babbitt alloy on the surface of the tile base. The anti weight bolt used to support and adjust the thrust pad is screwed into the thrust pad bracket. The locking plate is used to lock the height of the load-bearing bolts and thrust pads.

To meet the operating conditions of the thrust bearing, the entire thrust bearing is immersed in the lubricating oil inside the oil cylinder. The axial thrust pad is equipped with an oil inlet edge, which forms an oil film during operation through an oil wedge between it and the rotating mirror plate. After the motor starts running, heat is generated between the mirror plate and the thrust pad due to relative motion friction, which is carried away by the lubricating oil and transferred to the lubricating oil in the oil cylinder. There is an oil-water cooler installed inside the oil cylinder, and circulating water flows into the oil-water cooler externally to cool the lubricating oil in the oil cylinder, ensuring that the temperature of each part of the bearing is balanced and stable, and ensuring the normal operation of the thrust bearing.

There are two main forms of rigid support: Babbitt alloy pillar bolt support and small pillar double-layer support. The commonly used form of Babbitt alloy pillar bolt support structure was mentioned earlier. This structure involves brazing the thrust pad onto the support pad, which is supported by pillar screws. This support structure is simple and has low manufacturing costs, but it requires force adjustment of the thrust pad during installation or maintenance. Due to the difficulty of force adjustment, manual leveling is time-consuming and labor-intensive. During operation, the force on each thrust bearing cannot be automatically adjusted, and it cannot be linked. The frequent burning of the bearing limits its range of use. Therefore, it is generally used for small thrust bearings with low unit pressure.

Figure 1: Rigid Support Thrust Bearing

2.2.2 Elastic support bearings

The elastic supported thrust bearing, as shown in Figure 2, mainly consists of thrust head (including the mirror plate of the elastic supported thrust bearing in Figure 2), circular thrust pad, butterfly spring, bearing plate and other components. The thrust head is integrated with the mirror plate, forged from 45 # steel, and the mirror plate surface is precision machined and polished. The thrust pad is circular, and the pad surface is made of Babbitt alloy. There is a fixed shaft connected behind it, which plays a horizontal limiting role. The disc spring is formed by stamping thin steel plates into a hollow circular platform. The upper and lower planes have undergone mechanical precision machining and grinding, with a height error of within 0.01 mm. The small shaft behind the thrust pad is inserted into the bearing plate hole, and the sealing ring on the shaft can ensure that the thrust pad moves slightly in the horizontal direction, which is conducive to the formation and maintenance of an oil film between the thrust pad and the mirror plate.

Figure 2 Elastic Support Thrust Bearing

Due to its structural defects, rigid supports cannot effectively solve the uneven force distribution of thrust pads. Its elastic support is greatly reduced in stiffness due to the use of disc spring support at the bottom of the tile. When the height difference of the thrust pads is 0.02 mm, the disc spring can control the uneven force distribution of each thrust pad within 10%. In this way, the frictional force between the thrust pad and the mirror plate is uniform, the lubricating oil film is stable, and the heat generated by friction can be timely carried away by the lubricating oil. The temperature of the tile surface is not easy to accumulate, greatly reducing the possibility of tile burning.

2.2.3 Comparison of Two Support Structures for Thrust Bearings

The rigid support thrust bearing has a simple structure, low manufacturing cost, and is widely used in China. Elastic supported thrust bearings are new products developed by domestic bearing manufacturing companies that have introduced advanced design and manufacturing concepts from abroad. They have been widely promoted and used in China.

Comparing two types of thrust bearings with different support structures, the advantage of elastic support is that the thrust pads bear a relatively uniform load. The disc spring, as a supporting part, has excellent adaptive ability and damping characteristics. In this way, each thrust pad can be automatically leveled, and there will be no overloading of a certain thrust pad, thereby improving the overall bearing capacity of the thrust bearing. Elastic support has a good vibration absorption effect. When the motor vibration is too large or there is an impact load, the disc spring can play a good buffering role, greatly reducing the fatigue damage of the thrust pad surface. The deformation of the thrust pad surface of the thrust bearing with elastic support is relatively small. This is because the back of the thrust bearing with elastic support adopts circular ring contact, which reduces the comprehensive deformation of the pad surface and is conducive to the formation of lubricating oil film on the pad surface. Elastic support thrust bearings are easy and convenient to install, and the thrust pads can automatically adjust their height during operation. However, rigid support Babbitt alloy pads require scraping the thrust pads and leveling the mirror plate before installation, which takes a long time for debugging and maintenance.

2.3 Geometric shape of thrust pads

At present, traditional thrust bearings generally use fan-shaped thrust pads, as shown in Figure 3. This type of bearing has a wide range of applications, and most of the vertical water pump thrust bearings currently in operation in China use this shape of thrust bearing. In addition, the volume and weight of large, medium, or ultra large power generation units in the hydropower industry are enormous. On the premise of meeting the unit pressure requirements of the tile surface, the thrust pads are all in the form of large and narrow fan-shaped tiles, with a large number of tile blocks. However, the larger the area of traditional fan-shaped thrust pads, the more difficult it is to control deformation, and the operating temperature may also be higher. In actual operation, its deformation is difficult to control within a good range.

Figure 3 Fan shaped thrust pad

The emergence of circular thrust pads is the development of thrust bearing structures. The bearing capacity and lubrication performance of circular tiles are better than those of fan-shaped tiles, and it is inevitable that circular tiles will replace traditional fan-shaped tiles. In the 1980s, RENK, a German company, pioneered the world's first circular thrust pad sliding bearing with disc spring support and established industry design standards for this type of bearing. The innovation of circular tiles lies in the fact that the traditional fan-shaped thrust tile support point generally adopts a biased support point method, that is, the support line is located at the geometric center of the tile and deviates 10% from the oil outlet edge. Compared to the symmetrical support of the round tile, the deviation type support has a lower tile temperature of about 20 ℃ during normal operation, but the tile temperature is about 35 ℃ higher during reverse operation than during forward operation. The pads of the RENK thrust bearing are circular, and the fulcrum is located at the geometric center of the pads, which makes the pump unit have good adaptability to forward and reverse rotation.

Adopting circular tile surfaces eliminates edge effects when using fan-shaped tile surfaces. In addition, the thermal and elastic deformation of circular tiles are easy to calculate, making it easier to optimize the design of tiles and select a more reasonable ratio of tile diameter to its thickness. Circular thrust pads can automatically adjust the inclination angle during operation, which has better adjustment effect than traditional tilting pads, and their load-bearing capacity is larger than tilting pads, while the manufacturing cost is lower. Especially the circular thrust pad reduces the installation accuracy requirements, greatly avoiding faults such as tile burning and mechanical vibration caused by installation reasons, making installation and use more convenient. Based on fluid lubrication theory and considering the temperature viscosity effect of lubricating oil, Xiao Jianhai et al. established a thermal lubrication mathematical model for the block surface structure of oil lubricated thrust bearings. They used numerical analysis to analyze the oil film pressure distribution, temperature distribution, bearing capacity under different film thickness, inclination angle, and speed conditions, as well as the variation law of oil film temperature on six types of tile surface surfaces, including planar and cylindrical surfaces. Finally, it can be concluded that the cylindrical surface bearing has a higher load-bearing capacity, smaller oil film temperature rise, and the best lubrication performance, with the best overall performance.

3. Common faults

3.1 Burnout of thrust pads

Thrust pads are key components of large vertical motors. According to statistics, the burning failure of thrust pads accounts for about 40% of equipment failures in water pump units. The fundamental reason is that the oil film layer between the thrust pad and the mirror plate is damaged, and the frictional heat rapidly increases. The temperature of the pad surface exceeds the melting point of the Babbitt alloy on the pad surface, resulting in burning, as shown in Figure 4. At present, the main solutions include increasing the area of existing Babbitt alloy thrust pads, improving the overload capacity of thrust pads, using hydraulic lifting devices (opening holes in the center of the thrust pads, injecting high-pressure oil to promote oil film formation), ensuring the machining accuracy of the contact surface between the thrust head and thrust pads, and improving the cooling capacity of the water cooler in the fuel tank.

Figure 4 Burnt thrust pads

3.2 Bearing chamber oil splashing

Most large vertical water pump motors are equipped with thrust bearings lubricated with thin oil, and oil leakage problems often occur in the bearing chamber, seriously affecting the stable and safe operation of the unit. The problem of oil splashing in the bearing chamber is mainly caused by negative pressure and turbulence. The generation of negative pressure is due to the formation of a certain negative pressure inside the motor when the axial fan rotates during the operation of the motor. The oil and gas in the bearing chamber of the motor are sucked out by the negative pressure, condensed and condensed into oil droplets. Lubricating oil can easily overflow the top edge of the oil blocking bucket and flow into the motor rotor. The generation of turbulence is due to the motor driving the thrust bearing thrust head to rotate at high speed. The lubricating oil is rapidly stirred, and the oil flow becomes turbulent, appearing milky white with a lot of oil mixture, which enters the interior of the motor through the gap between the oil blocking bucket and the main shaft.

The current feasible solution to negative pressure is to increase the height of the oil blocking cylinder inside the thrust bearing, add a floating sealing ring outside the oil blocking cylinder, which can greatly reduce the oil and gas entering the rotor. At the same time, add an air filter on the oil tank of the bearing to balance the air pressure.

4. Conclusion

Vertical motor thrust bearings play a very important role in water pump units. This bearing bears loads such as the weight of the pump unit rotor and axial water thrust, and is a key component that affects the stable and safe operation of the unit. Multiple domestic and foreign bearing companies have their own unique characteristics and concepts in the design and manufacturing of vertical motor thrust bearings, which can serve as an important basis for subsequent research and reference. The core of thrust bearing structure design is how to choose the bearing structure, support form, and thrust pad geometry, which will be the focus of future bearing design work. In response to common faults of vertical motor thrust bearings such as burning of thrust pads and oil splashing in bearing chambers, the author fully considered the causes of the faults and adopted corresponding solutions, which achieved good results.

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