In the next decade, the offshore wind power market in China will develop rapidly. In view of the harsh working conditions of offshore wind power, wind power spindle bearings need higher power density, reliability and service life. This paper mainly expounds the present situation and future development direction of wind power spindle bearing technology from the aspects of bearing design, material, surface treatment and technology.
1, offshore wind power market and development trend of large MW units
Global Offshore Wind Power Report 2020 issued by Global Wind Energy Council (GWEC) predicts that by 2030, the installed capacity of global offshore wind power will increase from the current 29. 1GW to 234GW, and the Asia-Pacific region will become the most important market. On September 9th, 20021,the Global Offshore Wind Power Report 202 1 was released in Sholem, England. In 2020, the installed capacity of global offshore wind power will be 6. 1GW, slightly lower than 6.24 GW in 20 19, but GWEC predicts that 20021year will be a record of the installed capacity of global offshore wind power.
The report predicts that under the existing wind power policy, the installed capacity of offshore wind power will increase by 235GW in the next decade, which is equivalent to 7 times of the existing installed capacity of offshore wind power. Compared with the report in 2020, this forecast is raised by 15%.
In 2020, China will realize the new grid connection of offshore wind power above 3GW, and become the largest offshore wind power market in the world for the third consecutive year. The European market maintained steady growth. The Netherlands ranks second in the world and Belgium ranks third (706 MW) with the newly added installed capacity of nearly 1.5 GW.
According to the latest reports of the International Energy Agency (IEA) and the International Renewable Energy Agency (IRENA), if the global temperature rise is to be controlled within 65,438 0.5℃, the global installed capacity of offshore wind power needs to reach 2000GW in 2050, but the current installed capacity is less than 2% of this target, and the predicted installed capacity in 2030 is only 65,438 03% of this target.
2. Development status and technology of offshore wind turbine bearings.
Due to the special working conditions of offshore wind turbines, the main shaft bearing needs to be installed in the air tens of meters above the sea surface, which is extremely inconvenient to transport, install and replace, and the cost is high. At the same time, the environment of offshore wind power bearings is very bad, including typhoon and corrosion caused by high air humidity. Therefore, high performance, high reliability and long service life are essential qualities of spindle bearings. At present, wind power spindle bearings mainly rely on imports. The internationally famous manufacturers of wind power spindle bearings mainly include SKF in Sweden, Schaeffler in Germany and Timken in the United States. And occupy a dominant position in the global market. There is still a big gap between China's wind power bearings and foreign countries, mainly in materials, design, surface treatment, process level, process equipment and so on.
2. 1 Design of spindle bearing for offshore wind power
At present, the main shaft bearing in the wind turbine mainly bears most of the radial force, axial force and bending moment generated by the external wind in the transmission chain, and transmits the stable torque to the high-speed end of the wind turbine. Therefore, the bearing capacity, reliability and service life of spindle bearings are very critical indicators. At the same time, when the spindle bearing at the positioning end faces a large axial force or axial impact, its axial stiffness will determine its axial displacement under the action of external force, which has a great influence on the internal stress stability of the gearbox.
With the continuous improvement of megawatt level of offshore wind power, it is difficult to improve the bearing capacity, reliability and service life in a limited space, whether it is single-point support or double-point support. At the same time, it is accompanied by problems such as one-way eccentric load, system vibration and insufficient lubrication conditions.
At present, most installed wind turbines adopt the main shaft bearing support structure, and the main shaft bearing is generally divided into two-point support and three-point support.
Figure 1 Representative arrangement of spindle bearings
2. 1. 1 technical scheme of spindle self-aligning roller bearing
The two-point supporting form of self-aligning roller bearing with positioning end and floating end is the most typical arrangement form, in which the positioning end bearing plays an important role, which not only meets the main bearing requirements of radial and axial loads, but also has certain self-aligning performance (generally above 0.3), and also requires low-cost and stable operation for 20 years. The prior art scheme is as follows:
1) internal structure optimization
At present, the existing structure of large self-aligning roller bearing is shown in Figure 2. According to the structural form of the spacer, it can be divided into fixed spacer, floating spacer and non-spacer design. Compared with the design of floating middle diaphragm and non-middle diaphragm, the fixed middle diaphragm can effectively increase the axial stiffness and reduce the axial moving distance under the action of axial force, thus effectively reducing the influence of axial force on the gearbox. At the same time, the fixed middle spacer can effectively limit the swing angle of the rolling elements when they move. The advantage of the design without middle spacer is that it can make full use of the internal space to design larger rolling bodies and contact angles, and increase its axial bearing capacity.
Fig. 2 Structural types of large self-aligning roller bearings
2) Further improve the value of the rated dynamic load coefficient b m of the roller bearing.
According to ISO 28 1, the value of b m is defined as "rated dynamic load coefficient of contemporary common materials and processing quality" 2, which is used to calculate the basic rated dynamic load. As for the value of b m, due to the difference of smelting method of materials and manufacturing level of bearings, usually different manufacturers will provide it on the basis of test verification or experience. Self-aligning roller bearing, refined steel (vacuum degassed steel +0.0- 1. 15) defined by ISO 2865438, and electroslag remelting steel (advanced refined steel) is about 1.2- 1.5.
For large-size bearing products, with the improvement of material smelting method and manufacturing level, the use of higher purity bearing steel and superfinishing technology of rings and rollers greatly improves the surface and internal quality of each part of the bearing, improves the friction state, makes it possible to improve the b m coefficient, and thus improves the overall bearing capacity and service life of the bearing to a certain extent.
3) compression gap control interval
Bearing clearance has a great influence on bearing life and reliability. Excessive bearing clearance will reduce the total number of rollers carried by the bearing during operation and aggravate the pitting wear of rollers; If the clearance is too small, the bearing will easily generate friction and heat, the temperature will rise, the oil film will be damaged, and even the bearing will be stuck in serious cases.
Because the control clearance range of standard clearance group is large, especially for large bearings for wind power, a single standard clearance group often reaches more than 0.2 mm, while the axial clearance is more than 1 mm, which is too large for wind power applications with high reliability requirements, and it is easy to fail early due to unsatisfactory working clearance. The large clearance range will also adversely affect the self-aligning performance of self-aligning roller bearings.
Therefore, in the application of wind power spindle, considering the economy of actual machining, it is often suggested to use half of the standard clearance as the clearance of wind power, or to choose a special clearance according to the actual application data.
Fig. 3 Suggested table of clearance of large-scale wind power self-aligning roller bearing
4) By changing the rolling elements
Usually, logarithmic curve is a common modification curve for self-aligning roller, which can effectively avoid the generation of edge stress, optimize the uniform distribution of contact stress, reduce the PV value of friction factor and reduce the risk of early wear.
Figure 4? Comparison of PV value between modified and unmodified rolling bodies
5) Asymmetric bearing design 3
By setting the contact angles of the two rows of rollers to be different, the requirements of unidirectional bearing can be met. Compared with the traditional symmetrical structure, this design can effectively improve the axial bearing capacity and stiffness of the bearing under the same overall size, thus effectively avoiding the risk of another row of rollers slipping to some extent. For wind power applications, 240 series bearings are usually selected because they can be designed with larger contact angles to increase the axial bearing capacity. Asymmetric design can make full use of the one-way direction of wind force and improve the feasibility of increasing contact angle to the gearbox side. As shown in the figure, 230 series bearings can be used instead of 240 series bearings to reduce the size of bearings.
Fig. 5 Self-aligning roller bearing with asymmetric design
Asymmetric bearing design provides a new direction for fan manufacturers to have higher performance bearings without changing the existing main structure, thus greatly reducing the cost and difficulty of upgrading new models or existing models.
6) Ductile iron cage
For self-aligning roller bearings for large wind power spindles, machined brass cages are widely used because of their easy processing and molding, good mechanical properties, recyclability and certain self-lubrication. Among them, lead brass is widely used in cages because of its low cost and good machinability. However, there is lead leaching problem in the use of lead brass parts, which is easy to cause environmental pollution. Lead-containing brass cages will not be able to continue to be used in the near future, and lead-free brass will face greater cost pressure. It is urgent to find a material that can replace the existing brass cage.
At present, Schaeffler has developed a ductile iron cage suitable for large-scale self-aligning roller bearings, which has better mechanical properties and considerable manufacturing cost.
Fig. 6 Ductile iron cage
Because of its greater material fatigue strength, a certain number of rolling elements can be added on the basis of the original brass cage design, which can increase the bearing capacity and service life of the bearing to a certain extent. At the same time, in the past, 240/24 1 series bearings were mostly used in the design of brass cages in wind power. Because of its large width, the cage is often installed inside the bearing after passing through the minimum diameter of the bearing outer ring, and the outer diameter of the cage cannot be too large, otherwise it cannot be installed smoothly. At the same time, it should not be too small, otherwise the strength of the cage is low and it is easy to fail prematurely. Ductile iron cage can reduce the risk of strength problems after the outer diameter is reduced to a certain extent.
2. 1.2 technical scheme of spindle tapered roller bearing
For large MW offshore wind turbines, it has become an industry trend to choose double tapered roller bearings with better axial positioning and higher load. In addition to the existing technical schemes such as self-aligning roller bearings, including proper roller modification to reduce the risk of edge stress and further improve the bm coefficient of bearing capacity, tapered roller bearings will face greater challenges, mainly due to the difficulty in machining after large size, the difficulty in ensuring machining accuracy, the complex cage structure, the complex heat treatment process and the low production efficiency. Facing the challenge, the existing technical solutions are as follows:
Structure optimization of 1) cage
The existing large tapered roller cage structure is shown in the following figure.
Fig. 7 Tapered roller bearings with different structural types
Machined steel cage is characterized by high machining accuracy, large lubrication space, bearing assembly and hot installation of auxiliary heating device, and high overall cost.
The biggest feature of pin-piercing cage is that it can make full use of the circumferential space to fill more rollers to maximize the load, and its lubrication space is limited, especially the inner diameter surfaces of pins and rollers are often poorly lubricated, which is easy to cause abnormal wear. Secondly, its processing technology is complex, and the rolling body needs nitrocarburizing treatment, and its overall cost is also very high.
Segmented cage has the characteristics of easy assembly, low production difficulty and high efficiency, but at present, because there is usually no connecting device between segments, it can only be used on double-row tapered roller bearings.
2) Selection of heat treatment process
Seamless induction quenching can effectively prevent white cracks in large-size bearings, and has the advantages of small workpiece deformation, high dimensional stability and high production efficiency. After surface quenching, the ferrule has higher surface hardness and higher core impact toughness. At present, the biggest difficulty is that the parameters of induction quenching heads with different sizes cannot be accurately predicted, and it needs to be determined by continuous experiments, and the development cycle is long.
2.2 Offshore wind power bearing materials
Material is an important factor that directly affects the final performance of bearings. Due to the special reliability requirements of offshore wind power, the quality requirements of bearing materials used are very high. As we all know, the main factors affecting the quality of bearing steel are oxygen content, carbide, segregation and inclusions.
Inclusions in steel are closely related to oxygen content, and increase with the increase of oxygen content, which basically determines the contact fatigue life of bearing steel. At present, international steel manufacturers, such as Japan Sanyo and Sweden OVAKO, have controlled the oxygen content of traditional steel below 5× 10-6. On this basis, the two companies developed ultra-high purity bearing steel (EP steel) and isotropic bearing steel (IQ steel) by improving the ultra-high purity smelting process, and even controlled the oxygen content of the steel at (2-3 )× 10. In addition, in order to meet the requirements of long life, high precision and high temperature resistance of bearings, foreign countries have successively developed new bearing materials such as special heat-treated bearing steel (SHX steel), low-density bearing material (60NiTi), high-temperature bearing steel CSS—42L and high-corrosion bearing steel Cronidur 30.
Domestic steel mills need to shorten the gap with foreign countries in the future, further improve the cleanliness of bearing steel and reduce the content and size of inclusions in steel; Through process optimization, the uniformity of carbide is further improved, and liquid phase precipitation, network and banded carbide are reduced and eliminated; Further improve the grain size of matrix structure and further refine the grain size of bearing steel; Reduce macro-structural defects; Further reduce the central porosity, central shrinkage cavity and central component segregation in bearing steel, and improve the uniformity of macro-structure.
2.3 Surface Treatment of Offshore Wind Power Bearings
Surface coating technology includes physical vapor deposition (PVD), chemical vapor deposition (CVD), radio frequency sputtering (RF), ion spraying (PSC), electroless plating and so on. It can improve the wear resistance and contact fatigue resistance of bearing parts and reduce the surface friction coefficient. At present, according to the technical trends of several major bearing manufacturers, the coatings mainly applied to wind power spindle bearings are as follows:
2.3. 1 black coating
After the black coating treatment, the bearing will have better running performance, slight corrosion resistance and wear resistance, and at the same time, the coating will enhance the resistance to white corrosion cracking (WEC) to some extent. In the past, in the practical use of onshore wind power, the surface of rolling elements was often treated with blackening coating. However, after the wind turbine was transferred from land to sea, it was suggested that the rings and rolling bodies should be blackened because of the more complicated and harsh working conditions.
DLC coating
DLC coating is a superhard coating with properties very similar to diamond coating, such as extremely high hardness, resistivity and thermal conductivity. 5. This coating can reduce the friction and wear under mixed friction conditions, greatly improve the bearing life and wear resistance, and avoid the adhesive wear (smearing) of roller bearings caused by sliding between rolling contact surfaces.
2.3.3 Columnar hard chromium coating
The coating mainly adheres to the inner diameter surface of the inner ring, which can provide high wear resistance (high hardness), especially the mating surface that is prone to fretting corrosion.
Phosphating coating
This coating is often used on the outer diameter surface of floating end bearings, mainly for improving emergency lubrication and wear protection. For example, in order to prevent fretting corrosion or friction corrosion, the corrosion resistance can be temporarily improved by passivation or corresponding post-treatment of oiling.
2.4 Domestic offshore wind power bearing manufacturing status
Compared with foreign countries, there is still a big gap in the manufacturing level of domestic wind power bearings, especially the large MW bearings are limited by processing equipment and process level. With the increasing demand of foreign-funded enterprises for localization of high-end products, major bearing manufacturers are also accelerating the localization process. For example, Schaeffler Group has four factories in Nanjing, specializing in the production of large-scale wind power bearings, which can process self-aligning roller bearings, cylindrical roller bearings and tapered roller bearings with outer diameters of 800~2000 mm and above 2,000 mm respectively. By introducing foreign large-scale production equipment and technology, mass production of various kinds of bearings has been realized.
The improvement of domestic production level is conducive to the rapid development of domestic wind power market. Under the condition of ensuring that the product quality meets the highest standards of wind power, it can realize fast delivery and lower cost, and ensure the interests of customers to the greatest extent.
3 Conclusion
At present, the special application conditions of offshore wind power put forward higher requirements for carrying capacity, reliability and service life. For large bearings of offshore wind power, bearing design, materials, surface treatment and technology can be improved in the future. For bearing design, it is necessary to further improve the overall bearing capacity, including better structural characteristics, including contact optimization, structural form and material selection of cages, especially tapered roller bearings, and how to simplify processing technology and heat treatment methods. For materials, how to shorten the gap with foreign countries, including further improving the cleanliness of bearing steel, reducing the content and size of inclusions in steel and improving the uniformity of carbides. For surface treatment, develop better surface treatment technology, including how to solve the boundary friction and lubrication problems after the intervention of external pollutants.