Analysis of the Effect of Grease Containing Magnesium Hydroxysilicate in Wind Power Bearing Field Tests

Abstract

Ultra-high-power wind turbine generator bearings are susceptible to micro-spalling and electrical erosion in long-cycle operation, which seriously affects the operating efficiency and service life of the unit. For this reason, this paper adopts a kind of composite grease containing nano-hydroxy magnesium silicate powder and, through the wind turbine assembly machine test and raceway surface analysis, systematically investigates its impact on bearing temperature rise, bearing vibration, and wind turbine power under actual working conditions to meet the lubrication requirements of wind turbine generator bearings. The results of the study showed that the composite grease significantly reduced the operating temperature of the wind turbine bearings under full operating conditions. It is worth noting that the reduction in generator bearing temperature varied among the three turbines due to uncertain environmental factors. In addition, the grease effectively increased the output power of the turbine under medium wind speed loading conditions, further verifying its potential value and practical effect in the application of wind turbines.

1. Introduction

Bearings are the critical components of wind turbines that bear the greatest loads, and their performance directly affects the reliability and service life of the entire unit [1,2]. Under prolonged high contact pressure, the metal surface is prone to fatigue, resulting in microcracks and the detachment of surface material, a wear phenomenon known as micro-spalling [3,4]. At the same time, the shaft current generated during the operation of the motor will trigger an arc discharge in the contact surface between the raceway and the roller, resulting in a sharp rise in local temperature and the destruction of the lubricant film, ultimately triggering abnormal wear and tear or even failure of the bearings. This seriously threatens the stable operation of the wind turbine [4,5]. In recent years, with the continuous growth of installed wind power capacity, its conversion efficiency, operating costs, and service life are facing more serious challenges [2,5,6,7,8]. Therefore, a new type of grease is particularly important for improving the service life and operational efficiency of wind turbines, and it has been an important research topic in recent years.

Nano-magnesium hydroxysilicate (MSH) powders are a type of silicate mineral whose unique crystal structure makes them effective in reducing friction and lowering wear when subjected to external forces, making them ideal lubricant additives or grease components [9,10,11]. Previous studies have shown that nano-MSH powder has the ability to self-repair worn metal surfaces, forming a high-hardness repair film on the surface of the metal contact interface, effectively repairing metal surface damage caused by friction and wear [11,12,13,14]. Most of the current studies focus on the synthesis and dispersibility of MSH, while the evaluation of friction and wear properties is usually carried out using experimental equipment such as four-ball and ring block [12,15,16,17]. However, the working conditions of this kind of experimental equipment are often relatively fixed, and it is difficult to fully reflect the multiple complex working conditions faced by wind power bearings in the actual service environment, such as high humidity and high salt environments, thermal stress changes caused by alternating high and low temperatures, impact- load fluctuations caused by frequent starting and stopping, and other factors [13,18]. In addition, field tests—referring to the on-site validation of lubricating grease performance under real-world service conditions—are usually long and costly, and few studies have been reported in this area [19]. For this reason, there is an urgent need to investigate the practical application effect of nano-MSH powder grease in wind turbine bearings, especially in improving the bearing temperature rise, reducing the bearing vibration, and increasing the wind turbine power, in order to provide an important reference for the practical application of grease.

Based on the above research background, this paper applied composite grease and commercial grease to three 2 MW wind turbine generator bearings in an actual operating environment to compare their lubrication performance. Figure S1 shows the adopted wind turbine generator bearing. The performance differences between the greases were evaluated through a comparative analysis of operational data, such as wind speed, active power, bearing temperature rise, and bearing vibration, before and after grease filling. In addition, the feasibility of applying the composite grease in wind turbines was further investigated through a series of analyses of the generator bearing raceway repair surfaces.

2. Experimental Section

2.1. Compound Grease Formulation

The composite grease was formulated from MSH powder and wind turbine commercial grease, where the mass ratio of nano-MSH powder to commercial grease was 1:42. Specific procedures for the preparation of nano-MSH powders and composite greases are detailed in the ‘Supplementary Materials’ Section. The commercial grease used in this study is LGHP 2/0.4 high-speed and high-temperature grease manufactured by SKF, with its physicochemical properties listed in

Table 1. Physicochemical properties of SKF grease LGHP 2/0.4.

Items	Parameters
NLGI consistency grade	2–3
Soap type	Polyurea-based
Color	Blue
Base oil type	Mineral oil
Kinematic viscosity at 40 °C, mm2/s	96
Kinematic viscosity at 100 °C, mm2/s	10.5
Worked penetration, 0.1 mm	245–275
Dropping point, °C	>240

. In addition, commercial grease was used as a control group for evaluating the performance difference between them.

2.2. Test Methods

To evaluate the performance of commercial and composite greases under actual operating conditions, three wind turbines (1#, 2#, and 3#), each with a rated power of 2 MW, from one wind farm were selected for a comparative test on generator bearing lubrication. Each turbine consists of complete components including the tower, nacelle, pitch system, main shaft, gearbox, and generator. The selected turbines are similar in terms of model, years of operation, operating conditions, and environmental factors. The test period was set to one month, during which the operation of the unit was accompanied by alternating loads, eccentric loads, wind speed fluctuations, and other uncertain environmental factors. The temperature data, vibration data, and power curves of generator bearings using commercial grease and composite grease were retrieved to compare and analyze the performance differences. The generator DE-end (A-end) and NDE-end (B-end) bearing type used was NU1030M/C3. An orthogonal metallographic microscope (HT-1000, Dongguan, China) was used for microscopic observation of the outer raceway cross-section to analyze the thickness of the repair layer. The surface roughness of the raceway was measured using a roughness tester (TR200, Shanghai, China) with a maximum measuring range of 320 µm and a resolution of 0.001 µm. Specific test steps: First, 400, 800, 1000, and 2500 grit sandpaper were used to sand in sequence, followed by diamond polishing agent for polishing. Next, it was corroded with 2% nitric acid–alcohol solution and cleaned thoroughly with anhydrous ethanol. After the samples were dried, the thickness of the repair layer of the outer raceway section after the use of composite grease was observed by an orthogonal metallographic microscope (magnification of 100×). In addition, the hardness and modulus of the outer raceway surface with composite grease were tested using a micro–nanomechanical testing system (iMicro, Milpitas, CA, USA), which has a range of 0 to 1000 mN and 0 to 80 μm. The test method was the continuous stiffness method, and the test conditions were 1000 mN load and 2500 nm indentation depth. To ensure the repeatability and accuracy of the test results, three indentation tests were performed on representative areas of the sample surface, and the average value was taken as the final result.

3. Results and Discussion

3.1. Bearing Temperature Rise

Micro-spalling and electrical erosion in generator bearings cause localized surface damage, increasing frictional forces and heat generation, thereby further affecting bearing temperature rise and vibration characteristics [20]. Meanwhile, damage to the bearing surface can also lead to the breakdown of the lubricating oil film, increasing wear and ultimately reducing the operational efficiency of the wind turbine [3,4]. To investigate this effect in depth, this experiment compared and analyzed the temperature rise differences in the generator’s A-end and B-end bearings under rated speed conditions when using composite grease versus commercial grease, as shown in Figure 1. The test results indicate that the temperature rise of generator bearings using composite grease was significantly lower than that using commercial grease across multiple units (e.g., wind turbine units 1#, 2#, and 3#). In contrast, the performance of commercial grease was slightly inferior in this regard, resulting in a higher bearing temperature rise. This is mainly due to the excellent anti-wear properties of the composite grease, which effectively reduces the heat generated by friction and wear. It should be noted that in Figure 1b,c, the temperature rise curves of the two greases are closer at certain periods of time, but on the whole, the composite grease still shows better temperature rise control under most operating conditions.

3.2. Bearing Vibration

The vibration monitoring system data for bearings using composite grease and commercial grease were extracted, as shown in Figure 2, Figure 3 and Figure 4. Significant changes in the vibration characteristics of the generator bearings were observed in wind turbine units 1#, 2#, and 3#. Specifically, in the time-domain analysis, the data for commercial grease exhibited noticeable high amplitudes and irregular fluctuations, reflecting surface defects such as micro-spalling or electrical erosion. These defects increased the instability of rolling friction, leading to intensified fluctuations in the time-domain waveforms. In contrast, the time-domain signals for composite grease showed significantly reduced fluctuations, lower amplitudes, and overall smoother waveforms. This change indicated that the composite grease effectively eliminated surface defects in the bearing raceways, thereby significantly reducing vibrations caused by surface irregularities and friction imbalance. In the frequency-domain analysis, the spectral data for commercial grease displayed numerous high-frequency components and significant broadband noise, resulting from resonant phenomena induced by micro-spalling or electrical erosion defects on the raceway and roller contact surfaces. Conversely, the high-frequency components for composite grease were markedly reduced, with the spectral distribution becoming more concentrated and broadband noise significantly diminished. Among the units, wind turbine units 1# and 2#, which had larger initial defects, showed particularly notable improvements in vibration amplitude and high-frequency components. In contrast, unit 3#, which had a relatively better initial condition, exhibited a slightly smaller improvement, though the overall trend remained consistent. This further demonstrates the effectiveness of composite grease in eliminating contact surface defects and optimizing lubrication conditions, thereby improving the operational state of the friction pair and significantly suppressing the generation of high-frequency vibrations.

3.3. Worn Surfaces

The surface conditions of the bearing raceways using composite grease and commercial grease show significant differences, as illustrated in Figure 5. On the raceway surfaces using commercial grease, numerous deep scratches, pits, and localized fatigue spalling are observed, indicating that the bearings experienced severe frictional wear during operation. This damage mainly stems from the lack of lubricant film formation ability; under high-speed and long-time running conditions, the lubricant film is prone to rupture, which exacerbates the plastic deformation of the surface of the material and ultimately leads to surface degradation. In contrast, the raceway surfaces lubricated by the composite grease are smoother overall, with significantly fewer scratches and pits, and the integrity of the surface morphology is significantly enhanced, demonstrating the advantages of maintaining the dynamic balance of the lubricant film.

As shown in Figure 6, the surface of the raceway with composite grease formed a repair layer with thicknesses of 14.88 and 11.07 μm, and the surface roughness was 0.45 μm, indicating that a uniform and stable boundary lubrication film was formed during the lubrication process, which helped to inhibit the expansion of microscopic damage. Additionally, as shown in

Table 2. Hardness and modulus of bearing raceway surfaces using composite grease.

Test Point	Hardness (Gpa)	Modulus (Gpa)
1	6.017	201.400
2	6.027	198.66
3	6.478	213.250
Average value	6.174	204.439

, the average surface hardness and modulus of the bearing raceways using composite grease are 6.174 and 204.439 GPa, respectively. The measurement results indicate that the hardness and modulus of the raceways using composite grease exhibit good uniformity, with minimal fluctuations among the three measurement points. The raceway hardness values and modulus of the composite grease in the repair layer showed good numerical levels, helping to resist wear and fatigue during rolling contact [21,22]. These mechanical properties reflect that the composite grease has good stability and reliability under long-term high-speed operation conditions, providing a strong guarantee for the long-term stable operation of wind turbines.

3.4. Wind Turbine Power

The power curves of units 1#, 2#, and 3# were extracted, as presented in Figure 7. At a wind speed of 10 m/s, all three units operating with composite grease achieved full power output. This enhancement is attributed to the formation of a stable boundary lubrication film by the composite grease on the frictional interfaces, which effectively mitigates the adverse effects of wear and surface roughness on turbine performance. Additionally, it minimizes energy losses and mechanical degradation caused by friction. In contrast, when using commercial grease, units 1#, 2#, and 3# required wind speeds of 11.5, 11, and 11 m/s, respectively, to reach full power output. This difference primarily results from the inferior strength and shear resistance of the lubrication film formed by commercial grease under high-load and high-shear conditions, limiting its ability to reduce frictional losses at lower wind speeds. More importantly, under the same wind speed conditions, units using commercial grease relied on higher wind speed inputs to compensate for power deficits caused by frictional losses. Therefore, composite grease demonstrates clear advantages in minimizing frictional losses and enhancing the operational efficiency of wind turbine units.

4. Conclusions

Through field testing and surface analysis, this study systematically evaluates the performance advantages of composite grease in wind turbine applications, offering valuable insights for meeting the lubrication requirements of generator bearings. The primary conclusions are as follows: First, compared to commercial grease, composite grease exhibits significant advantages in reducing bearing temperature rise, lowering vibration amplitude, and suppressing high-frequency vibration signals. Second, wind turbine units utilizing composite grease are capable of achieving full power output at a wind speed of 10 m/s, whereas those using commercial grease require higher wind speeds to attain equivalent performance. By minimizing frictional losses, composite grease notably enhances the overall operational efficiency of wind turbine units. Third, surface analysis further confirmed that composite grease significantly improved the mechanical properties of bearing raceways. Specifically, it formed a uniform and continuous repair layer that increased the surface hardness to 6.174 GPa and the elastic modulus to 204.439 GPa.

Although this study verified the performance advantages of the composite grease, certain limitations still exist. First, a direct comparison with commercial greases in terms of raceway repair layer thickness, hardness, and modulus is lacking. Second, the service life and aging behavior of the grease were not investigated, making it difficult to comprehensively assess its long-term stability. In addition, this paper has not revealed the interfacial mechanism of additives from the microscopic level. In the future, we will introduce commercial greases to carry out systematic comparisons, combined with molecular dynamics simulations to deeply explore the interfacial behavior of MSH, and provide theoretical support for grease optimization and life prediction.