Main Shaft Bearings Research Articles

Study on corrosion behavior and first-principle analysis of M50 steel in lubricating oil contaminated by salt water

The corrosive wear of M50 steel in lubricating oil contaminated with saline is a form of wear that often occurs within the main shaft bearings of aviation engines carried by aircraft working at sea. In the present paper, the corrosion behavior of M50 steel in lubricating oil contaminated with saline was studied. The open-circuit potential, polarization curve and impedance spectrum were analyzed by rotating electrochemical corrosion wear test. A three-layer interface microscopic molecular model was established with Materials Studio to simulate the dynamic corrosion evolution process of M50 steel self-matching pairs and the adsorption energy was calculated. The results show that the corrosion type is pitting corrosion. With the increase of saline concentration, the corrosion area becomes wider and shallower, and the size of the pitting core gradually decreases. The weight loss of M50 steel increases over time, but the trend slows down, which is also shown in the MS simulation results. Electrochemical tests show that the tribocorrosion of M50 steel is related to load, speed. As the load increases, the corrosion rate decreases. And, as the speed increases, the corrosion rate increases. This has practical significance for extending the understanding of tribocorrosion of M50 steel in marine atmospheric environments.

Thermal error prediction and reliability analysis of the main shaft bearing at wind turbines

During the operation of the gearless wind turbine, the phenomenon of heat generation in the main shaft bearing is inevitable and further affects the assembly preload. It is crucial to determine the effect of the thermal error on bearing assembly preload. In this paper, a reliability analysis method for main shaft bearing is proposed. Firstly, a finite element model for the thermal analysis of wind turbines is established based on heat transfer theory, and the thermal error of the preload is calculated. Subsequently, a reliability analysis of the main shaft bearing is conducted through Quasi-Monte Carlo simulation (QMCS) considering the influence of uncertainty factors. To further improve the computational efficiency, a surrogate model based on stochastic configuration network (SCN) is established to analyze the reliability and sensitivity of the bearing. Finally, the numerical example shows that the proposed model has high accuracy and applicability.

Spontaneous process of dispersed salt water droplets in lubricant oil establishing wetted areas: Settling, spreading, coalescing and de-wetting

Main shaft bearings determine aircraft engine's reliability and performance benchmark. Corrosion induced by salt water contamination in lubricant oil (SWCLO) is a big threat to bearing especially in coastal aircraft's engine. The recent progress in harm, purification and monitoring of SWCLO mixture contrasts the inadequate understanding about the critical process how droplet in SWCLO mixture is settled and distributed on bearing steel surface before corrosion occurrence. In present work, the complete processes towards reaching equilibrium states of single and multiple droplets were investigated. Hydrodynamic pressure involved model well predicts the overall falling of droplet over several millimeters. Smaller droplet has longer resting while faster spreading processes. Statistically, smaller droplet takes more time to be deposited accompanied by less probability, further smaller droplet keeps resting or drifting. There is a critical droplet size for deposition. Spreading induced coalescing causes droplet reshaping or de-wetting depending on causal and sessile droplet sizes. De-wetting indicates weakened corrosion. Spatial distribution of eventual sessile droplets is determined by deposition and coalescence at early stage of standing. These results are crucial for understanding, predicting and preventing aircraft engine's bearing corrosion from SWCLO mixture.

Understanding the thermal stability and friction behavior of ultrasonic shot peening-introduced gradient nanostructured M50 bearing steel

The M50 steel is a common material for main shaft bearings in aerospace applications. The thermal stability of gradient ultrafine grains/nanograins steels is of critical importance for engineering applications. Here, we fabricated gradient nanostructured M50 steel using ultrasonic shot peening and subsequent annealing. This study investigated the effect of annealing treatment on the microstructure evolution of gradient nanostructured M50 steel and the thermal stability of the USPed sample post-annealing. Furthermore, the friction behavior of gradient nanostructured M50 steel was also examined. After annealing at 350 °C for 30 min, ferrite phase at the depth span of 0–100 μm has better thermal stability, and formed a high-density dislocation network and cells at the top surface of the gradient layer. When annealed samples were continued annealed at 350 °C for 100 h, the average ferrite phase size at the depth span of 0–100 μm reduced to a fixed value (∼434 nm). High-density dislocation cells with smaller and more dispersed ordered B2 phases have a stronger pinning effect of dislocation. Hetero-deformation induced hardening by a dislocation network of grain interior and dislocation wall (∼200 nm thick) and precipitation hardening of dislocation cells with more dispersed ordered B2 phase is the main reason for the high microhardness of the samples annealing at 350 and 450 °C for 30 min. The USPed sample within the 100–200 μm depth span exhibits better friction properties attributed to the strengthened martensitic matrix, partial decomposition of spherical carbides, and the presence of high-density dislocations, which reduce the cracking of carbides and matrix, and carbide spalling. Although heat treatment results in a slight decrease in the friction properties of the USPed sample, they still surpass those of the tempered sample. This reduction in friction properties post-annealing primarily arises from the annihilation of high-density dislocations.

Compound fault diagnosis of wind turbine bearing under ultra-low speed operations using generalized sparse spectral coherence

Wind turbines are widely used in wind power systems, and have become the central pillar in the transition to renewable and low-carbon energy. As a key component, condition monitoring of mainshaft bearing (MB) is critical to ensure the safe and stable operation of wind turbines. Nevertheless, due to harsh operating environments and ultra-low rotating speeds, conventional diagnostics based on vibration analysis always perform poorly. More importantly, naturally damaged MB usually presents compound faults, which further increases the diagnostic difficulty. To resolve this issue, a generalized sparse spectral coherence using acoustic emission (AE) is proposed for compound fault diagnosis of MB. Firstly, to obtain synchronized rotating speeds, physics-inspired sparse learning is innovatively developed to estimate rotational speed from AE signals themselves. Subsequently, by introducing the lp/lq norm, a generalized sparse cyclic spectrum is constructed for compound fault diagnosis. On this basis, the data-driven optimization scheme is then designed to improve diagnostic accuracy. Finally, the superiority of the developed method is validated by simulation analysis and engineering applications. The results demonstrate that the proposed method can effectively perceive compound faults of MB operating under ultra-low speed, which may provide a useful tool for the health management of wind turbines.

Electrostatic discharge impacts on the main shaft bearings of wind turbines

Abstract. This paper studies the electrostatic discharge effect in wind turbines, a possible trigger source of the main bearing current. A lab setup with a charge generator and downsized wind turbine was built to verify the impact of electrostatic discharge on the main bearing current. In the test, a fatal amplitude for the bearing current was found at only −93 mV driven voltage on the shaft. Compared with the bearing current driven by the common-mode voltage, the electrostatic discharge effect triggers the bearing breakdown at a lower shaft voltage but much higher bearing current amplitude. The results demonstrate that the electrostatic discharge effect is a pattern of the bearing current in wind turbines and is much more dangerous to the bearing.

Analysis of the effect of maneuvering overload on the reaction force of the main shaft bearing in aero-engine

In order to study the effects of maneuvering overload on the main shaft bearing (abbreviation: bearing) reaction force in an aero-engine, a bending-torsion coupling dynamic model of the dual rotor system under maneuvering overload was established. Combined with the output parameters of the system model, the bearing reaction force analysis model was established. The maneuvering overload parameters are input into system model. Newmark-β and Newton-raphson methods are used to obtain the bearing reaction force response, and the change laws of bearing reaction force under the different maneuvering overloads are analyzed. The results show that gravity will make the transient response of bearing reaction force offset, and maneuvering overload will make the offset shift. Maneuvering overload will cause bearing to produce a circumferential asymmetric reaction force. For bearing 1, the additional gyroscopic torque in maneuvering overload has a significant effect on the dynamic eccentricity, resulting in a larger offset of reaction force of y direction. For bearings 3 and 4, reaction force of x direction has a larger offset. Under the influence of bent-torsional coupling, the combined frequencies of base frequencies of rotational speed appear in the reaction force spectrum diagram.

Using acoustic emission for condition monitoring of the main shaft bearings in 4-point suspension wind turbine drivetrains

ABSTRACT The continuous growth of wind power technology makes condition monitoring of wind turbine components crucially important for their operational efficiency. The main shaft bearings in wind turbines have been identified as one of the most critical components in the system, especially with the ongoing increase in rotor size and weight. This increase made the 4-point suspension drivetrain more preferable. In this study, we present a novel approach for condition monitoring of the main shaft bearings in a 2 Megawatt wind turbine with 4-point suspension drivetrain using primarily acoustic emission (AE). The focus was on the analysis of time and frequency domains of the AE signal, where the dominant frequency of each AE hit was identified and plotted back in the time domain to create the so-called dominant frequency map in specific time intervals for each bearing. A comparison between the two dominant frequency maps of the two bearings gives valuable insights into the condition of the two bearings. The distinctive nature of the dominant frequency bands in the dominant frequency maps presented promising potential for this method. The presented method is straightforward and can be automated and then integrated into a planned predictive maintenance programme for this wind turbine.

Time-dependent reliability-based design optimization of main shaft bearings in wind turbines involving mixed-integer variables

Main shaft bearings are essential transmission components in wind turbines. The failure of the main shaft bearings is inevitably accompanied by catastrophic consequences. In this regard, this paper presents a time-dependent reliability-based design optimization (TRBDO) approach for main shaft bearings involving mixed-integer variables. The maximization of fatigue life and the elastohydrodynamic film thickness under the load spectrum are selected as optimization objectives. The time-dependent reliability and geometric correlation are determined as nonlinear constraints. An efficient two-stage enrichment strategy is introduced to handle the time-dependent probabilistic constraint with mixed-integer design variables. A mixed-integer nonlinear optimization method is developed based on a meta-heuristic algorithm to solve the formulated TRBDO problem. Eventually, the effectiveness and robustness of the proposed approach are demonstrated by a real application in a 5 MW wind turbine.

Reliability Sensitivity Analysis of Main Shaft Bearings of Wind Turbines Subject to Subsurface Stress

As the core component of a wind turbine, the performance of main shaft bearings directly affects the transmission efficiency and reliability of wind turbines. To the best of our knowledge, few reliability analyses of wind power bearings have been carried out with the consideration of mechanical properties. In this paper, a finite element model is established to calculate the subsurface stress of the main shaft bearing of a wind turbine, considering the structural thicknesses, the friction conditions, and the interference conditions of the bearing. The randomness of several factors is considered, including structural thicknesses, material parameters, friction coefficients and the interference of the bearing. Latin hypercube design is used to get sample points, and the bearing’s mechanical responses of these sample points are analyzed. Through the data of these sample points, a Kriging model is established. The comparison with the finite element results shows that the Kriging model greatly improves the computational efficiency of the finite element model, with a relative error result of only 3.80 × 10−5. The Monte Carlo simulation method is applied to analyze the reliability and sensitivity of the bearing’s subsurface stress. The results show that an increase in the inner ring thickness will improve the bearing’s stress reliability, while an increase in other parameters will reduce the bearing’s stress reliability, including outer ring thickness, roller length, material elastic modulus, density, bearing and stationary shaft interference, and friction coefficients. The research results provide a reasonable reference for optimizing the design of the structure, assembly and material selection for main shaft bearings of wind turbines.

Correlative Method for Diagnosing Gas-Turbine Tribological Systems

Lubricated tribosystems such as main-shaft bearings in gas turbines have been successfully diagnosed by oil sampling for many years. In practice, the interpretation of wear debris analysis results can pose a challenge due to the intricate structure of power transmission systems and the varying degrees of sensitivity among test methods. In this work, oil samples acquired from the fleet of M601T turboprop engines were tested with optical emission spectrometry and analyzed with a correlative model. Customized alarm limits were determined for iron by binning aluminum and zinc concentration into four levels. Two-way analysis of variance (ANOVA) with interaction analysis and post hoc tests was carried out to study the impact of aluminum and zinc concentration on iron concentration. A strong correlation between iron and aluminum, as well as a weaker but still statistically significant correlation between iron and zinc, was observed. When the model was applied to evaluate a selected engine, deviations of iron concentration from the established limits indicated accelerated wear long before the occurrence of critical damage. Thanks to ANOVA, the assessment of engine health was based on a statistically proven correlation between the values of the dependent variable and the classifying factors.

Application of Life Cycle of Aeroengine Mainshaft Bearing Based on Digital Twin

Aeroengine mainshaft bearings are key components in modern aeroengines, and their main functions are to support the rotation of the main shaft of the aeroengine in harsh environments, such as high temperature, heavy load, high speed and oil break; reduce the friction coefficient during the high-speed rotation of the main shaft; and reliably ensure the rotation accuracy and power transmission of the aeroengine’s main shaft during operation. The manufacture of aeroengine mainshaft bearings requires complex processes and precise machining to ensure high performance and reliability, and how to intelligently complete the production and manufacture of mainshaft bearings and ensure the strength and accuracy of the bearings, quickly distinguish the fault types of the bearings and efficiently calculate, analyze and predict the life of the bearings are the current research hotspots. Therefore, building a high-fidelity and computationally efficient digital twin life cycle of aeroengine mainshaft bearings is a valuable solution. This paper summarizes the key manufacturing technology, manufacturing mode and manufacturing process based on digital twins in the life cycle of aeroengine mainshaft bearings, including the metallurgical process, heat treatment process and grinding process of aeroengine mainshaft bearings. It presents a fault diagnosis and life analysis of mainshaft bearings of aeroengines, discussing the key technologies and research directions of the life cycle of mainshaft bearings based on digital twins.

Understanding the recrystallization and phase transformation of gradient nanostructured M50 steel under the athermal effect of electrical pulses

M50 steel is a commonly used material for aeroengine main shaft bearings. In this study, a gradient nanostructured M50 steel was fabricated by an efficient surface nanocrystallization method, i.e., ultrasonic shot peening (USP) technology. The recrystallization behavior and phase transformation of gradient nanostructured M50 steel under the athermal effect of electrical pulses were investigated. Compared with the USPed sample, the ferrite structure size was 32.5% smaller, and the structure size uniformity was 51.5% greater in the 0–50 μm region after USP treatment and electrical pulsing treatment (EPT) with 38 A/mm2–1 s. Athermally induced ferrite recrystallization by EPT accompanied with obvious precipitation of VC and Mo2C carbides, and decomposition of austenite occurred during ferrite structure coarsening. EPT has different responses to different depths of gradient M50 steel, and there is an optimal current density to achieve structure refinement in different depths. However, the coarse-grain tempered M50 steel did not undergo obvious ferrite structure refinement under the same EPT parameters. The results revealed that the EPT promoted static recrystallization of incompletely recrystallized structures after USP processing by activating the release of stored deformation energy. Static recrystallization under the athermal effect of EPT was achieved by promoting the migration of the subgrain boundary to absorb the surrounding dislocations and increase the misorientation between itself and the surrounding ferrite structure. The mechanism of the athermal effect of EPT was based on electron-defect interactions, such as dislocations and lattice distortions. High-density nonequilibrium grain boundaries, acting as “active sites”, accelerating the process of recrystallization under the effect of electric pulses. This study proposed a method for optimizing EPT parameters through the change in instantaneous resistance to evaluate the degree of recrystallization of the gradient structure.

Health monitoring of the shaft bearings in a micro turbojet based on vibration analysis

In a recently implemented aerial target, the propulsion system consists of two JetCat micro turbojet engines. Due to the limited durability of the rotor support bearings, the engine operation time has been limited to two hours or two flights. In addition, after each flight, an engine bench test with vibration measurement is required. The adopted maintenance system is also labor-intensive and costly due to obligatory shaft bearing inspections performed frequently in the repair shop. These problems result from the limitations of the available high-speed bearing technology and the high rotor loads related to aircraft maneuvers in the air. The goal of the work is to develop a method for assessing the health of the main shaft bearings in the micro turbojet based on vibration signals. Methods known from full-scale jet engines were adapted for monitoring the health of the main shaft bearings. An accelerometer and rotational speed sensor were mounted on the engine. Vibration and other parameters of the engine were measured and analyzed, and then symptoms of damage were sought. The vibrations of seven JetCat engines were analyzed, selecting the one for further destructive testing. In all three axes X, Y and Z, the vibration components related to the rotational frequency and its orders are dominant, especially EO2 and EO3 related to rotor dynamics. Continuous engine vibration monitoring is a promising way of extending engine life and avoiding the deficiencies of microturbines related to their size and low-cost technology. The obtained results indicate that the tested JetCat engines have a life reserve, which has so far been used to a small extent.

Description of an HAWT hub enabling teetering about two out‐of‐plane axes with modeling showing significant reductions in cyclic behavior and fatigue due to turbulence and wind shear

AbstractA three‐bladed, dual‐axis teetering hub (3T) has been developed and compared to a three‐bladed rigid hub (3R) using computer modeling. Both hubs were fitted to a hypothetical 5‐MW offshore wind turbine mounted on an offshore floating platform. Cyclic analysis showed the 3T hub significantly reduced the impact of wind shear on the blades and virtually eliminated the impact of both turbulence and wind shear on the main shaft bearing and yaw bearing. Lifetime fatigue studies for the two hubs were performed using a rainflow‐counting program at various levels of turbulence and wind shear. Studies showed the significant reductions in cyclic variation provided by the 3T hub resulted in significant reductions in lifetime fatigue in comparison to the 3R hub. Simulations with the 3T hub showed an increase in turbulence caused a very small increase in blade fatigue but no increase in fatigue for the main shaft bearing and yaw bearing. In contrast, fatigue studies with the 3R hub showed an increase in turbulence caused significant damage to the blades and a combined increase in turbulence and wind shear caused extremely significant damage to the blades. This paper introduces a recently patented hub design that is superior to the ball‐and‐socket design described in a prior paper by this author.

Extreme-Low-Speed Heavy Load Bearing Fault Diagnosis by Using Improved RepVGG and Acoustic Emission Signals.

With the worldwide carbon neutralization boom, low-speed heavy load bearings have been widely used in the field of wind power. Bearing failure generates impulses when the rolling element passes the cracked surface of the bearing. Over the past decade, acoustic emission (AE) techniques have been used to detect failure signals. However, the high sampling rates of AE signals make it difficult to design and extract fault features; thus, deep neural network-based approaches have been proposed. In this paper, we proposed an improved RepVGG bearing fault diagnosis technique. The normalized and noise-reduced bearing signals were first converted into Mel frequency cepstrum coefficients (MFCCs) and then inputted into the model. In addition, the exponential moving average method was used to optimize the model and improve its accuracy. Data were extracted from the test bench and wind turbine main shaft bearing. Four damage classes were studied experimentally. The experimental results demonstrated that the improved RepVGG model could be employed for classifying low-speed heavy load bearing states by using MFCCs. Furthermore, the effectiveness of the proposed model was assessed by performing comparisons with existing models.

Comparative Wear Test of Journal Sliding Bearings with Sintered Bronze and Babbitt Alloy Bushes Lubricated by Environmentally Acceptable/Adapted Lubricants (EAL)

A growing awareness of the negative effects of mineral oils on the natural environment has resulted in the introduction of new regulations related to environmental protection. One of these regulations requires the use of environmentally acceptable/adapted lubricants (EAL) to lubricate marine main shaft bearings, in place of the mineral lubricating oils that have been used for decades. Classification Societies, which supervise the technical condition of ships, record a certain number of failures each year of heavy loaded stern tube bearings lubricated with modern, environmentally friendly lubricants. The reason often given for such failures is that the operating parameters of EAL lubricants are worse than those of mineral oils. The aim of the experimental research reported here was to compare the operating parameters and wear of EAL lubricated journal sliding bearings. For this purpose, two classic mineral lubricating oils and a group of four alternative lubricants from the EAL group were selected. The tests carried out for two types of bearing bushes—sintered bronze and Babbitt—did not show significant differences in terms of operating parameters or journal and bush wear for the entire group of tested lubricants.

Compound faults diagnosis method for wind turbine mainshaft bearing with Teager and second-order stochastic resonance

Low rotational speed and heavy load lead to the weak vibration energy and complexity of signals captured on wind turbine mainshaft bearing. Compared with single fault diagnosis, compound faults diagnosis is a challengeable task for the wind turbine mainshaft bearing. To extract fault features accurately, one fault diagnosis method based on Teager energy operator and second-order stochastic resonance (TSSR) is proposed. Firstly, Teager energy operator (TEO) is employed to achieve the purpose of enhancing impact energy in vibration signal. Then, under the assistance of residual noises, second-order stochastic resonance (SSR) is utilized for processed signal to enhance the fault features. Meanwhile, the modified signal-to-ratio (MSNR) index is carried out to select optimal parameters of SSR system automatically. Finally, the fault characteristic frequencies are identified by fast Fourier transform (FFT) spectrums of the SSR output. The test-rig artificial faulty bearing signal and wind turbine mainshaft bearing signal from wind farm are employed to illustrate the superiority and effectiveness of the proposed method. The contribution of the method provides a certain reference for the heavy-load and low-speed bearings fault diagnosis in other fields.

Dynamic reliability analysis of main shaft bearings in wind turbines

Tapered roller bearings (TRBs) are widely employed in large wind turbines as main shaft supports. The reliability of TRBs is directly related to the operational efficiency and safety of wind turbines. In this paper, a method for dynamic reliability analysis of TRBs used for supporting the main shafts of wind turbines is presented. First, a new quasi-static TRB model is established based on the meta-heuristic optimization algorithm. Then, the influence of preload, bearing clearance, and friction on the fatigue life of the TRB is investigated in detail. Moreover, considering the impact of uncertain factors, a dynamic reliability analysis of TRBs under fatigue load is conducted based on the composite limit state and adaptive double-loop Kriging theories. The effect of preload on the dynamic reliability of TRBs is assessed to guide the main shaft bearing assembly of wind turbines. Finally, a practical application example is provided to demonstrate the effectiveness and benefits of the proposed method.

Microstructure and tribological performances of M50 bearing steel processed by ultrasonic surface rolling

To prolong service life of M50 steel for the main shaft bearing, ultrasonic surface rolling process (USRP) was utilized to improve surface performances. The effects of USRP on surface integrity and tribological behavior were investigated. The results show the effect of USRP parameters on surface performances follows the order: feed rate > load > rolling times. During the USRP treatment, plastic deformation layer with 24 µm thickness is formed, and fine grain strengthening increases the microhardness (15.56%) and residual stress (79.61%) and decreases the surface roughness (−68.09%), respectively. Friction coefficient and wear rate of USRP sample are lower than those of untreated sample, because USRP helps form a smooth tribolayer and changes the main wear mechanism to slight adhesive wear.