Investigation of the physical mechanisms in rolling bearings during the passage of electric current

With the increasing use of frequency converters as control units for electric motors, bearing failures caused by the passage of electric current through the bearings are more common. This paper discusses the most relevant parasitic current loops in frequency converter‐driven motors that can lead to damage of bearing raceways or deterioration of lubricants. Electrical stray capacitances, which are inevitably present in any electric motor, in combination with the high‐frequency components of the drive voltages, have been identified as the source of electric discharge machining (EDM) currents. Based on an EDM equivalent electric circuit, the passage of electric current through the rolling contact of a bearing is simulated. A set‐up is introduced to initiate EDM processes in bearings under well‐controlled electrical and mechanical conditions. The effect of EDM currents on bearing elements in the presence of different lubricants is studied. Initial results are presented.     

Tribology in electrical environments

This paper presents an overview of the author and others' work on the analysis of bearings and lubricants in electrical environments, and discusses recent advances in the study of the behaviour and response of rolling‐element bearings, hydro‐dynamically lubricated journal and thrust bearings, and lubricants, under the influence of an electric current. Bearings operating in electrical environments undergo specific and complex phenomena that affect bearing life, e. g., through crater formation and lubricant deterioration, and thus have to be studied in particular ways.     

Effect of operating parameters on the threshold voltages and impedance response of non-insulated rolling element bearings under the action of electrical currents

In the present work, the effects of operating parameters on the threshold voltages and impedance response of non-insulated rolling element bearings under the influence of varying levels of electrical currents have been studied. The voltage-current relationships in the bearings have been established.Investigations reveal that first and second threshold voltages appear under the influence of electrical currents in the bearings, depending on the lubricant resistivity, oil film thickness, bearing conditions and the operating parameters. The detected threshold voltages are primarily responsible for momentary flow of current and the further increase in current intensity with a slight change in potential drop across the bearings. The impedance of the bearings becomes negligible as the current intensity across the bearing increases. However, the impedance is more affected by the speed and film thickness than by the load on the bearings.This paper highlights the mechanism of the process of failure of the bearings, using lubricants with different characteristics. The investigations show that the experimentally evaluated threshold voltage coefficients of the bearings, using high resistivity lubricants, increase with speed and are independent of the loads on the bearings. This behaviour of the threshold coefficients of the bearings is used to predict the threshold voltages at different parameters of operation. However, bearings using low resistivity lubricants do not display the threshold voltage phenomenon.The investigation may also be used to determine the safe levels of potential drop across the bearing elements, to avoid damage of the bearings under different operating parameters and also to assess the film thickness by the measured impedance and current intensity response of the bearings.     

The deterioration of lithium greases under the influence of electric current—an investigation

In the present work, we report on the analysis of the deterioration of lithium greases used in rolling-element bearings by X-ray diffractometry (XRD), X-ray fluorescence spectrometry (XRFS), and atomic absorption spectrometry (AAS). These techniques give reproducible, reliable data with which to establish the severity of deterioration of greases recovered from the active zone of bearings. XRD appears more suitable for diagnosing quickly the chemical changes that have occurred in the soap residue of greases, even in small quantity, than other analytical and performance evaluation techniques. Fresh lithium greases, their chemical composition, and the formation of new compounds in the greases have been investigated after use in rolling-element bearings run in the presence of electric fields, and also under pure rolling friction conditions without electric fields on a roller-bearing test rig. The deteriorated greases recovered from various motor bearings have also been analysed and the results are compared with those for the greases from the test bearings. X-ray diffraction analysis shows that the chemical composition of the soap residue of the fresh lithium grease, lithium stearate (C₁₈H₃₅LiO₂), does not change in bearings operated without the effect of electric fields, under pure rolling friction. However, lithium iron oxide (Li₅FeO₂) peaks were detected after prolonged operation of the bearings. On the other hand, under the influence of electric fields, the chemical composition of the grease changed to lithium palmitate (C₁₆H₃₁LiO₂), and peaks of gamma lithium iron oxide (Γ-LiFeO₂) and lithium zinc silicate (Li_3.6Zn_0.2SiO₂) were detected. The surface of the bearings was found to be corroded after operation under electric fields. AAS showed a considerable increase in the lithium percentage, and lithium carbonate and lithium hydroxide in the used greases in aqueous solution, compared to the fresh grease The investigations reported in this paper, together with study of damaged/corrugated bearing surfaces, offer potential in diagnosing the cause of bearing failure in teh presence of electric fields, and in establishing the severity of deterioration of the lithium greases used in the bearings.     

超高速时电主轴轴承的动态支承刚度分析

基于滚动轴承受力分析的拟静力学和拟动力学模型,利用数值计算方法对超高速时电主轴轴承的内部动力学状态进行计算机模拟仿真,在求解每一个球滚动体动力学基本参量的基础上,计算电主轴轴承对转子支承的动态刚度,并结合具体算例,分析转速、轴承预载荷、径向外载荷等工况条件,以及套圈滚道曲率半径、球滚动体的直径和数量等轴承的内部结构尺寸、球材料的物理性能等方面的因素对轴承动态支承刚度的影响。分析结果表明,外部工况条件以及轴承内部的结构尺寸、球材料的物理性能等方面的因素对超高速时电主轴轴承的动态支承刚度影响较大,超高速时电主轴轴承的动刚度较静态和低速情况有着显著的变化。     

脂润滑轴承静置状态下漏油机理及对策

脂润滑轴承在静置状态下的漏油问题一直是一个难题。枯文旨在地润滑脂的分油及基础油在固体壁面的迁移分析，来探求脂润滑轴承润滑剂泄漏机理；同时介绍了在润滑脂中加入氟表面活性剂、改善轴承壁面结构设计两项措施来降低脂润滑轴承在静置状态下的漏油。     

Magnetic Flux Density Distribution on the Track Surface of Rolling-Element Bearings–-An Experimental and Theoretical Investigation

This paper brings out the theoretical model to determine the developed magnetic flux density on the track surfaces of inner race, outer race and rolling elements of a rolling-element hearing operated under the influence of electric current. The flux density, analytically determined, is compared with that of the flux density developed on the track surface of races and rolling elements of the bearings tested on the bearing test-up under the influence of electric current. Also, the residual magnetic flux density on the damaged bearings of motors and alternators has been measured and the theoretical model is used to determine the amount of current flow through the damaged bearings. The current flow through the bearings, thus established, has been compared with that of the current evaluated by the measurement of shaft voltage and bearing resistance.     

滚动轴承动力特性测试方法

轴承的动力特性对转子－轴承系统的动力特性有重要的影响。但至今，滚动轴承的动力特性仍缺乏系统的资料，在许多情况下，将滚动轴承简化为无阻尼的定常弹性元件。本文分析了滚动轴承动力特性的主要影响因素，介绍了滚动轴承动力特性的两种测试方法，并以某航空轴承为例作了具体测试，取得了比较一致的结果。本文提出的方法可作为建立滚动轴承动力特性数据库的基本手段。     

Occurrence of brittle flaking on bearings used for automotive electrical instruments and auxiliary devices

The higher transmission efficiency of engine accessory drive belts has resulted in a new type of bearing damage, called “brittle flaking” by NTN. This new type of bearing damage has been observed in bearings used in automotive electrical instruments and auxiliary devices. This paper summarizes recent test results which clarify the initiation mechanism of the brittle flaking damage.the type of grease used in the bearing was found to have a significant effect on the occurrence of brittle flaking. Thus, brittle flaking can be avoided by using specific greases. The amount of hydrogen measured in the inner rings, outer rings and balls of bearing run under acceleration-deceleration tests, was highest in the stationary ring, where brittle flaking was likely to occur. Brittle flaking was reproduced in rolling contact fatigue tests when the test specimens were charged with a sufficient amount of hydrogen prior to testing.The test results concluded that brittle flaking is caused by the diffusion of hydrogen into the steel due to the decomposition of the grease by a tribochemical reaction.     

Properties and applications of synthetic greases

Mineral oils are still predominant as base oils for lubricating greases, although increased numbers of greases have been developed with synthetic oils because of their good performance properties, including thermal/oxidation stability, low‐temperature fluidity, and plastics compatibility. Long life under high‐temperature conditions and good low‐temperature fluidity are required of greases used in bearings of engine electrical components. As many plastics are used in place of metals for the purpose of weight saving, synthetic hydrocarbon greases are used that do not have adverse effects on plastics. Various other special synthetic greases are also in use depending on specific requirements, such as conductivity and vacuum conditions. The properties and applications of various synthetic greases are discussed here.     

Determination of magnetic flux density on the surfaces of rolling-element bearings as an indication of the current that has passed through them—an investigation

This paper introduces a theoretical model to determine the developed magnetic flux density on the inner and outer surfaces of the inner race and outer race, and on the surface of rolling-elements of a rolling bearing operating under the influence of an electric current. The flux density, analytically determined, is compared with that of the flux density developed on the surfaces of races and rolling-elements of bearings tested on the bearing test-up under the influence of an electric current. The magnetic flux density on the surfaces of damaged bearings of motors and alternators has also been measured and the theoretical model is used to determine the amount of current flow through the damaged bearings. The current flow through the bearings, thus established, has been compared with that of the current evaluated by the measurement of shaft voltage and bearing resistance. The model has the potential to ascertain the cause of failure by current passage and to establish the amount of flow of leakage current through the bearings by determining the magnetic flux density on the surfaces of rolling-element bearings. The current flow, thus established, together with the measurement of the shaft voltage, allow the bearing impedance to be established.     

Variation and recovery of resistivity of industrial greases — an experimental investigation

In the present work, variation in the resistivity of different greases with time in static conditions, under the influence of applied potential drops, is established. The change in resistivity and physico-chemical characteristics of the greases in electric fields, and recovery of the resistivity within the test period in the absence of applied potential drop, have been found. The properties of the greases are compared, and their correlation with resistivity is analysed. The paper also gives the results of analysis using an SRV tester, IR spectra, chemical analysis and spectrophotometry, of changes that occurred periodically in the greases, normally used in non-insulated rolling-element bearings in the presence of a potential drop across the inner and outer races. The paper highlights the behaviour and mechanism of the process of bearing failure under electric fields.     

Rolling element bearings absolute life prediction using modal analysis

In this paper, the authors introduce an experimental procedure for predicting the fatigue life of each individual rolling element bearing separately using vibration modal analysis. The experimental procedure was developed based on a statistical analysis. A statistical analysis was performed to find an empirical model that correlates the dynamic load capacity of rolling bearings to their dynamic characteristics (Natural frequencies and damping). These dynamic characteristics are obtained from the frequency response function of each individual bearing that results from vibration modal analysis. A modified formula to the already known Lundberg-Palmgren life formula is proposed for rolling element bearings. Given the modified formula, one can predict the fatigue life of each individual rolling element bearing based on its dynamic characteristics. The paper compares the results from the modified formula with those from Lundberg-Palmgren formula. The modified formula provides an accurate prediction for the fatigue life of each individual bearing based on its dynamic characteristics. The experimental validation of the modified formula is considered for future work. Therefore, it can be used in various applications of rolling element bearings in machinery systems.     

Effect of fluid film wave bearings on attenuation of gear mesh noise and vibration

The attenuation of the gear mesh noise/vibration by fluid film wave bearings relative to rolling element bearings was experimentally investigated. Tests were performed on a gearbox that can accommodate both rolling element bearings and wave bearings. It was found that at specific speeds and torques, the wave bearings could significantly reduce the noise/vibration compared to rolling element bearings. Because the gear noise is accompanied by noise from other sources, a method was developed to extract from the original signal only the mesh harmonic components.The wave bearing dynamic coefficients were also predicted. It was found that adjusting the wave bearing parameters could considerably increase the capacity of the wave bearings to attenuate the gear mesh noise and vibration.     

Lubricant Replenishment in Rolling Bearing Contacts

It is generally assumed that starved lubricated contacts in rolling element bearings are replenished by side flow. The rolling elements partly push the lubricant to the side of the contacts after which the re-flow will provide fresh lubricant for the next rolling element. This re-flow is driven by surface tension and restricted by the viscosity. Hence, thick oils or greases may yield problems here, which is generally observed in practice. This paper quantifies this re-flow by means of a numerical model using a so-called thin layer/film assumptions. The results here show that this form of replenishment may happen to some extent in single contacts but for sure not in rolling element bearings. There are two reasons for this: the time between successive overrollings is too short and secondly, the centrifugal effects on the inner ring will drive the flow in a vertical direction rather than transversely. This applies to cylindrical roller bearings (CRB)-type of surfaces. This may be different for tapered or spherical bearings, which should be the topic of future research.     

A finite element model for rolling contact fatigue of refurbished bearings

Bearing refurbishing has become a popular method of extending the life of rolling element bearings. In the refurbishing process the raceways of the bearing may be ground to remove any surface damage prior to repolishing and reassembly with larger sized rolling elements. In the current study a continuum damage mechanics finite element model was developed to quantify the damage in original and refurbished bearings. After calculating the damage accumulation for a set number of contact cycles with the original bearing geometry, refurbishing is simulated by removing a layer of the original surface. The refurbished microstructural model is then subjected to additional computational contact cycles until a fatigue crack reaches the surface, signifying failure. This model preserves the fatigue damage accumulated prior to refurbishing and evaluates its influence on the refurbished bearing fatigue life. All refurbished bearing surfaces showed a significant amount of life after refurbishing with L₁₀ lives from the point of refurbishment, varying from 20% to 94% of the original L₁₀ life. The results indicate that the remaining life of the refurbished bearing population is inversely related to the time before refurbishing and is proportional to the depth of the regrinding. Results obtained from this investigation are in good agreement when compared to the Lundberg-Palmgren bearing life equation modified for analyzing the life of a refurbished bearing.     

Experimental measuring procedure for the friction torque in rolling bearings

The reduction of the power loss generated in mechanical transmissions and the use of low friction biodegradable lubricants has been attracting considerable attention in recent times. Therefore, it is necessary to develop methods to test and evaluate the performance of such lubricants and compare them with conventional ones. In this sense, a Four‐Ball Machine was modified allowing the test of rolling bearings. A 51107 thrust ball bearing was used to test two different greases and the corresponding base oils. Friction torque and operating temperatures were continuously monitored to quantify the power loss and the heat evacuation for each lubricant tested. Copyright © 2010 John Wiley & Sons, Ltd.     

Vibration response of rigid rotor in unloaded rolling element bearing

Rolling element bearings appear in nearly 90% of all rotating machinery. Their dynamic performance is often the limiting factor in the performance of the machines that use them. The specific construction of a bearing has a decisive influence on its dynamic behaviour. The paper defines a new vibration model of a rigid rotor supported by rolling element bearings. By application of the defined model, the parametric analysis of the influence of internal radial clearance value and number of rolling elements influence on rigid rotor vibrations in unloaded rolling element bearing was performed. The defined vibration model and parametric analysis were verified experimentally. The results of experimental analysis are also presented in this paper.     

轴承电蚀机制和防护技术的试验研究进展

随着高速铁路、新能源汽车、风电等行业的发展，轴承不可避免地工作于各种电场环境，电的介入使轴承产生了轴承电蚀这一新的损伤形式。阐述轴电压、轴电流的产生机制，分析轴电流引起的轴承材料性能弱化、表面烧蚀和润滑性能衰退等危害及机制；概述轴承电蚀失效试验方法和单接触点润滑滚动电接触等效试验方法；总结轴承电蚀防护的3种技术，包括源头抑制、绝缘抑制及导电通路，并展望轴承电蚀机制和防护技术的研究前景。     

Influence of Currents from Electrostatic Charges on WEC Formation in Rolling Bearings

White etching crack (WEC) early bearing failures occur when the rolling contact is subjected to a so-called additional load such as an electrical current flowing through the bearing, in addition to the pure rolling load (p_Hz). Tests on rolling bearings showed that a low electrical direct current flow, such as that resulting from electrostatic charges, can lead to WEC failures in oil-lubricated roller bearings and greased ball bearings.

The WEC formation in the performed tests was dependent on the current, electrical polarity, load type (rotating or stationary ring load), and bearing load. A black oxidation of the WEC critical bearing ring led to a significant increase in lifetime. Based on the findings, the failure hypothesis “cathodic WEC fatigue” for electrical direct current-initiated WEC failures was established.