Vibration analysis of rolling element bearings with various defects under the action of an unbalanced force

In this paper, a method based on the finite element vibration analysis is presented for defect detection in rolling element bearings with single or multiple defects on different components of the bearing structure using the time and frequency domain parameters. A dynamic loading model is proposed in order to create the nodal excitation functions used in the finite element vibration analysis as external loading. A computer code written in Visual Basic programming language with a graphical user interface is developed to create the nodal excitations for different cases including the outer ring, inner ring or rolling element defects. Forced vibration analysis of a bearing structure is performed using the commercial finite element package I-DEAS under the action of an unbalanced force transferred to the structure via a ball bearing. Time and frequency domain parameters such as rms, crest factor, kurtosis and band energy ratio for the frequency spectrum of the enveloped signals are used to analyse the effect of the defect location and the number of defects on the time and frequency domain parameters. The role of the receiving point for vibration measurements is also investigated. The vibration data for various defect cases including the housing structure effect can be obtained using the finite element vibration analysis in order to develop an optimum monitoring method in condition monitoring studies.     

变工况下滚动轴承故障注入静电监测方法研究

采用静电监测方法研究变工况下滚动轴承不同故障注入程度对应静电信号特征.将自主研发的磨损区域静电传感器(WSS)搭载滚动轴承实验平台,利用电火花加工技术(EDM)对实验轴承进行不同程度和位置故障注入,在变工况条件下进行了一系列滚动轴承静电监测实验.对实验中各阶段静电信号进行处理,提取均方根值(RMS)作为结果分析的特征值.实验结果表明,同一工况下轴承静电RMS随故障程度的增大而增大,证明静电感应方法可用于在线监测滚动轴承故障程度的变化,且比振动更为敏感;不同工况下轴承静电RMS受转速影响大于载荷;轴承故障注入位置的改变对轴承静电信号也存在影响.进一步利用多元回归分析给出该轴承静电RMS与各参数间的具体关系式,其判断系数R2达到97.7％且满足F检验和T检验,证明拟合效果良好.该研究结果为进一步利用静电监测方法进行轴承的故障诊断与寿命预测及实现仪器产品化奠定了基础.     

Effect of rotational speed fluctuations on the dynamic behaviour of rolling element bearings with radial clearances

The behaviour of rolling element bearings is of major importance to the vast majority of rotating machines. Due to the varying assembly compliance and the internal clearance existing in the bearing in most cases, a highly non-linear behaviour of the rotor bearing system appears. Thus, a dynamic model of a horizontal rotor supported on ball bearings with radial internal clearance is considered, taking into account contact forces between the balls and the races, as well as the effect of varying compliance and of the internal radial clearance. However, since rotational speed fluctuations of various levels practically appear in all rotating equipment, their effect is now additionally considered. Their effect is examined in two characteristic rotor bearing cases. The analysis includes all the characteristic states of the dynamic behaviour of the rotor, such as periodic, unstable periodic and chaotic responses, using methods like frequency spectra, phase spaces, higher-order Poincare maps and Lyapunov exponents. All results presented show a dominant stabilization effect of the speed fluctuations with respect to the system behaviour. From the analysis performed, it is concluded that even a minimum fluctuation of the rotor speed may result in major changes of the system dynamics, indicating that speed fluctuations of the rotor are a governing parameter for the dynamic behaviour of the system.     

Faults diagnosis of rolling element bearings based on modified morphological method

In order to effectively smooth noise and extract the impulse components in the vibration signals of defective rolling element bearings, a new modified morphology analytical method has been proposed. In this method, average of the closing and opening operator has been used as the morphology operator. Being the flat and zero adopted as the shape and the height of structure element (SE), respectively, the optimized length of SE is defined by a new proposed criterion (called SNR criterion). The effect of the new method is validated by both simulated impulsive signal and vibration signal of three defective rolling bearings with an outer, an inner and a rolling element faults and compared with Nikolaou’s method. The result shows that the proposed method has the superior performance in extracting impulsive characteristics of vibration signals, especially for the high level noise signals, and can implement better in diagnosis of defective rolling element bearing.     

基于动力学模型的滚动轴承磨损特性分析

本文建立了考虑轴承游隙、摩擦力和打滑的非线性滚动轴承动力学模型,并将动力学模型与Archard磨损理论结合,对轴承滚道磨损特性进行研究。首先通过动力学方程,计算出滚动轴承运行过程中的非线性接触力与接触表面滑移速度;然后将计算结果带入磨损模型中,获得滚动轴承外滚道磨损分布;根据磨损后的滚动轴承径向游隙,更新动力学模型,研究轴承磨损特性以及轴承振动响应,并研究不同表面粗糙度下轴承磨损分布的变化趋势,结果表明：表面粗糙度对轴承磨损性能有较大影响,轴承表面粗糙度σ=0.42μm时外圈磨损速度是σ=0.2μm的6.9倍。本文研究成果为滚动轴承系统运行状态评估与寿命预测提供了理论基础。     

An analysis of the mechanisms of grease lubrication in rolling element bearings

There are over eighteen billion bearings in use in the world, and most are grease lubricated. Grease is chosen for a variety of reasons: it gives low friction, is easily confined, and has a long lubricating life at low cost. In many applications, the grease is expected to lubricate effectively over a wide range of conditions and for extended periods. For such applications, lubricant failure leads to instantaneous and rapidly increasing damage of the components and eventually breakdown. Thus, an improved understanding of the fundamental aspects of grease lubrication has far-ranging technical and economic implications. Despite the overwhelming importance of this subject, very little is known about the mechanisms of grease lubrication and the relationship between composition, lubrication performance, and life. This paper attempts to summarise current understanding of grease lubrication mechanisms in rolling element bearings and to consolidate the different aspects of this complex topic.     

基于谱峭度法的滚动轴承故障诊断研究

共振解调是滚动轴承故障诊断中最常用的方法之一,然而其带通滤波器参数的选取通常比较困难.谱峭度法可以根据峭度最大化原则自动确定带通滤波器参数.采用一种基于峭度最大化的谱峭度法设计最优带通滤波器,利用包络分析进行故障诊断,通过实际轴承故障振动信号分析表明,该方法具有良好的效果.     

Appearance of craters on track surface of rolling element bearings by spark erosion

A theoretical investigation has been carried out to determine the time span for the appearance of craters by spark erosion on the track surfaces of rolling-element bearings operating under the influence of shaft voltages. An analytical model has been developed using the continuum theory of Griffith to determine energy per unit area required for the development of craters on the track surfaces. Also, an expression is deduced for the net energy transmitted per unit area on the track surfaces by the leakage of stored charge energy resulting in the appearance of craters under the effect of shaft voltage and bearing capacitance depending on operational and dimensional parameters of a bearing in a given span of operation. By the formulation of capacitive stored energy in a bearing using high resitivity lubricant, along with the developed model and expressions using the ratio of cycles to accumulate and discharge of the accumulated charge energy at the various levels of bearing to shaft voltage, the time span/cycles for the appearance of craters of specified dimensions by spark erosion has been determined. Matching time span/cycles has been ascertained by the investigation of a case history of the diagnosis of bearings that failed by crater formation. The analysis can also be used for determination of shaft voltage if the time span/cycles for the formation of known size craters and bearing capacitance are established.     

基于HHT的机电系统滚动轴承故障诊断

滚动轴承是旋转机械中最常用的零件,它也是最容易损坏的零件之一.滚动轴承的质量直接影响整个机械系统的运行.采用经验模态分解(Empirical Mode Decomposition,EMD)与Hilbert变换相结合的HHT(Hilbert-Huang Transform)方法,对滚动轴承的故障机理和故障特征进行分析.通过实际应用与传统的时域分析、频谱分析方法相比较,该方法更能提取滚动轴承故障特征,并且所得结果与理论上滚动轴承的故障特征是一致的,因此,HHT方法对滚动轴承故障诊断是有效的、可行的.     

Vibration response of spalled rolling element bearings: Observations, simulations and signal processing techniques to track the spall size

Fatigue in rolling element bearings, resulting in spalling of the races and/or rolling elements, is the most common cause of bearing failure. The useful life of the bearing may extend considerably beyond the appearance of the first spall and a premature removal of the bearing from service can be very expensive, but on the other hand chances cannot be taken with safety of machines or personnel. Previous studies indicated that there might be two parts to the defect vibration signal of a spalled bearing, the first part being originating from the entry of the rolling element into the fault (de-stress) and the second part being due to the departure of the rolling element from the fault (re-stress). This is investigated in this paper using vibration signatures of seeded faults at different speeds. The acceleration signals resulting from the entry of the rolling element into the spall and exit from it were found to be of different natures. The entry into the fault can be described as a step response, with mainly low frequency content, while the impact excites a much broader frequency impulse response. The latter is the most noticeable and prominent event, especially when examining the high pass filtered response or the enveloped signal. In order to enable a clear separation of the two events, and produce an averaged estimate of the size of the fault, two approaches are proposed to enhance the entry event while keeping the impulse response. The first approach (joint treatment) utilizes pre-whitening to balance the low and high frequency energy, then octave band wavelet analysis to allow selection of the best band (or scale) to balance the two pulses with similar frequency content. In the second approach, a separate treatment is applied to the step and the impulse responses, so that they can be equally represented in the signal. Cepstrum analysis can be used to give an average estimate of the spacing between the entry and impact events, but the latter can also be assessed by an arithmetic estimation of the mean and standard deviation of the event separation for a number of realizations, in particular for the second approach. In order to determine the effects of various simulations and signal processing parameters on the estimated delay times, the entry and exit events were simulated as modified step and impulse responses with precisely known starting times. The simulation was also found useful in pointing to artefacts associated with the cepstrum calculation, which affect even the simulated signals, and have thus prompted modifications of the processing of real signals. The results presented for the two approaches give a reasonable approximation of the measured fault widths (double the spacing between the entry and impact events) under different speed conditions, but the method of separate treatment is somewhat better and is thus recommended.     

A study on the sound of roller bearings operating under radial load

The purpose of the paper is to numerically investigate the sound characteristics of roller bearings operating under radial load. For the sake of simplifying the analysis, it was assumed that the roller bearings are infinitely long, a noise source due to pressure fluctuation of oil film is taken as a line noise source, and acoustic energy losses in the bearing are neglected. To obtain sound characteristics of the bearing, the rolling contact load and the sound pressure level distributions were calculated for various operating conditions, and the results were compared with each other. How much the bearing noise can be affected by the design parameters, such as radial clearance of the bearing, viscosity of lubricant, and the number of rollers was also investigated. The results and discussions of the present paper could aid in the design of low-noise roller bearings and to solve the noise problems related to such bearings.     

A fault diagnosis method for rolling element bearings based on ICEEMDAN and Bayesian network

As commonly used components in rotating machinery, rolling element bearings (REBs) can fail due to complex working conditions and high-speed rotation. The failure of bearings may cause great damage. It is necessary to identify the faults of bearings to prevent property losses and heavy casualties. This paper proposes a fault diagnosis approach based on improved complete ensemble empirical mode decomposition with adaptive noise (ICEEMDAN) and Bayesian network. The intrinsic mode functions (IMFs) extracted by ICEEMDAN algorithm are applied to construct feature vectors based on the energy entropy, and then the fault diagnosis model of the bearing is constructed by Bayesian network. The influence of load and sampling frequency on diagnostic accuracy of the bearing with different fault types is studied in this paper. And the research results show that the ICEEMDAN-BN method can improve the uncertainty reasoning ability and accuracy of the developed fault diagnosis model.

     

双半外圈角接触球轴承滚动体裂纹分析

针对某双半外圈角接触球轴承在装机115min后钢球产生裂纹的情况,对故障轴承进行了一系列的测试和模拟试验工作。试验结果证明,钢球裂纹属材料外伤在冲压加工后造成的缺陷。     

Application of acoustic emission technique for the detection of defects in rolling element bearings

The usefulness of acoustic emission (AE) measurements for the detection of defects in roller bearings has been investigated in the present study. Defects were simulated in the roller and inner race of the bearings by the spark erosion method. AE of bearings without defect and with defects of different sizes has been measured. For small defect sizes, ringdown counts of AE signal has been found to be a very good parameter for the detection of defects both in the inner race and roller of the bearings tested. However, the counts stopped increasing after a certain defect size. Distributions of events by ringdown counts and peak amplitudes are also found to be good indicators of bearing defect detection. With a defect on a bearing element, the distributions of events tend to be over a wider range of peak amplitudes and counts.     

Grease lubrication of rolling element bearings — role of the grease thickener

The contribution of grease thickener to lubricant film formation was examined in this paper. Lubricant film thickness and friction were measured for different grease thickener types in a bearing simulation device. The results showed that the greases formed thick (20–80nm), low friction surface layers at low speeds, which were much greater than the corresponding base oil film. These films appeared to be formed by the physical deposition of thickener in the track during overrolling of the grease. This was confirmed by infrared reflection analysis, which showed the deposited films to have increased thickener content. The ability of grease to form renewable physically deposited solid films has significant implications for optimising lubricant formulation for certain applications, e.g. bearings operating at high temperatures and low speeds where a conventional elastohydrodynamic lubricating film would be inadequate. Copyright © 2007 John Wiley & Sons, Ltd.     

Vibration monitoring of rolling element bearings by the high-frequency resonance technique — a review

Vibration monitoring of rolling element bearings by the high-frequency resonance technique is reviewed. It is shown that the procedures for obtaining the spectrum of the envelope signal are well established, but that there is an incomplete understanding of the factors which control the appearance of this spectrum. Until the envelope spectrum can be fully explained, use of the technique is limited     

An adaptive SK technique and its application for fault detection of rolling element bearings

In this paper, we propose an adaptive spectral kurtosis (SK) technique for the fault detection of rolling element bearings. The primary contribution is adaptive determination of the bandwidth and center frequency. This is implemented with successive attempts to right-expand a given window along the frequency axis by merging it with its subsequent neighboring windows. Influence of the parameters such as the initial window function, bandwidth and window overlap on the merged windows as well as how to choose those parameters in practical applications are explored. Based on simulated experiments, it can be found that the proposed technique can further enhance the SK-based method as compared to the kurtogram approach. The effectiveness of the proposed method in fault detection of the rolling element bearings is validated using experimental signals.     

Analysis by the finite element method of hydrodynamic bearings operating in the laminar and superlaminar regimes

By adopting the non-linear theory proposed by Elrod and Ng, the generalized Navier-Stokes equations were modified and solved numerically for the flow field in the clearance space of a journal bearing.The performance characteristics of a finite circular hydrodynamic bearing (aspect ratio L/D = 1) were studied in terms of the Sommerfeld number, attitude angle, oil flow, friction coefficient and temperature rise parameter at various eccentricities for Reynolds' numbers up to 13 300.     

Sync-period frequency analysis and its application to the diagnosis of multiple element defects of rolling bearings

A method for diagnosing multiple element defects in rolling bearings has been investigated. The method combines the time-synchronous averaging and envelope spectral analysis techniques to produce spectra of synchronously averaged envelope signals with a range of synchronous frequencies. The spectra are displayed in the synchronous period versus frequency domain, to result in the sync-period versus frequency distribution. The distribution separates the characteristic defect frequencies and their associated sidebands in the synchronous period axis. This analysis technique makes it possible to detect and diagnose multiple defects appearing in different elements of rolling bearings. Another main benefit of the method is the significant noise reduction by both the enveloping and the synchronous averaging processes. Results from both computer synthesised data and experimental simulated data are presented.