2609 多軸鉆床設計,鉆床,設計 中文翻譯：軸承的振動分析Dr. S. J. Lacey舍夫勒(英國)有限公司工程經(jīng)理摘要：在生產(chǎn)過程中,滾動軸承振動產(chǎn)生的復雜幾何缺陷可能會導致滾動表面上的缺陷或相關組件的幾何誤差。噪音和振動在所有類型的設備中正變得越來越關鍵,因為它常常被認為是質量的代名詞并常被用于預測和維護。這篇文章是關于一些軸承振動的不同來源是如何隨著缺陷的頻率特征而展現(xiàn)出來的。文中給出了一些例子來說明了如何利用振動來進一步分析探測機器狀態(tài)的惡化。關鍵詞：軸承振動，維修工程，可靠性工程，振動探測。介紹：幾乎所有類型的旋轉機械都要用到滾動接觸軸承，他們的可靠性依賴于軸承選定的類型以及所有相關組件的精度,即軸、墊片、螺帽等。在假定軸承正確安裝、操作和維護時，軸承工程師一般使用疲勞和正常的失效模式。今天，由于制造工藝和材料的改進，一般情況下軸承疲勞壽命（與表面應力相關）并不是限制因素而且不必考慮那些不足3%的誤差。不過，盡管許多軸承因為污染、潤滑不良、極端溫度、惡劣的裝修、不平衡和不合理的維修而過早失效，所有這些因素導致軸承振動的增加，而且在使用了多年之后的狀態(tài)監(jiān)測仍然會對軸承造成災難性失效(與停工期的相關費用或身體機器其他部位的重大損害)。滾動軸承通常被用于噪聲敏感的電器中，如家電用電動機往往使用中小型軸承。因此軸承振動不論是從它對機器質量的重要性方面，還是從環(huán)境方面考慮，它都變得越來越重要。現(xiàn)在人們普遍認為安靜的運行形式是滾動接觸表面光潔度的代名詞。因此，軸承制造商已經(jīng)開發(fā)它們作為衡量質量和振動試驗的有效方法。一個常用的方法是把它安裝在一個安靜運行的主軸軸承上和對軸承的外圈點和三個頻段，分別是徑向速度。 50-300，300-1800和1800-10000赫茲。軸承必須符合三個頻段的均方根速度的限制。由于軸承振動信號是由機械結構修改的，所以在大多數(shù)情況下軸承振動不能被直接測量。這種情況下因為其它設備（如電動馬達，齒輪，皮帶）的振動進一步復雜化，會使那些未受過訓練的專家很難對振動數(shù)據(jù)做出解釋，在某些情況下會導致誤診斷，造成不必要的停機時間和成本增加。振動源：滾動軸承振動是一個復雜的系統(tǒng)，其組成部分包括：滾動體，內(nèi)滾道，外圈滾道和籠，相互作用產(chǎn)生復雜的振動。雖然滾動軸承是采用高精密機床加工，進行嚴格的質量控制和清潔，像其他制成品一樣，通過互動的滾動和滑動組合，他們有缺陷且表面產(chǎn)生震動。如今，雖然表面缺陷的幅度在納米級別，仍然可以在整個可聽頻率范圍（20赫茲- 20千赫）內(nèi)產(chǎn)生顯著的振動。該振動水平將取決于許多因素，包括沖擊能量，在哪個振動測量點和軸承的結構。變量合規(guī)： 即使軸承有非常完美的幾何外形，在承受徑向載荷下，軸承振動也是滾動軸承的固有特點，而非質量差的原因。這樣的振動類型通常被稱為變量合規(guī)，而且外部負載發(fā)生的原因是其位置由一個方面的負荷隨時間不斷變化造成的。（見圖1）圖1變剛度振動嚴重依賴于配套的滾動體外所加的負載數(shù)目，滾動體的數(shù)量越多，它的振動就越小。由于徑向加載或錯位軸承的游隙決定了外部負載區(qū)域，因此，一般情況下隨著間隙變合規(guī)，運行間隙不應與徑向內(nèi)部游隙（里克）相混淆，由于過盈配合運行中的內(nèi)、外圈熱膨脹的原因，前者通常是低于里克。變剛度振動水平可以比粗糙度和表面波紋度的滾動產(chǎn)生的更高。然而，在應用中至關重要的是振動可以通過使用正確的水平軸向預緊力來減少到一個可以忽略的水平。幾何缺陷：由于生產(chǎn)過程中使用的性質，生產(chǎn)軸承零件的幾何缺陷將始終存在，其不同程度根據(jù)軸承精度等級而定。對于受壓球軸承在中等速度運行下的臨界軋制表面形式和表面光潔度是軸承的噪音和振動的最大來源。因此控制部件波紋和在生產(chǎn)過程中的表面光潔度是至關重要的，因為它可能不僅有顯著影響，而且振動可能會影響軸承的使用壽命。為方便起見，考慮到滾動體，滾道的接觸寬度不完善之處幾何缺陷的波長 被稱為粗糙表面特征，而波長較長的特點被稱為波紋（見圖2）。圖2 表面粗糙度：表面粗糙度是振動的一個重要的來源，當其表面粗糙度高時，在滾動體與滾道的接觸處（見圖2）產(chǎn)生較厚的潤滑油膜。在此條件下表面粗糙可以突破潤滑膜，并與對方互動的表面，造成金屬與金屬的接觸。由此產(chǎn)生的振動，組成了一個激發(fā)所有的軸承和支撐結構的自然模式隨機序列。表面粗糙度產(chǎn)生的振動主要發(fā)生在六十次頻率軸承轉速上。因此頻譜的高頻部分通常顯示為一個系列的共振。用于估計相互作用的粗程度一個常見的參數(shù)是 lambda 比（Λ）。這是復合潤滑膜厚度，表面粗 糙度糙度比率，由表達式給出Λ = h (σЪ 2 + σr 2)0.5 其中 Λ=粗糙程度的相互作用h=油膜厚度σЪ=粗糙度球σr=滾道表面粗糙度 圖3如果我們假定滾道表面光潔度是滾動體的兩倍，那么對于一個典型的潤滑油膜厚度為0.3μm 的表面飾面比0.06微米更加需要實現(xiàn)三 Λ值和較低的相互作用發(fā)生率。如需0.1_m 表面潤滑油膜厚度比0.025_m完成須達到 Λ= 3。 Λ 對軸承的壽命的影響見圖3。如果 Λ 小于1，軸承將因破損而不可能達到估計的設計使用年限，這可能迅速導致滾動表面疲勞破壞。一般來說，Λ 比值大于三顯示完整的表面分離。從全彈流（彈流潤滑）過渡到混合潤滑（有些粗糙接觸彈流潤滑膜部分）發(fā)生在1至3Λ 范圍。波紋：對于更長波長的表面特征，與赫茲觸點相比峰值是低的，滾動運動是隨著表面輪廓與滾動體相連續(xù)的。表面間的幾何形狀和振動水平的關系是復雜的，視軸承和接觸幾何以及載荷和速度情況而定。波紋能夠產(chǎn)生振動頻率高達約三百次的旋轉速度，但通常是低于六十次的頻率轉速為主。上限是由于滾動體的滾道接觸的有限區(qū)域最終得到更短特點的平均波長。在滾動方向上，接觸時彈性變形減輕對簡單的諧波波形接觸寬度見圖4。 圖4在極限情況下，波長隨衰減水平的減小而提高，直到一個波長等于接觸寬度，波紋幅度在理論上是零。接觸長度也是短波長的衰減表面特征。通常較差的相關性可能存在于平行面之間高度的軌道，在不同的點抽取不同的配置文件，并全波紋度測量幅值平均值低一級。對于典型的軸承表面的平行面只有在更短的波長存在較差的相關性。即使是現(xiàn)代精密加工技術波紋也不能被完全消除，盡管在相對較低的水平，但波紋元素將始終存在。對于軸承本身，相關的零部件的質量也會影響軸承的振動而且任何軸的幾何外徑的誤差，可能在振動軸承的滾道中反映出來。因此，需要特別注意形式和所有相關的軸承零件的精度。離散缺陷：而表面粗糙度和波紋度的結果直接從軸承零部件生產(chǎn)制造工藝，離散的缺陷是指表面的滾動由于污染，操作，安裝，保養(yǎng)不善等而造成的過大的損害。這些缺陷可能會非常小，難以察覺，但可能對振動的關鍵設備產(chǎn)生重大影響或可能導致軸承壽命降低。此類缺陷可能有很多種類的形式。壓痕，劃痕和整個沿軋制表面，坑，碎片及潤滑油中的顆粒。軸承制造商對成品缺陷的檢測都采用簡單的振動測量，但這些往往受到軸承的類型和大小的限制。這方面的一個類型的測量例子如圖5（a）和5（b）所示，與一個良好的軸承相比，在軸承外圈滾道離散損害產(chǎn)生了典型的脈沖振動，具有較高的峰值/有效值比。圖5（a）圖5（b）在大量的缺陷發(fā)生個別峰值處不具有很明確的定義，但在 RMS 振動水平數(shù)倍于通常與軸承相關的良好條件的總和。參考文獻1.Harris T A, Rolling Bearing Analysis (4th Ed), Wiley, New York, 2001.2.Lacey S J, Vibration monitoring of the internal centreless grinding process, Part 1: mathematical models. Proc Instn Mech Engrs. Vol 24. 1990.3.Lacey S J, Vibration monitoring of the internal centreless grinding process, Part 2: experimental results. Proc Instn Mech Engrs. Vol 24. 1990.4.Wardle F P and Lacey S J, Vibration Research in RHP. Acoustics Bulletin.外文資料：An Overview of Bearing Vibration AnalysisDr. S. J. LaceyEngineering Manager, Schaeffler (UK) Ltd AbstractVibration produced by rolling bearings can be complex and can result from geometrical imperfections during the manufacturing process, defects on the rolling surfaces or geometrical errors in associated components. Noise and vibration is becoming more critical in all types of equipment since it is often perceived to be synonymous with quality and often used for predictive maintenance. In this article the different sources of bearing vibration are considered along with some of the characteristic defect frequencies that may be present. Some examples of how vibration analysis can be used to detect deterioration in machine condition are also given.Keywords: Bearing Vibration, maintenance engineering, reliability engineering, Vibration detectionINTRODUCTIONRolling contact bearings are used in almost every type of rotating machinery whose successful and reliable operation is very dependent on the type of bearing selected as well as the precision of all associated components, i.e. shaft, housing, spacers, nuts etc. Bearing engineers generally use fatigue as the normal failure mode, on the assumption that the bearings are properly installed, operated and maintained. Today, because of improvements in manufacturing technology and materials, it is generally the case that bearing fatigue life, which is related to sub-surface stresses, is not the limiting factor and probably accounts for less than 3% of failures in service.Unfortunately though, many bearings fail prematurely in service because of contamination, poor lubrication, temperature extremes, poor fitting/fits, unbalance and misalignment. All these factors lead to an increase in bearing vibration and condition monitoring has been used for many years to detect degrading bearings before they catastrophically fail (with the associated costs of downtime or significant damage to other parts of the machine).Rolling element bearings are often used in noise sensitive applications, e.g. household appliance electric motors which often use small to medium size bearings. Bearing vibration is therefore becoming increasingly important from both an environmental consideration and because it is synonymous with quality.It is now generally accepted that quiet running is synonymous with the form and finish of the rolling contact surfaces. As a result, bearing manufacturers have developed vibration tests as an effective method for measuring quality. A common approach is to mount the bearing on a quiet running spindle and measure the radial velocity at a point on the bearing’s outer ring and in three frequency bands, viz. 50-300, 300-1800 and 1800-10000 Hz. The bearing must meet RMS velocity limits in all three frequency bands.in most situations bearing vibration cannot be measured directly and so the bearing vibration signature is modified by the machine structure, this situation being further complicated by vibration from other equipment on the machine, i.e. electric motors, gears, belts, hydraulics, structural resonances etc. This often makes the interpretation of vibration data difficult other than by a trained specialist and can in some situations lead to a mis-diagnosis, resulting in unnecessary machine downtime and costs.In this paper the sources of bearing vibration are discussed along with the characteristic vibration frequencies that are likely to be generated.SOURCES OF VIBRATIONRolling contact bearings represents a complex vibration system whose components – i.e. rolling elements, inner raceway, outer raceway and cage – interact to generate complex vibration signatures. Although rolling bearings are manufactured using high precision machine tools and under strict cleanliness and quality controls, like any other manufactured part they will have degrees of imperfection and generate vibration as the surfaces interact through a combination of rolling and sliding. Nowadays, although the amplitudes of surface imperfections are in the order of nanometres, significant vibrations can still be produced in the entire audible frequency range (20 Hz – 20 kHz).The level of the vibration will depend upon many factors, including the energy of the impact, the point at which the vibration is measured and the construction of the bearing.Variable complianceUnder radial and misaligning loads bearing vibration is an inherent feature of rolling bearings even if the bearing is geometrically perfect and is not therefore indicative of poor quality. This type of vibration is often referred to as variable compliance and occurs because the external load is supported by a discrete number of rolling elements whose position with respect to the line of action of the load continually changes with time (see Figure 1).圖 1Variable compliance vibration is heavily dependent on the number of rolling elements supporting the externally applied load; the greater the number of loaded rolling elements, the less the vibration. For radially loaded or misaligned bearings ‘running clearance’ determines the extent of the load region, and hence, in general, variable compliance increases with clearance. Running clearance should not be confused with radial internal clearance (RIC), the former normally being lower than the RIC due to interference fit of the rings and differential thermal expansion of the inner and outer rings during operation.Variable compliance vibration levels can be higher than those produced by roughness and waviness of the rolling surfaces. However, in applications where vibration is critical it can be reduced to a negligible level by using ball bearings with the correct level of axial pre-load.Geometrical imperfectionsBecause of the very nature of the manufacturing processes used to produce bearing components geometrical imperfections will always be present to varying degrees depending on the accuracy class of the bearing. For axially loaded ball bearings operating under moderate speeds the form and surface finish of the critical rolling surfaces are generally the largest source of noise and vibration. Controlling component waviness and surface finish during the manufacturing process is therefore critical since it may not only have a significant effect on vibration but also may affect bearing life.It is convenient to consider geometrical imperfections in terms of wavelength compared with the width of the rolling element-raceway contacts. Surface features of wavelength of the order of the contact width or less are termed roughness, whereas longer wavelength features are termed waviness (see Figure 2).圖 2SURFACE ROUGHNESSSurface roughness is a significant source of vibration when its level is high compared with the lubricant film thickness generated between the rolling element-raceway contacts (see Figure 2). Under this condition surface asperities can break through the lubricant film and interact with the opposing surface, resulting in metal-to-metal contact. The resulting vibration consists of a random sequence of small impulses which excite all the natural modes of the bearing and supporting structure.Surface roughness produces vibration predominantly at frequencies above sixty times the rotational speed of the bearing. Thus the high frequency part of the spectrum usually appears as a series of resonances.A common parameter used to estimate the degree of asperity interaction is the lambda ratio (Λ ). This is the ratio of lubricant film thickness to composite surface roughness and is given by the expressionΛ = h (σЪ 2 + σr 2)0.5 where Λ = degree of asperity interactionh = the lubricant film thicknessσЪ = RMS roughness of the ballσr = RMS roughness of the racewayIf we assume that the surface finish of the raceway is twice that of rolling element, then for a typical lubricant film thickness of 0.3μm surface finishes better than 0.06 μm are required to achieve a Λ value of three and a low incidence of asperity interaction. For a lubricant film thickness of 0.1_m surface finishes better than 0.025 _m are required to achieve Λ=3. The effect of Λ on bearing life is shown in Figure 3.圖 3If Λ is less than unity it is unlikely that the bearing will attain its estimated design life because of surface distress, which can lead to a rapid fatigue failure of the rolling surfaces. In general, Λ ratios greater than three indicate complete surface separation. A transition from full EHL (elastohydrodynamic lubrication) to mixed lubrication (partial EHL film with some asperity contact) occurs in the Λ range .WavinessFor longer wavelength surface features, peak curvatures are low compared with that of the Hertzian contacts and rolling motion is continuous with the rolling elements following the surface contours. The relationship between surface geometry and vibration level is complex, being dependent upon the bearing and contact geometry as well as conditions of load and speed. Waviness can produce vibration at frequencies up to approximately three hundred times rotational speed but is usually predominant at frequencies below sixty times rotational speed. The upper limit is attributed to the finite area of the rolling element raceway contacts which average out the shorter wavelength features.In the direction of rolling, elastic deformation at the contact attenuates simple harmonic waveforms over the contact width (see Figure 4).圖 4The level of attenuation increases as wavelength decreases until, in the limit, for a wavelength equal to the contact width, waviness amplitude is theoretically zero. The contact length also attenuates short wavelength surface features. Generally poor correlation can exist between parallel surface height profiles taken at different points across the tracks and this averages measured waviness amplitudes to a low level. For typical bearing surfaces poor correlation of parallel surface heights profiles only exists at shorter wavelengths.Even with modern precision machining technology waviness cannot be eliminated completely and an element of waviness will always exist albeit at relatively low levels. As well as the bearing itself, the quality of the associated components can also affect bearing vibration and any geometrical errors on the outside diameter of the shaft or bore of the housing can be reflected on the bearing raceways with the associated increase in vibration. Therefore, careful attention is required to the form and precision of all associated bearing components. Discrete defectsWhereas surface roughness and waviness result directly from the bearing component manufacturing processes, discrete defects refer to damage of the rolling surfaces due to assembly, contamination, operation, mounting, poor maintenance etc. These defects can be extremely small and difficult to detect and yet can have a significant impact on vibration-critical equipment or can result in reduced bearing life. This type of defect can take a variety of forms, viz. indentations, scratches along and across the rolling surfaces, pits, debris and particles in the lubricant. Bearing manufacturers have adopted simple vibration measurements on the finished product to detect such defects but these tend to be limited by the type and size of bearing. An example of this type of measurement is shown in Figures 5(a) and 5(b) where, compared to a good bearing, the discrete damage on a bearing outer ring raceway has produced a characteristically impulsive vibration which has a high peak/RMS ratio. Where a large number of defects occurs individual peaks are not so clearly defined but the RMS vibration level is several times greater than that normally associated with a bearing in good conditio圖 5（a）圖 5（b）Bearing characteristic frequenciesAlthough the fundamental frequencies generated by rolling bearings are expressed by relatively simple formulas they cover a wide frequency range and can interact to give very complex signals. This is often further complicated by the presence on the equipment of other sources of mechanical, structural or electro-mechanical .REFERENCES1.Harris T A, Rolling Bearing Analysis (4th Ed), Wiley, New York, 2001.2.Lacey S J, Vibration monitoring of the internal centreless grinding process, Part 1: mathematical models. Proc Instn Mech Engrs. Vol 24. 1990.3.Lacey S J, Vibration monitoring of the internal centreless grinding process, Part 2: experimental results. Proc Instn Mech Engrs. Vol 24. 1990.4.Wardle F P and Lacey S J, Vibration Research in RHP. Acoustics Bulletin.