滾動軸承故障振動分析

1、Detecting rolling element bearing faults with vibration analysisDetecting rolling element bearing faults is the highest priority for most vibration analysts. Detecting the fault at the earliest opportunity should be the priority, however in reality most analysts do not detect the fault in the first

2、or even the second stage of failure. This article is going to help you to detect faults at stage one so that you can truly be in control of your maintenance program.In this article I will describe the four stages of bearing failure and how to understand and successfully utilize the airborne ultrasou

3、nd, Shock Pulse, Spike Energy, PeakVue, enveloping/demodulation, time waveform analysis and spectral analysis methods. I will also explain why you should not rely on trending overall level readings.Reducing bearing faultsNo article of this nature can be complete without a discussion of the reasons w

4、hy bearings fail in the first place. Your first priority should be to minimize the causes of bearing failure. If you can do that successfully, then you will not need to rely on the vibration analysis techniques as much. That is not to say that I want to put vibration analysts out of work, or that yo

5、u should even consider downsizing your vibration monitoring program (because there will always be bearing failures and other mechanical faults the point is that the path to equipment reliability does not begin with vibration analysis.The fact is that if you properly purchase, transport, store, insta

6、ll, and lubricate your bearings, and you operate machines that are balanced, aligned and operating well away from natural frequencies, your bearings will last longer.You may not have control over many of these factors, but if you are involved in vibration analysis then there are two things you can d

7、efinitely do: look for the presence of conditions that will cause bearings to have a reduced life, and perform root cause analysis when you detect bearing damage.I opened this article by pointing out that the detection of rolling element bearing faults is the highest priority for most vibration anal

8、ysts. The sad truth is that for too many analysts it is the only priority. Unbalance, misalignment, soft foot, and resonance often have a much lower priority. Although these faults conditions appear first on most wall charts, they can be the trickiest to diagnose. Phase analysis is a powerful, yetun

9、derutilized tool that can greatly help in the detection of these fault conditions but that was the topic of an earlier article.The point is that these conditions put additional stress on the bearings, thus reducing their life. If you do not take care of these conditions, it is inevitable that you wi

10、ll soon see the earliest stages of bearing damage.The pattern of bearing damageBefore we get into the specifics of the four stages of bearing failure, I would like to describe how the vibration changes in general terms. In classical teaching, bearing vibration is all about the four forcing frequenci

11、es: ball pass outer race (BPFO, ball pas inner race (BPFI, ball spin (BSF, and cage or fundamental train frequency (FTF. We will discuss these in more depth in a moment, but first I want to describe the movement of “broadband energy”.If a bearing is poorly lubricated, we can detect an increase in th

12、e level of “noise” at very high frequencies. It is not a specific, single frequency; instead it will depend on a number of factors to do with the machines construction. Suffice to say that you cannot hear it; it is well above your hearing range.As the state of lubrication worsens, the level of the n

13、oise will increase, but the frequency of the noise will slowly reduce it will move from very high frequencies to high frequencies. That is not to say that you cant detect the condition at lower frequencies; it is stronger at the higher frequencies.As the film of lubricant between the bearing surface

14、s is reduced further, we will have more and more metal-to-metal contact, causing “stress waves” to be generated. Stress waves (also referred to as “shock pulses” are like ripples in a pond; the moment the metal surfaces make contact, a wave of energy races away from the point of contact at the speed

15、 of sound. It all happens very quickly; possibly in less than a thousandth of a second!Even if the root cause of the bearing fault is not poor lubrication, if the bearings are damaged through poor installation, false brinelling (where the bearing has been vibrating whilst it is stationary, EDM, misa

16、lignment, or any one of a number of reasons, there will come a time when there is either metal to metal contact between two surfaces, or the stress waves will be generated from beneath the surface of the metal as subsurface defects develop.The subsurface defects will slowly develop due to the extrem

17、e forces experienced within the bearing. The difference is that these defects are likely to be localized; at the bottom of the outer race for example. The “noise” from the bearing due to poor lubrication is relatively constant (it is random, therefore not periodic, whereas when a fault condition dev

18、elops (e.g. a crack or spall, a new source of periodic vibration will be introduced. If the damage was on the outer race, each time a rolling element passes that location there will be a spike in the vibration. When the point of damageis between rolling elements, there is no vibration (well, less vi

19、bration. The good news is that we can calculate the frequency of this vibration (we can determine how often the rolling element will pass that point. The bad news is that the vibration is very, very low in amplitude Figure 1 vibration “spikes” that result from the rolling element coming into contact

20、 with the damaged area on the outer race.As the amount of damage increases, we witness more frequent contact between the metal surfaces. As cracks develop, or the subsurface defects grow and eventually break through to the surface, the vibration will change in three key ways:1.The vibration will be

21、periodic making it easier for us to understand the natureof the fault. Damage on the outer race has a different vibration pattern and frequency to damage on the inner race, rolling elements or cage.2.The “broadband energy” witnessed will reduce in frequency. It will slowlymove from the very high fre

22、quencies to the high frequencies, and eventually,to the low frequencies. When the bearing is quite badly damaged the vibration will be in your hearing range and it can be detected with conventional velocity spectra. When the bearing is very badly damaged we will see “hay stacks” in the spectrum and

23、it will cause the “noise floor” of the velocity spectrum to lift up3.The forces involved will increase in strength and thus the vibration willincrease in amplitude, making the fault easier to detect.The power of knowledgeThere may be two questions on your mind right now: why do I need to know that t

24、he bearing has very slight damage if it still has a number of months of life left in it, and why do I need to know whether the damage is in the inner race, outer race, rolling elements or cage?They are good questions!There are two basic goals in vibration analysis: stop machines (bearings from catas

25、trophic failure, and provide the intel that puts the maintenance (and production departments in control of the machines. If you know that a fault is developing at an early stage, you can decide what action is most appropriate. Based on the criticality of the machine, the availability of spares, the

26、demands on production, and the existing plans for maintenance, you can decide what action to take. Knowing the nature and severity of the fault condition puts you in control. And to clarify one point; the time to failure, and the nature of the failure is different for inner race, outer race, rolling

27、 element, and cage defects.ResonanceAt this stage we need to introduce another term resonance. When you strike a bell, it will vibrate at a specific frequency. In fact, in addition to the one dominant frequency you hear, it actually vibrates at a large number of frequencies. Bells of different sizes

28、 and shapes vibrate at different frequencies it all depends on their mass and stiffness. The amount of vibration (or sound in this case we hear is based on the amount of force used to strike the bell, and the amount of damping. Well, machines and bearings (and the accelerometer we use to measure the

29、 vibration, act in the same way they will all vibrate naturally when “excited”. Thanks to the “broadband energy” generated due to poor lubrication, the resonances will be excited. When metal-to-metal contact occurs, and when defects appear (subsurface, spalls, cracks, etc. these forces will again ex

30、cite the resonances.Now, at this point you may be wondering what resonances have to do with bearing faults. They are important for two reasons; the resonances amplify the vibration, making it easier to measure, and we can utilize the resonance in the accelerometer to further amplify the vibration. R

31、emember, in the early stages of bearing wear the vibrationamplitudes are very low, while in comparison, the vibration due to unbalance, misalignment and other sources is VERY high in comparison.Bearing fault detection technologiesThere are a number of technologies that can be utilized to detect bear

32、ing faults. The following is a summary of those techniques:Airborne Ultrasound:Also known as acoustic emission, the high frequency (above our hearing range vibration from the bearing can be monitored, either via a dB reading that can be trended, or by listening through headphones (the sound is heter

33、odyned so that it is in our audible range. If the bearing is in good condition, the bearing should make a muffled, smooth sound. If you hear a high-pitched rushing sound, or a crackly sound, the bearing may require lubrication, or it may be damaged. It is a simple method, so it can be used frequentl

34、y in order to detect a fault that can then be further examined by one of the vibration analysis methods described below. This method can even be used during the lube rounds to avoid under- or over-greasing as long as it is done with great care. Shock Pulse Method: SPM®The SPM company developed

35、a vibration sensor in the 1960s that will resonate in a predictable way at approximately 30 kHz. When there is inadequate lubrication, and in the earliest stage of a bearing failure, the sensor will resonate. The “carpet” level of the vibration is monitored, as are the peaks, or “spikes” that occur

36、duemetal-to-metal contact. If the sensor is mounted correctly, the Shock Pulse Method, also used by the PRÜFTECHNIK company, can indicate the nature of the lubrication problem and the severity of the bearing fault. It can also be used in a similar way to the enveloping technique, which provides

37、 a spectrum and waveform for detailed analysis.Spike Energy:Developed by the IRD Mechanalysis company (bought by Entek, then by Rockwell in the 1970s, the Spike Energy method utilizes the mounted resonance of the accelerometer in a similar way to the Shock Pulse Method. The vibration from the sensor

38、 (filtered around the resonant frequency goes through a process that holds the peak levels so that a gSE reading can be trended. In addition, a spectrum and time waveform can be displayed in a similar way to classical enveloping. It is very important that the same sensor is used for each measurement

39、, because a different sensor will produce different amplitude readings.Enveloping and DemodulationThese two names are used to essentially describe the same process. The process is used to perform two functions: remove the high amplitude, low frequency vibration (which would otherwise swamp the low a

40、mplitude bearing vibration, and convert the high frequency “spikes” into a low frequency signal so that waveform and spectrum analysis can be performed. In this example, the time between each “spike” is the time that it takes for each rolling element to roll over the damaged area on the outer race i

41、t relates to the ball pass outer frequency (BPFO as shown in figure 1. It is essential to set the high pass filter correctly so that the vibration that remains only comes from the bearing (and not from the gearbox, for example. Or The envelope spectrum will contain noise if there are no faults, and

42、peaks (and harmonics at the bearing forcing frequencies if there is a fault. The amplitude of these peaks will increase as the fault develops, and the noise floor will lift and swallow the peaks when the bearing is in the last stage of bearing failure.PeakVue®The PeakVue method, developed by CS

43、i (Emerson Process Management, provides a spectrum and waveform that is used to detect bearing faults at an early stage. This method does not relying on the sensor mounted resonance, and it has an important difference to the enveloping technique. The analog signal from the sensor is sampled at a ver

44、y high frequency (102 kHz in order to capture the short-duration stress waves. Using a peak hold algorithm, the waveform (and resultant spectrum viewed by the analyst retains the peak levels making it trendable. A high pass filter is used to remove the low frequency, high amplitude signals. As with

45、all the methods, it is important to select the correct filter setting.The four stages of bearing failureBearing failure has been classically described as occurring in four stages. (I personally prefer to expand the description because, from a vibration point of view, we see more than four changes.St

46、age oneIn stage one, the damage is minor the bearing still has 10% to 20% of its L10 life. If you were to remove the bearing at this stage you may not see any damage; the damage is predominantly sub-surface. At this stage you should continue to monitor the bearing, but you should also consider, and

47、remedy, the root causes: lubricate the bearing, check the balance and alignment, correct any resonance conditions, and so on. Stress waves will be generated when there is metal-to-metal contact, however it may be random (non-periodic until sub-surface defects develop. The airborne ultrasound (acoust

48、ic emission, Shock Pulse, Spike Energy, PeakVue and enveloping techniques can all be used at this stage, however the level of success will depend greatly on the way the sensor is mounted, and the filter settings chosen. Stage TwoAs the fault continues to develop, the sub-surface defects will grow, e

49、ventually breaking through to the surface, causing spalls, cracks, flakes, etc. The vibration pattern will gradually change as a result. The force of the impacts will be greater, and there will definitely be periodicity to the vibration. Now there is only 5% - 10% of the L10 life remaining. Again yo

50、u should consider the root causes and check the lubrication; howeveryou should also monitor this bearing more frequently. The high frequency techniques such as Shock Pulse, Spike Energy, airborne ultrasound, and PeakVue will continue to be effective. Enveloping (demodulation will also be effective,

51、with peaks visible at the bearing forcing frequencies (BPFO, BPFI, BSF and FT depending on the nature of the fault along with harmonics. Harmonics of the bearing forcing frequencies may also be visible in the acceleration spectrum, and time waveform analysis may show signs of the fault (especially f

52、or lower speed machines. Peaks may be visible in the velocity spectrum, but probably not until late stage two,when the damage is more pronounced. Stage ThreeNow the damage is more significant. If you removed the bearing you would see the damage. The bearing has less than 5% of its L10 life at this p

53、oint. It is now time to replace the bearing; unless the risk of failure can be offset by the need to continue running the machine. The high frequency techniques will still indicate the presence of a fault. The Shock Pulse and Spike Energy (gSE readings will still trend upwards. The peaks in the enve

54、lope spectrum will continue to grow in amplitude. In stage three you will definitely see peaks in the velocity spectrum that correspond to the bearing forcing frequencies (BPFO, BPFI, BSF and FT depending on the fault condition.If there is damage on the outer race of the bearing (horizontally orient

55、ed machine, there will be harmonics of the BPFO frequency. Initially theharmonics may have a lower amplitude than the peak at BPFO, but as the fault develops the harmonics will grow to be greater in amplitude than the peak at BPFO.If there is damage on the inner race, there will be a peak at the BPF

56、I frequency.There will be harmonics of this frequency, and sidebands of the running speed will surround the fundamental and harmonics. The sidebands appear due to aphenomenon known as amplitude modulation. As the damaged area on the inner race moves in and out of the load-zone, the impacts will rise

57、 and fall (once perrevolution.If the rolling elements are damaged, there may be a peak at the BSF frequency, but more likely at twice that frequency (because damage on the ball or roller will impact the inner race and outer race per revolution. Again we will see harmonics and sidebands, however this

58、 time the sidebands spacing will equal to the FT (cage frequency because each ball/roller moves in and out of theload-zone with the cage. Figure x A rolling element bearing highlighting the load-zoneTime waveform analysis will also indicate the presence of the modulation, and impacts will be visible

59、 in the waveform. The time waveform shows you exactly what is happening inside the bearing each impact is visible. Care must be taken to select the correct record length and resolution so that it is possible to see the detail required. Stage FourNow the bearing has substantial damage. The bearing should be replaced; you are taking a significant risk of catastrophic failure to leave it in