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The Role of Vibration Monitoring in Predictive Maintenance
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The Role of Vibration Monitoring,in Predictive Maintenance. Dr S J Lacey Engineering Manager Schaeffler UK Limited. Unexpected equipment failures can be expensive and potentially catastrophic resulting in unplanned. production downtime costly replacement of parts and safety and environmental concerns. Predictive Maintenance PdM is a process for monitoring equipment during operation in order to identify. any deterioration enabling maintenance to be planned and operational costs reduced Rolling bearings. are critical components used extensively in rotating equipment and if they fail unexpectedly can result in. a catastrophic failure with associated high repair and replacement costs Vibration based condition. monitoring can be used to detect and diagnose machine faults and form the basis of a Predictive. Maintenance strategy, 1 Introduction The cost of not having a robust maintenance strategy should. not be underestimated It should not be looked at simply as. As greater demands are placed on existing assets in terms of an upfront cost but viewed as an investment to safeguard and. higher output or increased efficiency the need to understand protect key assets reducing the need for costly repairs and. when things are starting to go wrong is becoming more protecting the output of key processes In some industries. important Add to this the increasing complexity and maintenance is now the second largest or even the largest. automation of plant and equipment it becomes more element of operating costs As a result it has moved from. important to have a properly structured and funded almost nowhere to the top of the league as a cost control. maintenance strategy There is also a need to understand the priority in the last two or three decades. operation of equipment so that improvements in plant output. and efficiency can be realised In today s increasingly The need to contain costs and run plant for longer more. competitive world all of these issues are of key importance reliably means that there is a growing awareness of the need. and can only be achieved through a properly structured and to prevent unnecessary equipment failures Central to this is a. financed maintenance strategy that meets the business needs maintenance strategy which is based on monitoring key assets. to detect when things are starting to go wrong enabling plant. Maintenance can often be a casualty as businesses seek to outage to be better planned in terms of resource availability. save costs How often have we heard the words we have had spare components repairs etc As a result the risk of missing. no problems since the equipment was installed so we don t important contract deadlines is reduced and customer. need condition monitoring This is often borne out of confidence is improved. ignorance and not undertaking a proper risk assessment to. identify the criticality of existing assets so that the potential Until recently many industries have and still do take the. return on investment ROI of a properly funded maintenance reactive approach to maintenance since this has no upfront. strategy can be determined costs but can result in many hours or days of plant downtime. and or lost production While this may have been acceptable. The need to run a plant at a higher efficiency yet often with in the past the increasing complexity and automation of. fewer people puts increasing pressure on all concerned when equipment has meant this is not now a cost effective option. equipment fails prematurely When equipment does fail it is. often at the most inconvenient time either in the middle of a Having a clear and robust maintenance strategy fully. key process at a weekend or in the middle of the night when supported by senior management is becoming more important. obtaining replacement parts may be difficult and labour costs particularly in industries where it not only has a major impact. are high due to overtime While there is never a good time for on costs but also on the health and safety of employees and. equipment to fail the technology available today means that in situations where secondary damage and a catastrophic. there is simply no excuse for not taking the necessary steps failure may result. to protect key assets This can be achieved by minimising the. risk of early and unexpected failures through a properly. structured and funded maintenance strategy which will. ultimately reduce overall operational costs,2 Maintenance Approach 2 1 Reactive Maintenance. Maintenance is traditionally performed in either time based Reactive maintenance of machinery often referred to as the. fixed intervals as so called preventive maintenance or by run till failure approach involves fixing problems only after. corrective maintenance when a breakdown or fault actually they occur Of course this is the simplest and cheapest. occurs In the latter it is often necessary to perform the approach in terms of upfront costs for maintenance but often. maintenance actions immediately but in some cases this may results in costly secondary damage along with high costs as a. be deferred depending on the criticality of the equipment result of unplanned downtime and increased labour and parts. With predictive maintenance an advanced warning is given of costs Since there are no upfront costs it is often seen as an. an impending problem and repairs are only carried out when easy solution to many maintenance strategies or there is no. necessary and can be planned to avoid major disruption strategy at all. A summary of all three approaches is given in Figure 1 and. discussed briefly below In rotating equipment rolling element bearings are one of the. most critical components both in terms of their initial selection. Reactive Preventive Predictive and just as importantly in how they are maintained. Maintenance Maintenance Maintenance Bearing manufacturers give detailed guidelines as to what. DISADVANTAGES maintenance is required and when which is often overlooked. High risk of High replacement High upfront costs, catastrophic failure costs parts including equipment This can have disastrous consequences in terms of poor. or secondary replaced too early training quality output reduced plant efficiency or equipment failure. damage High repair, replacement costs Risk of early failure Monitoring the condition of rolling bearings is therefore.
infant mortality essential and vibration based monitoring is more likely to. Loss of key assets, due to high Human error during detect the early onset of a fault. downtime Lost replacement of,production missed repaired or new parts. contract deadlines 2 2 Preventive Maintenance,Parts may often have. Inventory high cost many years of, of spare parts or serviceable life With Preventive Maintenance PM machinery is overhauled. replacement remaining, on a regular basis regardless of the condition of the parts.
This normally involves the scheduling of regular machine plant. High labour cost, shutdowns whether or not they are required The process may. subcontracting cut down failures before they happen but it also leads to. High cost due to increased maintenance costs as parts are replaced when this. hire of equipment,is not necessarily required,Increased Health. Safety risks, There is also the risk of infant mortality due to human error. Environmental, concerns during the time the asset is taken out of service for repair. adjustment or installation of replacement parts Other risks. ADVANTAGES include installing a defective part incorrectly installing or. No upfront costs Maintenance is Risk of unexpected damaging a replacement part or incorrectly reassembling. e g equipment planned and helps to breakdowns are parts. training prevent unplanned reduced,breakdowns,Seen as an Equipment life is.
easy option Fewer catastrophic extended A frequent and direct result of preventive maintenance is that. failures resulting in much of the maintenance is carried out when there is nothing. expensive secondary Reduced inventory, damage labour costs wrong in the first place If the plant can be monitored in such. Maintenance can be, a way as to obtain advance warning of a problem significant. Greater control over, inventory planned and carried costs savings can be obtained by avoiding unnecessary repair. out when convenient work Such an approach is known as Predictive Maintenance. Reduced risk of,Health Safety,environmental,Opportunity to. understand why,equipment has failed,and improve,efficiency.
Figure 1 Comparison of different types of maintenance. 2 3 Predictive Maintenance Replacing a bearing in a gearbox is preferable to replacing the. whole gearbox and replacing a motor bearing is better than. Predictive Maintenance PdM is the process of monitoring the having to send the motor to a rewinder to make expensive. condition of machinery as it operates in order to predict which repairs and replace parts. parts are likely to fail and when In this way maintenance can. be planned and there is an opportunity to change only those At the heart of many Predictive Maintenance strategies is. parts that are showing signs of deterioration or damage Condition Monitoring which detects potential defects in critical. The basic principle of predictive maintenance is to take components e g bearings gears etc at the early stage thereby. measurements that allow for the prediction of which parts will enabling the maintenance activity to be planned saving both. break down and when These measurements include machine time and money and preventing secondary damage to. vibration and plant operating data such as flow temperature equipment which can often be catastrophic. or pressure,3 Identifying Asset Criticality, Continuous monitoring detects the onset of component. problems in advance which means that maintenance is Rolling bearings are used extensively in almost every type of. performed only when needed With this type of approach rotating equipment whose successful and reliable operation is. unplanned downtime is reduced or eliminated and the risk of highly dependent on the bearing type bearing fits and. catastrophic failure is mitigated It allows parts to be ordered installation and maintenance requirements such as. more effectively thereby minimising inventory items and relubrication. manpower can be scheduled thereby increasing efficiency and. reducing the costs of overtime, When rolling bearings deteriorate it can result in expensive. equipment failures with high associated costs Unplanned. The main benefits of PdM are downtime the costly replacement of equipment health and. safety issues and environmental concerns are all potential. Improved machine reliability through the effective prediction consequences of a maintenance strategy that fails to monitor. of equipment failures and predict equipment problems before they escalate into a. more serious situation,Reduced maintenance costs by minimising downtime. through the scheduling of repairs Assessing the criticality of an asset to the overall operation of. the plant is therefore essential in terms of determining the. Increased production through greater machine availability type of condition monitoring required and whether it is. necessary at all In some cases where a plant has a large. Lower energy consumption number of low cost assets where replacements are readily. available and or are not deemed critical a reactive or. Extended bearing service life preventive approach may well be appropriate. Improved product quality Even if an asset does warrant condition monitoring a decision. must be made not only on the technology but also on whether. Rolling bearings are often a key element in many different the asset warrants continuous online system or non. types of plant and equipment spanning all market sectors continuous patrol monitoring To help with this decision. On one hand they can be of a standard design readily assets are often assigned to one of three categories. available and low cost commodity items costing only a few depending on their criticality Figure 2. pounds such as those in electric motors fans and gearboxes. while on the other hand they can be of bespoke design with. long lead times and cost hundreds of thousands of pounds as. is the case in wind turbines steelmaking plant etc. However they have one thing in common if they fail. unexpectedly they can result in plant and equipment outage. resulting in lost production costing from a few thousand to. many millions of pounds With a Predictive Maintenance. strategy such large costs can be avoided by giving advance. warning of a potential problem enabling remedial action to be. planned and taken at a convenient time, Category Description Economics Category B assets are on the other hand essential assets and. include for example pumps or compressors where in the. A Equipment assets having a Failures are very event of a fault a standby unit is available this may then. large impact on plant expensive due to lost, output equipment that production health safety become a category A asset.
represents significant impacts or environmental, repair costs equipment impact Examples include Category C or non essential assets are at the far end of the. with significant health large horsepower high, safety impacts Failures energy density machines spectrum and the reasons for monitoring these if indeed they. can occur very suddenly with very high replacement are monitored at all might be to prevent failure by eliminating. and do not always give and maintenance costs root cause and allow more effective maintenance planning. advance warning Financial justification,prevention of lost 4 Return on Investment ROI. production reduced,maintenance costs, protection of life As already discussed equipment failure can be expensive. and environment and potentially catastrophic resulting in unplanned downtime. missed customer schedules costly machine, B Equipment assets having Similar to economics of replacements repairs as well as safety and environmental.
a lesser impact on plant critical equipment assets. output equipment with but of smaller magnitude concerns By initiating a Predictive Maintenance strategy. moderate repair costs Typical examples include unforeseen failures are minimised and this can yield an. equipment that can have medium horsepower,health safety machines with moderate. impressive ROI Another major benefit of introducing CM as. implications if failure replacement and part of a Predictive Maintenance program is that it enables a. occurs Failures can maintenance costs greater understanding of the equipment critical to the process. occur relatively quickly, but usually with some and also allows more time to be spent on improving the overall. advance warning condition of the assets and improving the efficiency of key. C Equipment assets having Typically include smaller. little or no direct impact on assets with small individual. plant output equipment replacement repair, When justifying PdM the following should be taken into. that represents limited costs little or no costs consideration. repair costs equipment related to lost production, that has minor safety However they collectively 1 Direct Costs. ramifications comprise a large,percentage of annual Labour.
maintenance costs Normal and overtime labour for,Small individual repair planned repair activities. replacement costs Costs,of failure do not exceed,unplanned repairs. costs to monitor or Materials,excessive payback periods Parts replaced. Machinery replaced,Figure 2 Asset categorisation, Category A assets are deemed to be critical and generally 2 Indirect Costs. fulfil one or all of the following criteria Lost production per hour. Outside services, Failure results in total or major interruption of the process Insurance costs.
Parts inventory, Failure represents a significant safety risk such as fire Total Potential Cost Reduction 1 2. toxic leak or explosion,Long lead times and or significant repair costs. 3 PdM Program Costs, A good example of this type of asset would be the main Site survey. turbine generator trains in a large power plant For such Cost of capital equipment. assets it is the cost of failure that is of primary concern Cost of any additional labour. Other examples would be the main rotor bearing or gearbox Cost of training. bearings in a wind turbine Due to the generally remote Initial setup and baseline. location failure of the main rotor bearing or gearbox bearings Scheduled data collection. which may lead to secondary damage makes replacement visits per year at per visit. costly in terms of replacement parts hire of equipment Total PdM Costs for One Year 3. and labour, By contracting out PdM there will be no capital equipment and. training costs and the benefits tend to be more immediate. because of the use of highly trained staff However it is often. more beneficial to keep the activities in house which allows. greater familiarity with plant equipment and processes and. gives the benefits not only of preventing unplanned downtime. but also enables more time to be spent on mitigating potential. failure modes and improving process efficiency, The cement industry is a good example where CM has been.
implemented and saved money both in terms of repair costs. and lost production In one case the failure of a large gearbox. caused a three week shutdown and extensive repair costs are. typically 50 000 to 100 000 To prevent such damage F IS. FAG Industrial Services Schaeffler Group installed an eight. channel FAG DTECT X1 system and trained the customer s. staff who received three months support at a total cost of. 18 000 Detecting deterioration of the gearbox early. resulted in a repair cost of 5000 saving the customer at Figure 3 Simple machine model. least 27 000 More importantly the company avoided lost At low speeds it is still possible to use vibration but a greater. production amounting to around 6000 hour degree of care and experience is required and other. techniques such as measuring shaft displacement or Acoustic. 5 Condition Monitoring, Emission AE may yield more meaningful results although the. Condition monitoring is a process where the condition of former is not always easy to apply Furthermore AE may. equipment is monitored for early signs of deterioration so that detect a change in condition but has limited diagnostic. the maintenance activity can be better planned reducing capability Vibration is used successfully on wind turbines. down time and costs This is particularly important in where the main rotor speed is typically between 5 and 30 rpm. continuous process plants where failure and downtime can In a wind turbine there are two main groups of vibration. be extremely costly frequencies generated gear mesh and bearing defect. frequencies This can result in complex vibration signals which. The monitoring of vibration temperature voltage or current can make frequency analysis a formidable task However. and oil analysis are probably the most common Vibration is techniques such as enveloping see section 5 1 3 which has a. the most widely used and not only has the ability to detect and high sensitivity to faults that cause impacting can help reduce. diagnose problems but potentially give a prognosis i e the the complexity of the analysis Bearing defects can excite. remaining useful life and possible failure mode of the machine higher frequencies which can be used as a basis for detecting. However prognosis is much more difficult and often relies on incipient damage. the continued monitoring of the fault to determine a suitable. time when the equipment can be taken out of service or relies Vibration measurement can generally be characterised as. on known experience with similar problems falling into one of three categories detection diagnosis and. 5 1 Vibration Monitoring, Detection generally uses the most basic form of vibration. Vibration monitoring is probably the most widely used measurement where the overall vibration level is measured on. predictive maintenance technique and with few exceptions a broadband basis in a range for example of 10 1000Hz or. can be applied to a wide variety of rotating equipment Since 10 10000Hz In machines where there is little vibration other. the mass of the rolling elements is generally small compared than from the bearings the spikiness of the vibration signal. to that of the machine the velocities generated are generally indicated by the Crest Factor peak RMS may imply incipient. small and result in even smaller movements of the bearing defects whereas the high energy level given by the RMS level. housing making it difficult for the vibration sensor to detect may indicate severe defects. Machine vibration comes from many sources e g bearings This type of measurement generally gives limited information. gears unbalance etc and even small amplitudes can have a other than to an experienced operator but can be useful for. severe effect on the overall machine vibration depending on trending where an increasing vibration level is an indicator of. the transfer function damping and resonances Figure 3 deteriorating machine condition Trend analysis involves. Each source of vibration will have its own characteristic plotting the vibration level as a function of time and using this. frequencies and can manifest itself as a discrete frequency to predict when the machine must be taken out of service for. or as a sum and or difference frequency repair or at least a more in depth survey must be performed. Another way of using the measurement is to compare the It is also easily influenced by other sources of vibration such. levels with published vibration criteria for different types of as unbalance misalignment looseness electromagnetic. equipment vibration etc, Although broadband vibration measurement may provide a In some situations the Crest Factor Peak to RMS ratio of. good starting point for fault detection it has limited diagnostic the vibration is capable of giving an earlier warning of bearing. capability and while a fault may be identified it may not give a defects. reliable indication of where the fault lies for example in. bearing deterioration damage unbalance misalignment etc As a local fault develops this produces short bursts of high. Where an improved diagnostic capability is required frequency energy which increase the peak level of the vibration signal. analysis is normally employed which usually gives a much but have little influence on the overall RMS level As the fault. earlier indication of the development of a fault and also its progresses more peaks will be generated until finally the. source Crest Factor decreases but the RMS vibration increases. Having detected and diagnosed a fault it is much more The main disadvantage of this method is that in the early. difficult to give a prognosis on the remaining useful life and stages of a bearing defect the vibration is normally low. possible failure mode of the machine or equipment This often compared with other sources of vibration present and is. relies on continued monitoring of the fault to determine a therefore easily influenced so any changes in bearing. suitable time when the equipment can be taken out of service condition are difficult to detect. and or on experience with similar problems,5 1 2 Frequency Spectrum. In general rolling bearings produce very little vibration when Frequency analysis plays an important part in the detection. they are free of faults and have distinctive characteristic and diagnosis of machine faults In the time domain the. frequencies when faults develop A fault that begins as a individual contributions such as unbalance bearings gears etc. single defect such as a spall on a raceway is normally to the overall machine vibration are difficult to identify In the. dominated by impulsive events at the raceway pass frequency frequency domain they become much easier to identify and. resulting in a narrow band frequency spectrum As the damage can therefore be much more easily related to individual. increases there is likely to be an increase in the characteristic sources of vibration. defect frequencies and sidebands followed by a drop in these. amplitudes and an increase in the broadband noise with It is not always possible to rely on the amplitude of bearing. considerable vibration at shaft rotational frequency Where discrete frequencies to provide defect severity because each. machine speeds are very low the bearings generate low machine will have different mass stiffness and damping. energy signals which may also be difficult to detect properties Even identical machines can have different system. Furthermore bearings located within a gearbox can be difficult properties and this can affect the amplitudes of bearing. to monitor because of the high energy at the gear meshing defects of similar size. frequencies which can mask the bearing defect frequencies. It is often the pattern of the bearing defect frequencies that. 5 1 1 Overall Vibration Level is most significant in determining the defect severity. The number of bearing related harmonic frequencies. This is the simplest way of measuring vibration and usually. frequency sidebands and characteristic features within the. involves measuring the RMS Root Mean Square vibration of. time waveform data can be much more reliable than amplitude. the bearing housing or some other point on the machine with. alone as a method of determining when action needs to be. the transducer located as close to the bearing as possible. The vibration is measured over a wide frequency range such. as 10 1000Hz or 10 10000Hz As already discussed a fault developing in a bearing will show. up as increasing vibration at frequencies related to the bearing. The measurements can be trended over time and compared. characteristic frequencies making detection possible at a. with known levels of vibration or pre alarm and alarm levels. much earlier stage than with overall vibration, can be set to indicate a change in the machine condition.
Alternatively measurements can be compared with general. Although this method represents a quick and low cost method. of vibration monitoring it is less sensitive to incipient defects. i e it is only really suitable for detecting defects in the. advanced condition and has limited diagnostic capability. 5 1 3 Envelope Spectrum Vibration monitoring can also be used to gain valuable. information about the condition of machining processes. When a bearing starts to deteriorate the resulting time signal In the manufacture of rolling bearings grinding of the. often exhibits characteristic features that can be used to raceways is a critical process in terms of achieving a high. detect a fault Furthermore bearing condition can rapidly surface finish and roundness essential to achieving the. progress from a very small defect to complete failure in a required service life. relatively short period of time so early detection requires. sensitivity to very small changes in the vibration signature Figure 4 shows cepstra of shoe force obtained during the. As already discussed the vibration signal from the early stage shoe centreless grinding of bearing outer ring raceways 2. of a defective bearing may be masked by machine noise. making it difficult to detect the fault by spectrum analysis alone. The main advantage of envelope analysis is its ability to extract. the periodic impacts from the modulated random noise of a. amplitude N,2 38 420 Hz, deteriorating rolling bearing This is even possible when the. signal from the rolling bearing is relatively low in energy and 0. buried within other vibration from the machine, Like any other structure with mass and stiffness the bearing. inner and outer rings have their own natural frequencies which. are often in the kilohertz range However it is more likely that 0 2. 1 28 9 86 18 44 27 02 35 60 44 18 52 76 81 34 69 92. the natural frequency of the outer ring will be detected due to quefrequency millisec. the small interference or clearance fit in the housing. a Diamond infeed 0 01mm rev, If there is a fault on the outer ring the natural frequency of the. ring may be excited as the rolling element hits the fault and 0 2. this will result in a high frequency burst of energy which. decays and is then excited again as the next rolling element. amplitude N, hits the defect In other words the resulting time signal will. contain a high frequency component amplitude modulated at 0. the ball roller pass frequency of the outer raceway In practice. this vibration will be very small and almost impossible to detect. in a base spectrum so a method of enhancing the signal is. required 0 2,1 28 35 60 69 92, By removing the low frequency components through a suitable quefrequency millisec.
high pass filter rectifying the output and then using a low pass b Diamond infeed 0 064mm rev. filter this leaves the envelope of the signal whose frequency. corresponds to the repetition rate of the defect This technique. is often used to detect early damage in rolling element 0 2. bearings and is also often referred to as the High Frequency. Resonance Technique HFRT or Envelope Spectrum,amplitude N. 5 1 4 Cepstrum Analysis, Vibration spectra from rotating machines are often very complex. containing several sets of harmonics and also sidebands as a. result of various modulations When trying to identify and. diagnose possible machine faults a number of characteristics of. the vibration signal are considered including harmonic 1 28 35 60 69 92. relationships and the presence of sidebands Cepstrum analysis quefrequency millisec. can simplify this because single discrete peaks in the cepstrum c Diamond infeed 0 125mm rev. represent the spacing of harmonics and sidebands in the Figure 4 Cepstra of top shoe force during the internal grinding. spectrum i e the cepstrum identifies periodicity within the of bearing outer ring raceways. spectrum Cepstrum analysis converts the spectrum back into. the time domain i e it plots amplitude versus time quefrency. and harmonics are known as rhamonics, In this case as the severity of the dressing process increases 6 1 Bearing Characteristic Frequencies. i e increasing diamond infeed the amplitude of the first peak. at 2 38ms increases along with the number of rhamonics Although the fundamental frequencies generated by rolling. The quefrency of 2 38ms corresponds to the wheel rotational bearings are related to relatively simple formulae they cover a. frequency of 420Hz This is because as the severity of the wide frequency range and can interact to give very complex. dressing operation increases it has a significant effect on signals This is often further complicated by the presence of. wheel form hence workpiece quality and the vibration signal other sources of mechanical structural or electromechanical. becomes more highly modulated at wheel rotational speed vibration on the equipment. 6 Rolling Element Bearings For a stationary outer ring and rotating inner ring the. fundamental frequencies are derived from the bearing. Rolling contact bearings are used in almost every type of geometry as follows. rotating machinery whose successful and reliable operation is. very dependent on the type of bearing selected as well as the fc o fr 2 1 d D Cos. precision of all associated components e g shaft housing fc i fr 2 1 d D Cos. spacers nuts etc Bearing engineers generally use fatigue as fb o Z fc o. the normal failure mode on the assumption that the bearings. fb i Z fc i, are properly installed operated and maintained Thanks to. fb D 2d fr 1 d D Cos 2, improvements in manufacturing technology and materials.
bearing fatigue life which is related to sub surface stresses fr Inner ring rotational frequency. is generally no longer the limiting factor and probably accounts fc o Fundamental train cage frequency relative to. for less than 3 of failures in service outer ring, fc i Fundamental train frequency relative to inner ring. Unfortunately many bearings fail prematurely in service due. fb o Ball Roller pass frequency of outer, to contamination poor lubrication misalignment temperature. raceway BPFO, extremes poor fitting fits unbalance and misalignment. All these factors lead to an increase in bearing vibration and fb i Ball Roller pass frequency of. condition monitoring has been used for many years to detect inner raceway BPFI. degrading bearings before they catastrophically fail with the fb Rolling element rotational frequency. associated costs of downtime or significant damage to other D Pitch circle diameter. parts of the machine d Diameter of roller elements. Rolling element bearings of small to medium size are often Z Number of rolling elements. used in electric motors for noise sensitive applications e g Contact angle. household appliances Bearing vibration is therefore becoming The bearing equations assume that there is no sliding and. increasingly important from both an environmental perspective that the rolling elements roll over the raceway surfaces. and because it is synonymous with quality In practice however this is rarely the case and due to a. number of factors the rolling elements undergo a combination. Vibration monitoring has now become a well accepted part of of rolling and sliding In addition the operating contact angle. many Predictive Maintenance regimes and relies on the well may be different to the nominal value As a consequence the. known characteristic vibration signatures which rolling actual characteristic defect frequencies may differ slightly from. bearings exhibit as the rolling surfaces degrade In most those predicted but this is very dependent on the type of. situations however bearing vibration cannot be measured bearing operating conditions and fits Generally the bearing. directly and the bearing vibration signature is modified by the characteristic frequencies will not be integer multiples of the. machine structure This situation is further complicated by inner ring rotational frequency which helps to distinguish them. vibration from other equipment on the machine such as from other sources of vibration. electric motors gears belts hydraulics structural resonances. etc Figure 3 Since most vibration frequencies are proportional to speed. it is important that data is obtained at identical speeds when. This often makes interpretation of vibration data difficult other comparing vibration signatures Speed changes will cause. than by a trained specialist and can in some situations lead to shifts in the frequency spectrum leading to inaccuracies in. a misdiagnosis resulting in unnecessary machine downtime both amplitude and frequency measurement In variable speed. and costs equipment spectral orders may sometimes be used where all. the frequencies are normalised relative to the fundamental. rotational speed This is generally called order normalisation. in which the fundamental frequency of rotation is called the. first order, Ball pass frequencies can be generated as a result of elastic These defects can be extremely small and difficult to detect. properties of the raceway materials due to variable compliance yet they can have a significant impact on vibration critical. or as the rolling elements pass over a defect on the raceways equipment or can result in reduced bearing life This type of. The frequency generated at the outer and inner ring raceway defect can take a variety of forms indentations scratches. can be estimated in approximate terms as 40 0 4 and 60 along and across the rolling surfaces pits debris and particles. 0 6 respectively of the inner ring speed multiplied by the in the lubricant During the early development of the fault the. number of rolling elements vibration tends to be impulsive but changes as the defect. progresses and becomes larger, Unfortunately bearing vibration signals are rarely.
straightforward and are further complicated by the interaction The type of vibration signal generated depends on many. of the various component parts but this can be often used in factors including the loads internal clearance lubrication. order to detect a deterioration of or damage to the rolling installation and type of bearing Since defects on the inner. surfaces ring raceway must travel across a number of interfaces such. as the lubricant film between the inner ring raceway and the. Analysis of bearing vibration signals is usually complex and the rolling elements between the rolling elements and the outer. frequencies generated will add and subtract and are almost ring raceway and between the outer ring and the housing they. always present in bearing vibration spectra This is particularly tend to be more attenuated than outer ring defects and can. true where multiple defects are present Depending upon the therefore sometimes be more difficult to detect. dynamic range of the equipment however background noise. levels and other sources of vibration bearing frequencies can When a defect starts a single spectral line can be generated. be difficult to detect in the early stages of a defect at the ball pass frequency and as the defect becomes larger. it allows movement of the rotating shaft and the ball pass. Over the years however a number of diagnostic algorithms frequency becomes modulated at shaft rotational speed. have been developed to detect bearing faults by measuring This modulation generates a sideband at shaft speed As the. the vibration signatures on the bearing housing These defect increases in size more sidebands may be generated. methods usually take advantage of both the characteristic until at some point the ball pass frequency may no longer be. frequencies and the ringing frequencies i e natural generated but a series of spectral lines spaced at shaft. frequencies of the bearing rotational speed occurs. By measuring the frequencies generated by a bearing it is A defective rolling element may generate vibration at twice. often possible to identify not only the existence of a problem the rotational speed as the defect strikes the inner and outer. but also its cause While it may be only be necessary to raceways The vibration produced by a defective ball may not. identify that a bearing is starting to deteriorate and plan when be very high or may not be generated at all as it is not always. it should be changed a more detailed analysis of the vibration in the load zone when the defect strikes the raceway. can often give some vital clues as to what caused the problem As the defect contacts the cage it can often modulate other. in the first place This can be further enhanced by inspecting frequencies i e ball defect frequency ball pass frequency or. the bearing after removal from the equipment especially if the shaft rotational frequency and show up as a sideband. fault has been identified at an early stage The cage rotational frequency can be generated in a badly. worn or damaged cage In a ball bearing the rolling elements. 6 2 Bearing Defects, may never generate ball rotational frequency or twice the ball. Rolling contact bearings represent a complex vibration system rotational frequency due to the combination of rolling and. whose components e g rolling elements inner raceway outer sliding and the constant changing of the ball rotational axis. raceway and cage interact to generate complex vibration In cylindrical roller bearings the damage often occurs all the. signatures 3 Although rolling bearings are manufactured way around the majority of the rolling element surface so the. using high precision machine tools and under strict cleanliness rolling element rotational frequency may never be generated. and quality controls they have degrees of imperfection like any. other manufactured part and generate vibration as the. 6 3 Variable Compliance, surfaces interact through a combination of rolling and sliding This occurs under radial or misaligned loads is an inherent. Although the amplitudes of surface imperfections are now of feature of rolling bearings and is completely independent of. the order of nanometres vibrations can still be produced in the quality Radial or misaligned loads are supported by a few. entire audible frequency range 20Hz 20kHz rolling elements confined to a narrow region and the radial. position of the inner ring with respect to the outer ring. Whereas surface roughness and waviness result directly from. depends on the elastic deflections at the rolling, the bearing component manufacturing processes discrete. element raceway contacts Figure 5 The outer ring of the. defects refer to damage of the rolling surfaces due to. bearing is usually supported by a flexible housing which. assembly contamination operation mounting poor, generally has asymmetric stiffness properties described by the. maintenance etc,linear springs of varying stiffness.
6 4 Bearing Speed Ratio, The bearing speed ratio ball pass frequency divided by the. shaft rotational frequency is a function of the bearing loads. and clearances and can therefore give some indication of the. bearing operating performance, When abnormal or unsatisfactory lubrication conditions are. encountered or when skidding occurs the bearing speed ratio. will deviate from the normal or predicted values If the bearing. speed ratio is below predicted values this may indicate. insufficient loading excessive lubrication or insufficient bearing. radial internal clearance which could result in higher operating. temperatures and premature failure Conversely a higher than. predicted bearing speed ratio may indicate excessive loading. excessive bearing radial internal clearance or insufficient. lubrication, For an experienced analyst vibration can be used not only to. detect deterioration in bearing condition but also to make an. Figure 5 Simple model of a bearing under radial load initial assessment of whether the equipment is operating. satisfactorily at initial start up, As the bearing rotates the individual ball loads and hence the. elastic deflections change to produce a relative movement In electrical machines two deep groove radial ball bearings are. between the inner and outer rings The movement takes the commonly used to support the shaft one is a locating bearing. form of a locus which is two dimensional and contained in a while the other is a non locating bearing that can be displaced. radial plane under radial load while it is three dimensional in the housing to compensate for axial thermal expansion of. under misalignment The movement is also periodic with a the shaft It is not unusual for bearings to fail catastrophically. base frequency equal to the rate at which the rolling elements due to thermal preloading or cross location where there is. pass through the load zone Frequency analysis of the insufficient clearance between the bearing outer ring and. movement yields the base frequency and a series of housing resulting in the non locating or floating bearing. harmonics So even a geometrically perfect bearing will failing to move in the housing i e the bearings become axially. produce vibration because of the relative periodic movement loaded. between the inner and outer rings due to raceway elastic. deflections The effect of this axial load is to increase the operating. contact angle which in turn increases the BPFO For a ball. Variable compliance vibration is heavily dependent on the bearing the contact angle can be estimated as follows. number of rolling elements supporting the externally applied. loads the greater the number of loaded rolling elements the Cos 1 1 RIC 2 ro ri D. less the vibration For radially loaded or misaligned bearings. running clearance determines the extent of the load region Contact angle. hence variable compliance generally increases with radial RIC Radial internal clearance. internal clearance A distinction is made between running ro Raceway groove radius of outer ring. clearance and radial internal clearance RIC When fitted to a. machine the former is normally smaller than the RIC due to ri Raceway groove radius of inner ring. differential thermal expansion and interference fit of the rings D Ball diameter. In high speed applications the effect of centrifugal force. should also be considered Since a deep groove ball bearing is designed to have a radial. internal clearance in the unloaded condition it can also. Variable compliance vibration levels can exceed those experience axial play Under an axial load this results in the. produced by roughness and waviness of the rolling surfaces ball raceway contact having an angle other than zero As the. In applications where vibration is critical however it can be bearing radial internal clearance and thus the axial play. reduced to a negligible level by using ball bearings with the increases so does the contact angle For a correctly. correct level of axial preload assembled motor under pure radial load the contact angle will. be zero and the BPFO will be given by,fb o Zfr 2 1 d D.
On the other hand if cross location occurs the outer ring The most probable reason was that the bearing had been. cannot move axially in the housing the bearing radial internal installed too tightly and could not move in the housing as the. clearance will be lost by the relative axial movement between shaft of the motor expanded and contracted. the inner and outer rings the bearings become axially loaded. and the BPFO will increase due to the increase in contact Shortly after installation the motor failed catastrophically. angle The amplitude of BPFO is likely to be small until the A photograph of the inner ring in Figure 11 shows the ball. bearing becomes distressed and it may not always be possible running path offset from the centre of the raceway towards the. to detect the BPFO particularly if using a linear amplitude shoulder After a thorough investigation of all the bearing fits. scale A log or dB amplitude scale may be better but care it was confirmed that there was insufficient clearance between. should also be exercised here because there may be other the outer ring and the housing of the non locating bearing. frequencies that may be close to the BPFO resulting in cross location thermal loading which was. consistent with the vibration measurements taken prior to. A good example of how the bearing speed ratio can be used installation. to identify a potential problem is given in Figure 6 which. shows a vibration acceleration spectrum measured axially at A number of harmonics and sum and difference frequencies. the drive end DE on the end cap of a 250kW electric motor relating to the BPFO 233 5Hz cage rotational frequency. The measurements were obtained during a run up test prior 21Hz and inner ring rotational frequency are also evident in. to installation in the plant the spectrum Figure 6. Once the motor had been rebuilt with new bearings and the. correct bearing fits the run up test was repeated prior to. installation Figure 7,a Base spectrum,a Base spectrum. b Base spectrum with zoomed amplitude and frequency scale. Figure 6 Axial vibration acceleration spectrum at the DE on the end. cap of a 250kW electric motor, For a nominal shaft speed of 3000 rpm the calculated. BPFO was 228 8Hz giving a bearing speed ratio of 4 576. The measured BPFO was 233 5Hz Figure 6 giving a bearing b Base spectrum with zoomed amplitude and frequency scale. speed ratio of 4 67 an increase of 2 The BPFO of 233 5Hz Figure 7 Axial vibration acceleration spectrum at the DE on the end cap. corresponds to a contact angle of 25 which strongly of a 250kW electric motor after fitting with new bearings. suggested that that the type 6217 bearing was subjected to. a high axial load, The base spectrum shows no characteristic bearing In the 0 5kHz spectrum there is a dominant discrete peak at. frequencies but when both the amplitude and frequency 1141 8Hz which neither corresponds with a harmonic of the. scales are expanded a discrete peak at 229Hz becomes rotor speed i e 1141 8 49 98 22 84 nor with any of the. evident Figure 6 b which matches very closely with the bearing generated frequencies On either side of 1141 8Hz. predicted BPFO fb o of 228 8Hz This motor went on to peak are sidebands spaced at the rotor speed 49 98Hz i e the. operate successfully 1141 8Hz frequency is amplitude modulated at the rotor speed. 7 Examples of Vibration Monitoring This is shown more clearly in Figure 9 a which shows that in. the range 0 650ms the signal is amplitude modulated at. In this section some examples are given of how vibration can. 20 2ms which within the measurement accuracy corresponds. be used to detect and diagnose problems on rotating. to 49 98Hz i e the rotor speed Expanding the time scale from. equipment ranging from electric motors to large crushing. 500 600ms Figure 9 b shows that the time between peaks. machines used for mining and processing Examples are also. is 0 87ms i e 13 051ms divided by 15 cycles which, taken from the FAG WiPro Condition Monitoring System used. corresponds to a carrier frequency of approximately 1149Hz. for monitoring the condition of wind turbine drive trains. Within the measurement accuracy of 0 0796ms this, 7 1 Electric Motor corresponds to the frequency of 1141 8Hz 0 876ms shown.
in Figure 8, An example of a vibration spectra measured axially on the. DE of a 250kW electric motor is shown in Figure 8 Dividing 1141 8Hz by the rotational speed of 49 98Hz gives. 22 85 which is not close enough for the frequency to be a. harmonic of the rotational speed One of the extensional. vibration modes of the outer ring was estimated to be 1158Hz. which is very close to the measured value of 1141 8Hz. One possible explanation is that the discrete peak at 1141 8Hz. is an excited natural frequency of the outer ring,a Vibration acceleration 0 650ms. Figure 8 Vibration acceleration spectra measured axially on the DE of a. 250kW electric motor, The nominal rotational speed was 3000 rpm and the rotor. was supported by two type 6217 C4 deep groove ball. bearings 85mm bore with grease lubrication The vibration. spectra are dominated by vibration at both harmonics and sub. b Vibration acceleration 500 600ms, harmonics of the rotor speed 49 7Hz The spectrum 0 1kHz. shows a number of harmonics and sub harmonics of the rotor Figure 9 Time signals of vibration acceleration measured axially on the. speed with no bearing characteristic frequencies being evident DE of a 250kW electric motor. The dominance of vibration at rotor speed and the absence of During a run up test prior to installation in the plant the RMS. any frequencies related to the rolling bearings suggest that the vibration level of the motor in the frequency range 0 1kHz. bearings have experienced such severe damage to the rolling before and after fitting the new bearings was 0 304g and. contact surfaces that this has resulted in an increase in radial 0 335g respectively. internal clearance allowing significant radial movement of the. rotor 7 2 Impact Crusher, Figure 12 shows another example of a vibration acceleration.
The envelope spectrum Figure 10 shows a dominance of. spectrum obtained from the housing of a type 23036. peaks related to the rotor speed with no evidence of any. 180mm bore spherical roller bearing located on the main. bearing characteristic frequencies, drive shaft of an impact crusher The spectrum shows a. number of harmonics of the BPFO 101Hz with a dominant. peak at 404Hz 4fb o and sidebands at the shaft rotational. frequency 9Hz, Figure 10 Envelope spectrum of vibration acceleration measured. axially on the DE of a 250kW electric motor,When the bearings were removed from the motor and. Figure 12 Vibration acceleration measured radially on the housing of. examined the NDE bearing had a ball running path offset from a type 23036 spherical roller bearing. the centre of the raceway towards the shoulder Figure 11. When the bearing was removed from the machine and, examined one part of the outer raceway had black corrosion. stains as a result of water ingress which had occurred during. Offset running band external storage of the machine Figure 13. Figure 11 Photograph of type 6217 inner ring showing running path. offset from centre of raceway, The DE bearing had significant damage all around both.
raceways and the rolling elements showed signs of severe. distress It was clear from the NDE bearing however that the. cause of the failure was too tight a fit between the outer ring. and housing This resulted in the bearing being unable to move. axially in the housing and compensate for axial thermal Figure 13 Type 23036 spherical roller bearing outer ring raceway. expansion of the rotor leading to a high axial load showing black corrosion stains. A number of the rollers also had black corrosion stains which. was consistent with the vibration at the cage rotational. frequency fc 4Hz in the envelope spectrum Figure 14. b Envelope spectrum, Figure 16 Vibration acceleration measured radially on the housing of. a type 23036 spherical roller bearing good machine. Figure 14 Envelope spectrum of the type 23036, spherical roller bearing suspect machine 7 3 Generator. The modulation of the time signal at the cage rotational During the initial running in phase of a 2MW generator on. frequency can be clearly seen in the time signal Figure 15 a test bed an intermittent rattling noise was evident. The generator was fitted with a type 6232 deep groove ball. bearing at the DE and a cylindrical roller bearing at the. non drive end NDE Both bearings were grease lubricated. The initial suspicion was that the rattling noise was related to. the cage because it was intermittent and became worse as. the bearings reached operating temperature, Vibration measurements obtained from the DE of the. generator are shown in Figure 17, Figure 15 Acceleration time signal of the type 23036. spherical roller bearing suspect machine, Figure 16 shows vibration measurements obtained from an.
identical machine considered to be operating satisfactorily. Both the base and envelope spectrum show no indication of. any vibration related to the type 23036 spherical roller bearing. Figure 17 Radial vibration acceleration measured at the DE end cap. The acceleration time signal shows what appears to be random. bursts of high frequency vibration but on closer inspection this. was in fact modulation at the cage rotational frequency. The time period between the pulses corresponds to the. revolution of the cage 84ms fc o 11 9Hz Also present. are pulses spaced at 9 3ms which correspond to the BPFO. fb o 107 9Hz of the type 6232 deep groove ball bearing. Dividing the time period for one revolution of the cage 84ms. by 9 3ms gives the number of rolling elements i e 84 9 3 9.


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