MAINTENANCE THE A-Z OF PREDICTIVE MAINTENANCE T I here is a very strong tendency, once a M predictive maintenance system has been purchased, to attempt to measure everything on the plant the system simply becomes swamped with paper. When assessing what to measure as part of a programme, the following' procedure should be used: a) make a list of all machines on the plant to be included in the predictive maintenance pro- gramme. b) make a list of failures that might occur on those machines. c) list the ways in which those failures could be detected. The above will cause you to focus on the real goal of the programme, that is to detect approaching failures in the machines, rather than becoming too focused on the data collection which has often his- torically been perceived as the goal of such a system. Once this has been done, we can begin to build a programme of measurements to be made on the plant, and the ways in which they could fail. What could I measure? Our goal is to monitor the machines regularly so that we only fix them when needed. What parameters could we measure to detect problems? Let us take the example of a motor driven pump system. On the motor we could measure the following parameters: bearing vibration; shaft vibration; speed; current; noise coupling alignment; state of vibration; bearing temperature; visual inspection; load. On the gear box we could measure: bearing vibration; gear mesh vibration; shaft vibration; coupling alignment; further lub oil analysis; gear noise; lub oil level; lub oil temperature; bearing temperature; visual inspection. On the pump we could measure: bearing vibra- tion; shaft vibration; cavitation; visual inspection; inlet/outlet pressure; temperature; flow. The one parameter that is common to all of these pieces of equipment is vibration. That is why vibra- tion has become the usual and most widely accepted means of making the necessary machine condition measurements. Predictive maintenance can save time, effort and money. But how should you go about tackling such a programme? Roger Hutton ofEntek Scientific Corporation explains. However, we should not lose sight of the require- ment for many of the other measurements. Vibration is consistent with all equipment, it is widely applic- able and very easy to measure from the outside of a machine. It is convenient, it gives early problem detection and it uses well established technology. It can be applied to machines such as pumps, fans, motors, compressors, gearboxes, turbines, ma- chine tools and a wide range of production ma- chinery. It is an early and convenient sign of a change in the machine's condition. There are a number of limitations to the vibration measurement. The major one is that we are not measuring the parameter we would like to measure. In an ideal situation we would measure forces inside the machine, for example the pulses from a bit on a rolling element in a bearing contracting with the races of the bearing, or the forces produced by balance on a rotating shaft. To measure these would be both extremely ex- pensive and involve large amounts of instrumenta- tion mounted directly into the machine. If we measure the resultant of these forces when they appear on the outside of the machine, which is the vibration pattern, then we can make a quick and easy assessment of the forces, but we need to be aware of the transmission path in between and the limita- tions this imposes. Once we have determined the measurements to be made, we need to set up the machines and the associated measurements on the database. This can be done on a PC. Once done, this becomes the heart of the system and, when properly used, can manage the organisation of the data collection as well as the data itself. When the points to be measured have been entered, they must be organised into circuits, tours or routes around the plant: this is simply a sequence of measurements to be made. When the sequence has been defined it can be loaded into a data collec- tor which is used to make the measurements and store the data during the tour of inspection of the plant. The route sequence is loaded from the com- puter into the data collector. Prior to data acquisition, the user is guided around the plant through plots that appear on the screen making the measurement simple, quick and easy to perform for even the lowest grade people on the plant. Once data has been collected the information is transferred from the data collector into the database where automated routines in the software can be used to evaluate the data, diagnose the problem and prepare any reports that may be required. The system should not be seen as simply indi- cating the machines that have vibration levels or other parameters that show a maximum possible lead time before a failure, this gives the time for scheduling, planning and achieving the best possible return on the investment made. Problems of the past The most basic approach to condition monitor- ing has historically been firstly to measure the ma- chine vibration, perhaps with a small portable vibra- tion meter. The measurements could be noted on a clipboard, for subsequent examination and perhaps plotting on a graph against time. Visual interpreta- tion is then used to look for significant upward trends in the levels indicating onset of a problem. Analysis of the data is time consuming and the transfer of data from meter to paper, and then to graph, subject to error. The measurements need to be carefully organised to ensure that those on ma- chines that quickly develop problems are performed regularly, while the system does not get burdened with excessive amounts of data from machines that need less frequent attention. The greater the number of different measurement intervals required, the more difficult the administration becomes. The process of splitting the vibration signal down into its various components is done using a MANUFACTURING ENGINEER OCTOBER 1991
Transcript
MAINTENANCE
THE A-Z OF PREDICTIVEMAINTENANCE
TI here is a very strong tendency, once aM predictive maintenance system has been
purchased, to attempt to measure everything on theplant the system simply becomes swamped withpaper. When assessing what to measure as part of aprogramme, the following' procedure should beused:a) make a list of all machines on the plant to be
included in the predictive maintenance pro-gramme.
b) make a list of failures that might occur on thosemachines.
c) list the ways in which those failures could bedetected.
The above will cause you to focus on the realgoal of the programme, that is to detect approachingfailures in the machines, rather than becoming toofocused on the data collection which has often his-torically been perceived as the goal of such a system.Once this has been done, we can begin to build aprogramme of measurements to be made on theplant, and the ways in which they could fail.
What could I measure?Our goal is to monitor the machines regularly so
that we only fix them when needed. What parameterscould we measure to detect problems? Let us takethe example of a motor driven pump system.
On the motor we could measure the followingparameters: bearing vibration; shaft vibration;speed; current; noise coupling alignment; state ofvibration; bearing temperature; visual inspection;load.
On the gear box we could measure: bearingvibration; gear mesh vibration; shaft vibration;coupling alignment; further lub oil analysis; gearnoise; lub oil level; lub oil temperature; bearingtemperature; visual inspection.
On the pump we could measure: bearing vibra-tion; shaft vibration; cavitation; visual inspection;inlet/outlet pressure; temperature; flow.
The one parameter that is common to all of thesepieces of equipment is vibration. That is why vibra-tion has become the usual and most widely acceptedmeans of making the necessary machine conditionmeasurements.
Predictive maintenance can save time, effort and money.But how should you go about tackling such a programme?
Roger Hutton ofEntek Scientific Corporation explains.
However, we should not lose sight of the require-ment for many of the other measurements. Vibrationis consistent with all equipment, it is widely applic-able and very easy to measure from the outside of amachine. It is convenient, it gives early problemdetection and it uses well established technology. Itcan be applied to machines such as pumps, fans,motors, compressors, gearboxes, turbines, ma-chine tools and a wide range of production ma-chinery. It is an early and convenient sign of achange in the machine's condition.
There are a number of limitations to the vibrationmeasurement. The major one is that we are notmeasuring the parameter we would like to measure.In an ideal situation we would measure forces insidethe machine, for example the pulses from a bit on arolling element in a bearing contracting with theraces of the bearing, or the forces produced bybalance on a rotating shaft.
To measure these would be both extremely ex-pensive and involve large amounts of instrumenta-tion mounted directly into the machine. If wemeasure the resultant of these forces when theyappear on the outside of the machine, which is thevibration pattern, then we can make a quick and easyassessment of the forces, but we need to be awareof the transmission path in between and the limita-tions this imposes.
Once we have determined the measurements tobe made, we need to set up the machines and theassociated measurements on the database. This canbe done on a PC. Once done, this becomes the heartof the system and, when properly used, can managethe organisation of the data collection as well as thedata itself.
When the points to be measured have beenentered, they must be organised into circuits, toursor routes around the plant: this is simply a sequenceof measurements to be made. When the sequencehas been defined it can be loaded into a data collec-tor which is used to make the measurements and
store the data during the tour of inspection of theplant. The route sequence is loaded from the com-puter into the data collector.
Prior to data acquisition, the user is guidedaround the plant through plots that appear on thescreen making the measurement simple, quick andeasy to perform for even the lowest grade people onthe plant.
Once data has been collected the information istransferred from the data collector into the databasewhere automated routines in the software can beused to evaluate the data, diagnose the problem andprepare any reports that may be required.
The system should not be seen as simply indi-cating the machines that have vibration levels orother parameters that show a maximum possiblelead time before a failure, this gives the time forscheduling, planning and achieving the bestpossible return on the investment made.
Problems of the pastThe most basic approach to condition monitor-
ing has historically been firstly to measure the ma-chine vibration, perhaps with a small portable vibra-tion meter. The measurements could be noted on aclipboard, for subsequent examination and perhapsplotting on a graph against time. Visual interpreta-tion is then used to look for significant upwardtrends in the levels indicating onset of a problem.
Analysis of the data is time consuming and thetransfer of data from meter to paper, and then tograph, subject to error. The measurements need tobe carefully organised to ensure that those on ma-chines that quickly develop problems are performedregularly, while the system does not get burdenedwith excessive amounts of data from machines thatneed less frequent attention. The greater the numberof different measurement intervals required, themore difficult the administration becomes.
The process of splitting the vibration signaldown into its various components is done using a
MANUFACTURING ENGINEER OCTOBER 1991
MAINTENANCE
Weighing up the benefitsof predictive maintenance- the costs are outweighedby the savings to be made
frequency analyser. Such instruments provide apowerful means of resolving the nature of the fault.Such analysers have in the past been large, bulkyinstruments, difficult to transport to, and then use ina typical machine environment. They also requiremains power.
Once these problems have been overcome, suchmachines yield complex pictures of the machine'svibration pattern, showing an enormous amount ofinformation. Almost every mechanical process with-in the machine will produce a small amount ofvibration, even when the machine is in perfect con-dition. In order to define and hence diagnose whatthe problem is, it is essential to know what is chang-ing in the pattern. This requires a record to beavailable of the vibration pattern of the machinewhen it is running smoothly so that comparisonscan be made.
This has traditionally been done by making hardcopy plots of the vibration pattern and then carryingout visual analysis of the differences. Any smallchange in the way that the analysis instrument isset-up can easily render this comparison im-possible. Many months or even years may passbetween the two measurements, and so not only isrecollection of the complex instrument setup almostimpossible, but finding old records in the depths ofa filing cabinet to allow the comparison may also bealmost impossible.
All of this assumes a high level of expertise andexperience to make the correct measurements in anappropriate way, and then to interpret them so thatoperators and maintenance managers can make de-cisions on whether to run an item of plant or to shutit down with a degree of confidence.
Typically on such a plani, an operator will havedetected a change in the noise made by a machine
and suspicion about its condition ensues. Unfortu-nately by this time it is often too late for real financialsavings to be made as the problem is already welladvanced and a decision on the continued runningof the machine is required immediately.
The vibration expert is called, who has to as-semble his bulky analysis equipment adjacent to themachine. He will make a measurement of the vibra-tion pattern which will probably be in a differentformat to any previous measurements, if any areavailable at all. The operators will be hustling for adecision on the machine's future running, but anaccurate prognosis is almost impossible as insuffi-cient information is available.
No wonder that many who have tried the vibra-tion analysis approach for machine maintenancescheduling and fault diagnosis have become scep-tical.
ObjectivesIf we are to be effective in running a system that
overcomes these problems, it is useful to definewhat we are trying to achieve.
The very subject itself suffers from a lack ofidentity due to the variety of titles applied to it.Machinery health monitoring adequately describesthe measurement process, while condition monitor-ing describes what we are trying to do.
However, a system is only viable if we can makesubstantial cost savings and these are usually bestmade through reductions in the maintenance bud-get. Hence predictive maintenance is now becomingadopted as a more suitable descriptor.
We have already seen how the task of datacollection and analysis can become so large that itis less than effective, so we need to set objectivesfor the system itself.
These include: automating, as far as possible,data collection, repetitive analysis and reporting;detecting significant changes in machine conditionat the earliest possible time, with a minimum of falsealarms; increasing the accuracy of record keepingand maintaining interest in the system and hencemorale.
If these objectives are met, then the chances ofsuccess are considerably improved.
System features
Data collectorsCurrent technology allows us to substantially
meet these objectives. Reduction in power con-sumption and size of micro-electronics, coupledwith improvements in battery technology, haveallowed the development of powerful and portabledata collectors. These act both as meters for a varietyof quantities and as electronic notebooks capable ofretaining the data for later transfer to a computer.They can also act as sophisticated vibration analysisinstruments capable of splitting a vibration signalinto its frequency components using the Fast Fou-rier Transform. Resulting vibration frequency spec-tra can similarly be stored.
More specific capabilities include: a facility toreceive from a host computer a series of measure-ments in an ordered series. The collector thenprompts the user with a descriptor of both the data tobe collected and where to measure. The systemautomatically sets up the instrument for the requiredanalysis. Measurements may include vibration,either from a handheld pickup, or from permanentlyinstalled transducers such as proximity probes, ortemperatures, pressures, flows or any process par-ameter.
Vibration analysis of paper making machinery at a paper mill
It should be possible to indicate invalid dataquickly. Previous values are displayed for instanta-neous comparison as well as alarm values.
There should be a faci I ity for noting observationssuch as loose guards or oil leaks.
FFT analysis may be carried out with a variety offrequency ranges and resolutions. Facilities existforoptimisation of signals such as averaging.
There should be sufficient memory and batterylife to allow continuous data collection for severalhours and facilities to overcome practical problemssuch as shutdown machines, need to review data,automatic shutdown on low battery with no data loss.
Last, but not least, the equipment should becompact and easy to carry, and easy to use by a widerange of users.
SoftwareThe software now represents the majority of the
power and facilities of the total system. It is wherethe system is organised, where data is stored andwhere decisions and interpretations are made. Morespecific facilities in the software include a databasethat allows organisation of data in an efficient formgiving fast access to the required data and also afacility to search for given types of informationallowing comparison of data from similar machines,even on different plants.
By using an industry standard file format, datacan be transferred to other systems such as widermanagement and data management and data anal-ysis systems. The system needs links to a variety ofdata collectors; different collectors address differenttypes of problem and there is often a requirement foran interface to more than one. For example, there is
a need for fast, portable collectors capable of col-lecting vast amounts of data in a short time tohighlight problem machines at an early stage. Oncea problem is identified, a more powerful analysismay be required using an FFT analyser with largedynamic range and zoom facility.
Automatic data analysis and report preparationreduces operator intervention to a minimum, whilepowerful alarm comparison capabilities ensure thatonly relevant data and significant changes in machinecondition are brought to the attention of the user.
Organisational capabilities are needed to ar-range data collection into logical sequences aroundthe machines, along with a facility to generate theseroute sequences quickly and to store, recall and editthem. Extracting the information subsequently re-quires many report formats.
Other facilities include representing trendsgraphically with the ability to project data forward, awide variety of plot formats with easy control overthese, a facility to compare trends in many parame-ters on a machine or of similar parameters acrosssimilar machines. Spectra can be plotted with a wideand flexible range of formats, including the ability tocompare multiple spectra overlaid and in waterfallformat.
Automatic comparison of spectrum data againstalarm values at all frequencies means alarms can begenerated quickly and easily from baselines col-lected from the machines, or from the archived datain the database. This can include statistical analysisof data.
A diagnostic system interprets spectrum dataand indicates mechanical components at fault in themachine. Easy input formats allow fast set-up, and
The ever growingcatalogue of success stories
will produce a rapid adoptionof the technology
there should be a facility for both visual and auto-matic interpretation.
Last, but not least, the software should providea growth path allowing the user to start at an appro-priate level for his expertise, and to increase hissystem capabilities in line with his own.
ComputersThe software needs to run on a low cost, widely
accepted computer, yet one that is powerful enoughto handle both today's as well as future require-ments. The IBM PC in its higher level variants iswidely accepted, and provides the necessary capa-bilities. A wide variety of add-on hardware items andsoftware enhancements such as high capacity disksystems and file sharing facilities provide expansioncapabilities for the future.
Key developmentsThe instruments and computer tools described
above are available now, but the instruments havechanged little, mostly due to the cost of productionchanges. The software can be updated at very lowcost on a regular basis, allowing a system to remainat the latest level of available techniques and tech-nology simply for the cost of the disks.
Thus, as experience has highlighted, any limita-tions in both the way that the system runs and thedata is handled, can be overcome by changes in thecode. This has allowed the software to advancerapidly and to move ahead as the dominant part ofthe system in allowing us to meet the objectives set.
AutomationThe large number of such systems in use has
meant that there have simply not been enough ex-perts to go around. There has thus been a need touse lower grade people to run the systems, oftenwith little or no vibration or computer expertise.
But, whoever runs the system, time equalsmoney, so a highly automated system is desirable.
Through use of the software to unload the datacollector, check the data against criteria or alarms,and prepare a report, including plots, computer timefor the everyday operators has been reduced almost
MANUFACTURING ENGINEER OCTOBER 1991
MAINTENANCE
to zero. A simple pictorial screen is presented to theoperator, who simply moves the cursor to indicatewhether he wishes to load or unload his data collec-tor, and the process proceeds. If he has chosen totransfer new data to the computer, having just re-turned from the plant, he initiates the transfer, andthen comes back after a break to find a report readyfor him.
DatabaseThe business world has made enormous invest-
ments in computer tools in recent years and, as aresult, considerable effort has been expended indeveloping powerful facilities such as database pro-grams allowing us to find and compare data withease. The requirements of a machinery operator tostore large amounts of information in a predictivemaintenance program is no different.
A hierarchical, well developed database, using astandard file format can be used, allowing easyinterface to many other systems as well as providingthe means to locate information with speed and ease.
Trend comparisonAn example of the use of the database system is
in comparing trends of different types of informa-tion. Large fluctuations in a machine's vibrationlevel may be observed that are nothing to do with adegradation of the machine's health. By searchingthe database for specific types of machine, visualcorrelation can be made. For example, is the vibra-tion level varying with load on the machine, or withthe pressure downstream of a pump etc? All thesecan be easily answered by simple visual comparisonon an overlay trend plot.
Setting trend alarmsA common question from the novice user is
"how do I know what level to use for my vibrationalarms?". The overlay trend plots allow the user torecall and overlay many trends of vibration level,perhaps for hundreds of good motors. These couldeven be sorted by size if this information is availablein the database. The plot gives a clear indication ofa reasonable alarm level just above the majority ofthe overlaid trend data.
Setting spectrum alarmsSimilar techniques can be used to create full
spectrum alarms from baseline data. The databasesearch facility is used to sift out those machines thatare to be included in the alarm generation process.The baselines for these machines can then be auto-matically recalled and an alarm generated using asmall number of parameters, such as the requiredpercentage change acceptable in any vibration com-ponent before an alarm condition occurs.
A potential problem with this is that operationconditions for a machine may change, causing achange in vibration level that is nothing to do withthe machine's health. By using the database to
search for historic information, alarms may be setusing statistical functions such as mean and stand-ard deviation. This significantly helps to avoid falsealarms that can destroy confidence in the systemvery rapidly.
DiagnosticsTypical output from a vibration spectrum ana-
lysis is "component at 1276Hz over alarm". Thishardly helps the plant engineer, who has no experi-ence of vibration analysis and who is responsible fordecisions on whether to run or shut down the ma-chine. There is therefore a need to relate the frequen-cies in the vibration pattern to mechanical faults.This can be achieved through software diagnosticroutines.
Ready, willing and ableThe instruments and computer tools described
above are available now. The software is usuallyupdated on a regular basis, allowing a system toremain at the latest level of available techniques andtechnology at minimal cost.
As instruments become more firmware driven,rather than hard wired, they too can be updated asthe available facilities expand.
This flexibility, plus the recognition that the newsystems offer substantially greater ability to meet theobjectives than ever before, has caused a rapidintake of these systems by many and diverse ma-chinery operators all over the world.
These users have seen financial savings oftenway in excess of those expected and system paybackhas usually been within the first year. The evergrowing catalogue of success stories will produce arapid and widespread adoption of the technology inthe near future. EQFor more information enter ME47
Mr Roger Hutton is European manager of EntekScientific Corporation. This is a paper he gave at aseries of seminars on condition based predictivemaintenance organised by Conference Comunica-tion. Contact David Wilson on (0251) 83111 formore information.
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