Condition Based Maintenance

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  Journal of Quality in Maintenance Engineering Condition ‐ based maintenance: tools and decision making  Albert H.C. Tsang  Article information: To cite this document: Albert H.C. Tsang, (1995), Condition#based maintenance: tools and decision making , Journal of Quality in MaintenanceEngineering, Vol. 1 Iss 3 pp. 3 - 17 Permanent link to this document: Downloaded on: 27 September 2014, At: 23:24 (PT)References: this document contains references to 23 other documents.To copy this document: permissions@emeraldinsight.comThe fulltext of this document has been downloaded 3078 times since 2006* Users who downloaded this article also downloaded: Lawrence Mann, Anuj Saxena, Gerald M. Knapp, (1995), Statistical#based or condition#based preventive maintenance? ,Journal of Quality in Maintenance Engineering, Vol. 1 Iss 1 pp. 46-59 A.K.S. Jardine, T. Joseph, D. Banjevic, (1999), Optimizing condition#based maintenance decisions for equipment subject tovibration monitoring , Journal of Quality in Maintenance Engineering, Vol. 5 Iss 3 pp. 192-202 Albert H.C. Tsang, (2002), Strategic dimensions of maintenance management , Journal of Quality in MaintenanceEngineering, Vol. 8 Iss 1 pp. 7-39 Access to this document was granted through an Emerald subscription provided by 540251 [] For Authors If you would like to write for this, or any other Emerald publication, then please use our Emerald for Authors serviceinformation about how to choose which publication to write for and submission guidelines are available for all. Pleasevisit for more information.  About Emerald Emerald is a global publisher linking research and practice to the benefit of society. The company manages a portfolio of more than 290 journals and over 2,350 books and book series volumes, as well as providing an extensive range of onlineproducts and additional customer resources and services. Emerald is both COUNTER 4 and TRANSFER compliant. The organization is a partner of the Committee on PublicationEthics (COPE) and also works with Portico and the LOCKSS initiative for digital archive preservation. *Related content and download information correct at time of download.    D  o  w  n   l  o  a   d  e   d   b  y   I  n   d   i  a  n   I  n  s   t   i   t  u   t  e  o   f   T  e  c   h  n  o   l  o  g  y  a   t   M  a   d  r  a  s   A   t   2   3  :   2   4   2   7   S  e  p   t  e  m   b  e  r   2   0   1   4   (   P   T   )  Condition-basedmaintenance 3 Condition-based maintenance:tools and decision making Albert H.C. Tsang The Hong Kong Polytechnic University, Hong Kong  Options for preventive maintenance Historically, maintenance activities have been regarded as a necessary evil bythe various management functions in an organization. However, in recent yearsthis attitude has increasingly been replaced by one which recognizesmaintenance as a strategic issue in the organization. The developments whichcontributed to this change include: environmental concerns, safety issues,warranty and liability factors, regulatory matters, ageing plant and equipment,drive for cost reduction and the like. To live up to the new expectationsdemanded of maintenance activities, maintenance programmes have to bedeveloped to ensure that physical assets will continue to fulfil their intendedfunctions at a minimum expenditure of resources. Obviously, maintenanceactivities which do not contribute to preserving or restoring the intendedfunctions of assets should be eliminated.Maintenance activities fall into two broad categories, namely correctivemaintenance and preventive maintenance. Corrective maintenance (CM), alsoknown as breakdown maintenance, is performed when action is taken to restorethe functional capabilities of failed or malfunctioned equipment or systems. This is a reactive approach to maintenance because the action is triggered bythe unscheduled event of an equipment failure. With this kind of maintenancepolicy, the maintenance related costs are usually high due to the followingreasons:  the high cost of restoring equipment to an operable condition under crisissituation;  the secondary damage and safety/health hazards inflicted by the failure;  the penalty associated with lost production.Preventive maintenance (PM), on the other hand, is the approach developed toavoid this kind of waste. Traditionally, PM takes mainly the form of equipmentoverhaul or item replacement at fixed intervals. This practice is known as time-directed (TD) maintenance. When only TD maintenance is performed, however,a lot of opportunities for enhancement of the PM programme will be lost.  Journal of Quality in MaintenanceEngineering, Vol. 1 No. 3, 1995, pp. 3-17.©MCBUniversity Press, 1355-2511  The author would like to thank Professor A.K.S. Jardine for helping to review the manuscript. This work was also supported by the Natural Sciences and Engineering Research Council of Canada under Grant A4174.    D  o  w  n   l  o  a   d  e   d   b  y   I  n   d   i  a  n   I  n  s   t   i   t  u   t  e  o   f   T  e  c   h  n  o   l  o  g  y  a   t   M  a   d  r  a  s   A   t   2   3  :   2   4   2   7   S  e  p   t  e  m   b  e  r   2   0   1   4   (   P   T   )   JQME1,3 4  To exploit the full potential of PM, the reasons for performing PM tasksshould be recognized. The reasons are[1]:  to prevent failure;  to detect the onset of failure;  to discover hidden failure. These reasons give rise to three types of PM task. The first and most commontype of PM task is time directed (TD). TD tasks, performed to prevent or retardfailures, are done at hard time intervals regardless of other information thatmay be available when the preset time occurs. A TD task also requires anintrusion into the equipment, thereby rendering it out of service until the task iscompleted. The second type of PM task is condition-based maintenance (CBM), which isalso known as on-condition maintenance, condition-directed maintenance, orpredictive maintenance. CBM is designed to detect the onset of a failure. It is anappropriate option for PM when the following conditions apply:  either failure prevention is not feasible, or how it can be achieved is notyet known, as in cases where the event leading to failure occurs in apredominantly random manner;  a measurable parameter which correlates with the onset of failure hasbeen identified, for example, the solids content in the lubricant is anindicator of the machine’s wearing condition;  it is possible to identify a value of that parameter when action may betaken before full failure occurs, such as the setting of warning limits forthe solids content of the lubricant.CBM is similar to TD maintenance in that the task is performed at presetintervals. However, unlike TD tasks, CBM does not normally involve anintrusion into the equipment and the actual preventive action is taken onlywhen it is believed that an incipient failure has been detected.Standby units, protective devices, or infrequently used equipment createspecial problems in preventive maintenance. Failures in this type of equipmentare known as hidden failures because they are not evident until the time whenthe proper function of the item is needed. Hidden failures cause operationalsurprises which may give rise to accidents. To reduce the risk exposed to hiddenfailures, fault-finding (FF) tasks are performed at scheduled intervals to checkthe state of items with dormant functions. The risk of failure  There is a widespread belief that corrective maintenance is always lesseconomical than preventive maintenance, and all failures can be prevented. Asa result, time-directed maintenance becomes the norm of preventivemaintenance action, motivating the indiscriminate use of overhaul or preventivereplacement procedures in PM programmes. Experience, judgement, vendor    D  o  w  n   l  o  a   d  e   d   b  y   I  n   d   i  a  n   I  n  s   t   i   t  u   t  e  o   f   T  e  c   h  n  o   l  o  g  y  a   t   M  a   d  r  a  s   A   t   2   3  :   2   4   2   7   S  e  p   t  e  m   b  e  r   2   0   1   4   (   P   T   )  Condition-basedmaintenance 5 recommendations and “the more the better” syndrome are the common basesfor determining the content and frequency of a TD task. This approach to PMwastes a lot of resources in doing unnecessary tasks which will not improveequipment or system availability. Furthermore, PM tasks which involveintrusion into the equipment (overhaul tasks) are potentially risky. According toa study on fossil power plants, 56 per cent of the forced outages occurred withinone week after an intrusive type of maintenance task has been performed[2]. The failure rate of an equipment measures the risk of its failure. It is definedas: h  ( t  ) = f  ( t  ) R  ( t  )where f  ( t  ) is the failure density, and R  ( t  p  ) = ∫  ∞ t  p  f  ( t  ) dt  is the reliability of an itemat age t  p  . According to the “bathtub model”, the failure rate, h  (·), decreases withage or usage when the equipment is new – the time during which this applies isknown as the infant mortality period, and it is usually relatively short. This isfollowed by a period of constant failure rate (the useful life period), and then a“wear-out” period. The latter is characterized by a failure rate which is anincreasing function of age or usage. TD tasks are effective in preventing failureswhen the equipment is operating within the wear-out region. In such cases, anoverhaul or preventive replacement task will revert the equipment to the “as-new” condition and, after a brief run-in period, the risk of failure will besignificantly reduced. If the goal of performing TD tasks is to minimize the totalmaintenance-related costs, the decision model for determining the optimalinterval between TD tasks is shown below[3]:where C  p  is the total cost of a TD task, C  f  is the total cost of a correctivemaintenance task, M  ( t  p  ) is the mean time to failure given that the failureoccurred before time t  p  , and C  ( t  p  ) is the total cost per unit time if the TD task isperformed when an item is of age t  p  .Obviously, performing a TD task will be a waste of resources when:  h  (·) is a non-increasing function of age;  the cost penalty of a corrective maintenance task, C  f  , is not greater thanthat of a TD task, C  p  .According to an extensive study conducted in the airline industry, the “bathtubcurve” is not a universal model that applies to all items, as most people havebelieved. In fact, as much as 89 per cent of all the airline equipment items do nothave a noticeable wear-out region throughout their service life, and hence theseitems will not benefit from TD tasks[4].  ()()[()]()()[()] C t C R t C R t t R t M t R t  p p p f p p p p p  =+ −+ − 11    D  o  w  n   l  o  a   d  e   d   b  y   I  n   d   i  a  n   I  n  s   t   i   t  u   t  e  o   f   T  e  c   h  n  o   l  o  g  y  a   t   M  a   d  r  a  s   A   t   2   3  :   2   4   2   7   S  e  p   t  e  m   b  e  r   2   0   1   4   (   P   T   )
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