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Applying Reliability- Centred Maintenance to Mechanical, Electrical, Electronic and Structural Systems Introduction In the March edition of Navy Engineering Bulletin, the benefits of applying Reliability-centred Maintenance to Naval assets were explored. It was shown that there is now a large body of experience in Defence to demonstrate that SAE JA 1011 compliant RCM (such as RCM2 and Def Stan 02 45) produces a maintenance programme which reduces the high cost of traditional naval maintenance without sacrificing system availability or reliability. Ongoing benefits in the Royal Navy for applying RCM to the majority of platforms and systems are expected to be in excess of £50M per annum. The article explained that true RCM involves answering seven structured questions about the asset or system under review: • What are the functions of the asset in its present operating context? • How can the asset fail to fulfil each function? • What would cause each functional failure? • What happens when each failure occurs? • In what way does each failure matter? • What can be done to predict or prevent each failure? • What should be done if no suitable proactive task can be found? Through the first four questions, RCM defines functional requirements, including performance standards, defines what we mean by ‘failure’ and completes a Failure Modes and Effects Analysis (FMEA) for the asset in question. The fifth question determines failure consequence and determines how each failure matters. The four RCM consequence types are ‘Hidden, Safety, Environmental, Operational (loss of mission in the naval sense) and Non- Operational. The last two questions enable the appropriate failure management policy to be developed. These seven questions can only be answered by people who know the asset best – the maintainers and operators supplemented by specialist input where appropriate. Readers of Navy Engineering Bulletin have requested more information about the applicability of this process to the complete range of systems and subsystems which make up modern platforms and weapon systems. This article shows that NAVY ENGINEERING BULLETIN SEPTEMBER 2003 the RCM process applies equally well to mechanical, electrical, electronic systems and naval structures and reviews the value of a comprehensive RCM database. RCM and Failure Management Before we can understand how RCM can be applied ‘across the board’, we must try to understand both the nature of failure and the decision-making logic involved in undertaking SAE compliant RCM. The six failure patterns The starting point is the six failure patterns described in the March issue, reproduced in Figure 1. These patterns, which are fundamental to understanding maintenance programme development, show the possible ranges of failure mode behaviour and are plots of conditional probability of failure (vertical axis) against time in service (horizontal axis). They arise from a large body of failure analysis conducted in BY DR ALUN ROBERTS, THE ASSET PARTNERSHIP 37
38 NAVY ENGINEERING BULLETIN SEPTEMBER 2003 A B C D E F FIGURE 1: THE SIX FAILURE PATTERNS the aircraft industry during the 1960s and 1970s. The significance of the Patterns is as follows: • Patterns A, B and C display agerelated failure. For Patterns A and B the ‘conditional probability’ increases sharply after a specific point in time, (whereas with Pattern C, the increase is steady and there is no one age at which a rapid increase occurs). These three patterns generally apply to simple items or to complex items which have a dominant mode of failure. Typically, they involve failure mechanisms such as wear, corrosion, erosion, evaporation and fatigue, often associated, although not exclusively, with mechanical systems. For aircraft systems, only 11% of failure modes could be assigned to this group with any confidence. • Patterns D, E and F, on the other hand, are not age-related, as there is no age at which there is a rapid increase in the conditional probability of failure. With the exception of the low or high conditional probability of failure in the early periods of service for Patterns D and F, the three patterns display essentially random failure. Patterns D, E and F are associated with systems such as hydraulics, pneumatics and electronics. (A good example 4% 2% 5% 7% 14% 68% Pattern A: The “Bathtub Curve” High infant mortality, then a low level of random failure, then a wear out zone. Pattern B: The “Traditional View” A low level of random failure, then a wear out zone. Pattern C: A steady increase in the probability of failure. Pattern D: A sharp increase in the probability of failure settling down to random failure. Pattern E: Random Failure No relationship at all between how old it is and how likely it is to fail. Pattern F: The “Reversed J” curve High infant mortality, then random failure. of a Pattern E failure is a rolling element bearing. Bearings are complex items which tend not to have a dominant failure mode. For a family of bearings, failures will occur through normal wear and tear, incorrect installation, inadequate lubrication, faulty materials, overloading (etc), this diversity being responsible for the demonstration of random failure behaviour). • In the world of aviation, Failure Pattern F accounted for approximately two thirds of all failure modes. This was a direct result of the level of invasive scheduled overhaul being conducted which often left the system concerned in a failed state. The reasons for infant mortality are shown in Figure 2: Managing Age-related failure Failure Patterns A and B, and to a lesser extent C, can often be managed through some form of scheduled maintenance activity which will be driven by the age at which we are confident the conditional probability of failure increases. In RCM, these strategies are labelled scheduled restoration and scheduled discard. Here, we either restore or remanufacture a part or piece of equipment to name plate levels of performance, or throw the item away and replace it at some point before the conditional probability of failure increases. Such items are often described as having a ‘Life’. Unfortunately, most industrial organisations do not possess the large body of failure data necessary to demonstrate beyond doubt the presence of agerelated failure. Indeed, in current overhaul-oriented programmes such as those used in the RAN, most plant will not reach the point at which any rapid increase in the conditional probability of failure would arise – the equipment will usually have been changed out far earlier based on the current invasive maintenance regimes in place. When an RCM analysis is being conducted, there is therefore some doubt about whether a failure is truly age related or not. Where FIGURE 2: PREMATURE FAILURE (PATTERN F) sufficient doubt exists, RCM does not sanction scheduled restoration or discard as valid strategies and instead seeks an alternative approach. Also, scheduled restoration and discard are both extremely costly, and can seriously impact future reliability by increasing the incidence of Failure Pattern F failures. For these reasons, they are selected relatively infrequently. Managing Random Failure For obvious reasons, random failure (as demonstrated by Failure Patterns D, E and F and the early age failures in Patterns A, B and C) cannot be managed through scheduled restoration and discard. This reality has led to the growth of condition-based maintenance (CBM) strategies over the last ten to fifteen years in most areas of industry. CBM is based on the fact that most incipient failures provide us with early warning signs, known in RCM as Potential Failures. Once we know a failure is occurring, we are to undertake corrective action at a time of our choosing. In managing random failure, RCM employs the concept of the P-F curve shown in Figure 4. This describes the deterioration of physical systems and the conditions which apply to selection of an appropriate failure management policy known in RCM as an ‘On-condition’ task. Frequencies of On-condition tasks are driven by the gap between Premature failures caused by: • incorrect functional specification (what it must do) • poor design (what it must be in order to do it) • poor quality manufacture • incorrect installation • incorrect commissioning • incorrect operation • unnecessary maintenance • excessively invasive maintenance • bad workmanship
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