What is Reliability Centred Maintenance?


We live in a changing world; change and management dominate all aspects of modern industry and commerce. Although perhaps not immediately apparent to some, the management of the physical assets underpinning today's industries has changed beyond recognition in under a generation. These changes have occurred at all levels; from the obviously technical shift as equipment and systems become increasingly complex, to strategic transformation in the way we understand failure and the rationale used to develop planned maintenance activities.

This paper looks at one of the most significant strategic change to impact the management of physical assets. This strategic change is known as Reliability Centred Maintenance (RCM) and incorporates within it several distinct shifts in the way we view physical assets and their upkeep. RCM as a subject has been widely written about and it is not the aim of this paper to provide a detailed account of the process here. Rather, its aim is to provide an introduction to RCM for new users and a quick history for the more experienced.


Before looking at where we going, lets see where we have come from. In the period up to and shortly after the First World War equipment was generally simple and robust. The ways in which it could fail were easily treated since the simplicity aided diagnostics and in some cases equipment failure was an acceptable reason for loss of production. In this environment, maintenance was largely reactive; simply to fix things when they tailed, supplemented with simple tasks such as lubrication. As an example, consider a steam train. The operating principle is well understood, the systems are simple with a low level of automation and low configurability (essentially doing one job), the construction is robust & contains redundant elements and operational tolerances are broad. There had to be a lot wrong before the train actually stopped running.

However, during the Second World War, things began to change and the availability of manpower declined in industrialized economies of the time. Equipment became more complex, thus replacing the need for manual intervention and reducing manpower requirements. Loss of production through equipment failure also became unacceptable leading to work on prevention of failures before they occurred. Conventional wisdom suggests that as equipment gets older it "wears out" and becomes more likely to fail. Using this model it was believed that failures could be avoided if equipment was maintained before items "wore out" and the failure occurred, i.e. planned intervention at the right time would prevent failures, all that had to be determined was the right time.

Interestingly, this line of thought yields an insight into the use of the principal maintenance performance indicators to this day i.e. the ratio of planned to breakdown maintenance. If the likelihood of item failure increases with age, then planned intervention before the failure should reduce the number of failures that occur. Using this model suggests that if we continue to see failures then we have not intervened early enough i.e. we do not yet know the right age. Therefore it would seem appropriate to measure the effectiveness of our strategy by measuring the amount of planned to unplanned maintenance. This is widely reported in industry; improvement targets are even established for this ratio (in most cases, the target is parity). However, as will shortly be shown, this takes no account of the technical characteristics of the failure and assumes that we want to prevent all failures. This is not the case and the measurement is essentially meaningless e.g. one German car plant has determined its most effective ratio of planned to unplanned maintenance as 1:64!

The growth of civil aviation in late 1940's and 50’s triggered the next step. At about the same time the Federal Aviation Administration (FAA), the body responsible for regulating airlines in the USA was worried about aircraft reliability. In an effort to reduce the number of failures, the industry concluded that the maintenance was being done too late based on the accepted "wear out" model of failure. So the frequency of scheduled maintenance was increased. This lead to higher maintenance costs which by the late 1950's prompted the industry to look at the concept of preventive maintenance. In addition the FAA was concerned that the reliability of some engines had not been improved by changing either the type or frequency of overhaul. The data available at the time indicated that although the frequency of occurrence of some failures had been reduced, many more had remained unchanged or actually increased! There was no way this finding could be explained using the model of failure accepted at that time.

A task force, consisting of representatives from both the FAA and the airlines, was established to investigate planned maintenance policies. What evolved was a statement from the committee that the reliability and the overhaul frequency of equipment was not necessarily directly related and the common belief that reliability declined with increasing age was not generally true. In fact:

1. Scheduled overhaul has little effect on the overall reliability of a complex item unless there is a dominant failure mode.

2. There are many items for which there is no effective form of scheduled maintenance.

It became obvious that too much emphasis had been placed on the 'right age’ model.

The task force went on to develop a propulsion system reliability program, each airline involved developed reliability programs for their own particular areas of interest. These became the Handbook for the Maintenance Evaluation and Program Development for the Boeing 747, more commonly known as MSG-1 (Maintenance Steering Group 1). MSG-1 was subsequently improved and became MSG-2. In 1979 the Air Transport Association (ATA) reviewed MSG-2 to incorporate further developments in preventive maintenance, this resulted in MSG-3, the Airline/Manufacturers Maintenance Program Planning Document.

United Airlines was sponsored by the US Department of Defense to write a comprehensive document on the relationships between Maintenance, Reliability and Safety. The report was prepared by Stanley Nowlan and Howard Heap, it was called ‘Reliability Centred Maintenance'. Outside the aerospace industries, the application of MSG-3 is generally known as RCM. The work of the airlines predated similar problems that spread throughout industry during the 1980’s, consequently industry has been fortunate in being able to use the airlines prior experience.

Why is RCM different?

The development of RCM has allowed us to literally re-define maintenance. It re-focuses our thinking by differing in four very significant ways from all that went before it:

1. The objective of a successful PM program is to prevent or mitigate the consequences of failures, not to prevent the failures themselves. Of the thousands of possible failure modes on any facility or installation, each has a different effect on e.g. safety, operations, environment or cost. It is the failure consequence that determines what, if any resources will be used to prevent their occurrence. This leads to the conclusion that that if the consequence of a failure does not have an adverse effect on safety; operations, environment or cost, then there is no need to carry out scheduled maintenance.

2. The consequences of failure differ depending on where and how items are installed and operated. For example, missing an appointment is a likely outcome of mechanical breakdown of a car usually driven in urban areas, however the same failure in the middle of the Sahara desert will have much more severe consequences. A formal review of failure consequences focuses attention on maintenance tasks that have most effect, and diverts energy away from those which have little or no effect. This helps ensure that whatever is spent on maintenance is spent where it will do the most good.

3. We no longer assume that all failures can be prevented by PM or that even if they could be prevented, that it would desirable to do so. Consider puncture of car tyres, what affects the number of failures rate is the number of nails in the road, not the age of tyre. PM is simply not applicable to this failure mode.

4. We are concerned principally with what we want the equipment to do not what it actually is. Say we need a hand held, portable writing instrument, capable of producing erasable text and lines in the width range 0.2 to 0.5 mm. Both a wooden and a mechanical (propelling) pencil broadly satisfy this simple description. The specification of what we need is independent of the method used to achieve it. By identifying what we actually want means we focus our maintenance on what matters and identify any gaps in the required performance and that which a system is capable of.

RCM builds on these simple ideas to determine applicable and effective maintenance for each failure. The mechanics of the RCM process itself are well described by other authors.

How does the RCM process fit into the Preventive Maintenance cycle?

The power of RCM is not in doubt. There is more than enough hard evidence from manufacturing, extractive, transport and process industries that prove the techniques value in establishing and improving system maintenance. It is however a sharp tool, and is usually best applied in selected areas rather than broadly across a facility. To achieve worthwhile results it must also be carried out by (or at least with) the actual operators and maintainers of the systems in question. RCM is not a "quick fix" solution, time and effort must be invested on training, raising awareness, execution and implementation.

It does however achieve an understanding of how plant works, what it can (or cannot) achieve, and the causes of failure. By doing so it focuses maintenance effort on those areas where it is beneficial. The analysis itself is carried out in groups consisting of experienced supervisors, and specialists (if needed). These groups set up maintenance tasks and an ownership concept is developed. The development of RCM had lead to a radical change of direction in our understanding of maintenance and its performance and has presented us with what were entirely new concepts, Today, maintenance directly influences the core aspects of modern business, safety and environmental integrity, energy efficiency, quality, uptime and costs. RCM forms the core of any effective maintenance policy and should therefore be at the heart of your business.

Your company can also benefit from RCM. Our 4 day RCM seminar will show you exactly how.

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RCM is a team activity. These teams are led by an RCM specialist known as a facilitator.  M2K provides an 8-day RCM Facilitator Course to enable companies to develop in-house RCM facilitators.

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