April 26, 2019, 11:55 pm

Reliability Centred Maintenance (RCM) vs Preventive Maintenance Optimisation (PMO)

In previous articles, we have described "What is Reliability Centred Maintenance (RCM)?" and "What is Preventive Maintenance Optimisation (PMO)?". In this article, we will discuss the similarities and differences between the two, and make some recommendations regarding which approach may be most appropriate for Preventive Maintenance program development, and the circumstances under which that may apply.


Similarities between RCM and PMO

There are a number of strong similarities between RCM and PM Optimisation. These include:

  • Both processes aim to produce an optimal PM program.
  • Both use the identification of underlying Failure Modes (causes) as the starting point for the decision making process.
  • Both processes use traditional RCM decision logic. The second half of both processes is essentially identical, using the same decision framework and RCM principles.
  • Both approaches focus on the business consequences of equipment failure, as well as the technical characteristics of those failures.RCM vs PMO

The Difference between RCM and PMO

The primary difference between RCM and PMO is in the initial stage of the analysis and relates to the way that failure modes are identified.

  • RCM starts with a blank sheet of paper and identifies the failure modes by first identifying the key functions required of the equipment (in its current operating context), and then the associated functional failures and failure modes. In this sense, it starts from a zero-base.
  • On the other hand, PMO uses existing Preventive Maintenance tasks and failure history to identify likely failure modes. It only explicitly considers functions when identifying Hidden failure modes. In this sense, it tends to use current experience and practice as the starting point.

The Potential Pitfalls with RCM

There are a few potential pitfalls associated with RCM (in comparison with PMO – there are other pitfalls which we will not discuss in this article). These include:

  • As it is a zero-based process, RCM tends to be more time-consuming. In particular, as much as one third of the analysis time can be spent simply identifying failure modes for analysis.
  • The zero-based nature of RCM also tends to mean that, where teams are used to perform the analysis, the team needs to be involved throughout the process. There are few opportunities to do pre-work prior to engaging the team. This makes the process more labour-resource-intensive.
  • When applying RCM, there can be a tendency, if not carefully facilitated, to over analyse equipment and systems and identify many functions and failure modes – many of which end up having preventive tasks that are either not technically applicable, or are not worth implementing (given the business context of the equipment).
  • There can also be a temptation, when using RCM, to inappropriately cut and paste analyses and the results of analyses from one equipment item to another. This can, however, be avoided by appropriately skilled and trained facilitators.
  • Because RCM analysis is comparatively more time-consuming, often the focus and priority is on completing the analysis, rather than on implementing the outcomes.

The Potential Pitfalls with PMO

There are a few potential pitfalls associated with PMO (once again, in comparison with RCM). These include:

  • PMO ideally requires an existing PM program and/or failure history for the equipment (or essentially identical equipment). If this does not exist, then PMO is very difficult, if not impossible, to perform.
  • The approach for identifying failure modes tends to be less rigorous. As a result, low frequency (but potentially high consequence) failure modes can potentially be overlooked. This may be an important factor when analysing highly critical equipment.
  • In addition, low frequency failure modes, even when low consequence, can sometimes be overlooked when using the PMO process. Failure history in CMMS from events greater than about 3-5 years ago generally does not exist (as these systems are often updated at this frequency), and as a result failures with an MTBF > about 5 years can be overlooked. If the current PM program does not address these less frequent failure modes, then a PMO review may also miss these.
  • Because the starting point for most failure modes is the existing Preventive Maintenance program, there can sometimes be a bias to maintain the status quo (i.e. retain the existing PM routines, or at best only vary them slightly), even when more effective tasks exist. This can be overcome through effective facilitation, but it does require greater vigilance and effort on the part of the facilitator to challenge this bias.

Which approach should you use?

Given the above, we suggest that:

  • RCM tends to be more appropriate for brand new equipment, particularly where this equipment is using new technology that does not exist elsewhere. In this situation, a "blank sheet of paper" approach is necessary in order to identify the likely failure modes although you could potentially use the FMEA developed during the design phase of the equipment as a starting point, if one exists, and if you have access to it. In addition, where it is absolutely vital that you make sure you identify all critical failure modes associated with the equipment (for example, those associated with failure with potentially catastrophic consequences), then the more rigorous approach applied by RCM in this area is likely to be appropriate.
  • PMO tends to be more appropriate for existing equipment, particularly where an existing PM program is in place and some operating and maintenance experience has been obtained. The use of the existing PM programs and experience base lends itself to optimisation. This is particularly true where the likely failure modes associated with the equipment are well known. In these situations, PMO will provide essentially the same, optimum Preventive Maintenance program as RCM will, but will do this in a more time- and resource-efficient manner.

Of course, there are many other approaches that can be taken for the development of Preventive Maintenance programs. We discuss these (as well as RCM and PMO) in our article "Alternative approaches for developing and optimising Preventive Maintenance".


I hope that this has given you a better understanding of the similarities and differences between RCM and PMO. Assetivity has been training and implementing RCM and PM Optimisation in organisations for 15 years. If you would like to benefit from our experience, then why not consider attending one of our two-day courses in Reliability Centred Maintenance and PM Optimisation. These run in various locations around Australia, or we can deliver the course in-house for your organisation. Our consultants can also facilitate RCM or PM Optimisation studies and help you to embed these processes within your organisation. Contact us if you would like to discuss how we may be able to help you.

Sandy Dunn


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