FMEA or RCM: Is There a Difference?

Q: Should I do a FMEA or RCM to resolve a machine issue?

A:The short answer is that it depends on several things, primarily frequency of occurrence and severity of the failure mode. I frequently am asked, what’s the difference between doing an FMEA (failure mode and effects analysis) and an RCM (reliability-centered maintenance) evaluation?

FMEAs were originally directed at improving product, machinery, and equipment at the design stage. Use of them began in the military in the late 1940s, were implemented in the aerospace industry in the 1960s, and spread to manufacturing in subsequent years. In 1983, Stan Nowlan (United Airlines) collaborated with John Moubray to deliver the message to industry that led to Moubray’s RCM II book (1977, Industrial Press), which is a key RCM reference.

In simplest terms, an FMEA is a standardized methodology used to identify potential failure modes for a product/process/machine. FMEAs use three scales (severity, occurrence, and detection), to assess each failure-mode risk. Failure modes are ranked, based on a risk-priority number (three scales multiplied together). The final step is to correct the undesired items. Note that, in some situations, the preferred option would be run to failure. After performing several FMEAs, it becomes evident (with some break-even calculations) which minimum risk-priority number (RPN) indicates required improvement actions. Often, a severity-versus-occurrence matrix is constructed to depict relative importance in terms of elimination or mitigation. FMEAs should be a living document that drive continuous improvement.

Reliability-centered maintenance

RCM is also a standardized methodology. It’s used to analyze the functions and potential failure modes of a physical asset (machinery, equipment, automobile, pump) to remove failures and develop an ongoing maintenance plan. RCM addresses risk, operations, and cost and attempts to remove all failures (not just those above a specified RPN number, as with FMEAs). For those reasons, RCM provides a more-detailed analysis than an FMEA. Details on RCM and FMEA can be found in “Evaluation Criteria for Reliability-Centered Maintenance (RCM) Processes” (JA1011_199909, updated_200908). That was how it started.

What I see today is that the majority of required work involves machinery and equipment carrying lesser failure consequences than airplanes and military assets. People are primarily doing FMEAs and predictive-maintenance optimization (PMO) to attain better failure-mode mitigation through engineering changes, or using a combination of condition-based monitoring, predictive technologies, and time-based and run-to-failure strategies. In addition, more engineers are writing FMEAs to a similar detail level of RCM (with function and performance details). Some have added parts availability, cost, and other items to the FMEAs for better decision making. With the higher level of detail and PMO done at the back end of FMEAs, there is not much difference between an industry-typical RCM and an FMEA.

You will also hear the term FMECA, which is an FMEA with a criticality analysis. FMEA is based on qualitative analysis (team consensus of the three scales). FMECA uses quantitative analysis (actual failure rate and failure severity), which is called a criticality analysis (CA).

Many of the FMEAs performed in manufacturing and process industries are still done on paper or an Excel spreadsheet, with some using software. While paper is a good way to begin and learn, it doesn’t lend itself to sharing knowledge or developing a reliable history. Remember that an FMEA should be a living document. As such, some form of digital storage and common formatting will be helpful to enable easy use of historical learning.

The answer to the original question is somewhat different than what may have been given in the 1950s or 1980s, and will be different again in a few years. FMEAs have evolved from mainly a design-in tool for new product/equipment to today’s more-detailed analyses and PMO (which is becoming similar to RCM). Future FMEAs will use machine learning/artificial intelligence to capture, store, and leverage historical information. This will result in better processes, products, equipment design-in, and, in general, ongoing improvement. EP

Based in Knoxville, Klaus M. Blache is director of the Reliability & Maintainability Center at the Univ. of Tennessee, and a research professor in the College of Engineering. Contact him at kblache@utk.edu.