Issue 76, June 2007

Basic Steps of Applying Reliability Centered Maintenance (RCM) Part III

This article continues the discussion of the basic steps of applying RCM that began in Issue 72.

Although there is a great deal of variation in the application of Reliability Centered Maintenance (RCM), most procedures include some or all of the seven steps shown below:

1. Prepare for the Analysis
2. Select the Equipment to Be Analyzed
3. Identify Functions
4. Identify Functional Failures
5. Identify and Evaluate (Categorize) the Effects of Failure
6. Identify the Causes of Failure
7. Select Maintenance Tasks

If we were to group the seven steps into three major blocks, these blocks would be:

• DEFINE (Steps 1, 2 and 3)

• ANALYZE (Steps 4, 5 and 6)

• ACT (Step 7)

The DEFINE stage was discussed in Issue 72 of Reliability HotWire and the ANALYZE stage was discussed in Issue 73. In this article, we examine the final stage: ACT. This stage, consisting of the Select Maintenance Tasks step, deals with the sixth and seventh questions in the SAE JA1011 standard, which describes the minimum criteria that a process must comply with to be called a Reliability Centered Maintenance (RCM) process:

• What should be done to predict or prevent each failure (proactive tasks and task intervals)?

• What should be done if a suitable proactive task cannot be found (default actions)?

At this juncture, our goal is to determine the list of applicable candidate tasks and then select the task that will allow us to meet the maintenance goals and give us the biggest return on investment.

Failure Management Strategies

There are a number of failure management strategies that can be implemented. These are discussed briefly in the following sections.

Scheduled Inspections:

The purpose of inspections is to discover failures that have already happened but have not yet surfaced or to detect failures that are about to happen.

• Failure-Finding Inspections: inspect the equipment on a scheduled basis to discover hidden failures in the equipment before an operation demands it. If the equipment is found to be failed, initiate corrective maintenance. An example of this would be checking a smoke detector to confirm that it still works.

• On-Condition Inspections: inspect the equipment on a scheduled or ongoing basis (condition monitoring) to discover conditions that indicate that a failure is about to occur. This can be done by measuring some parameter that correlates over time with incipient failure conditions. If the equipment is found to be about to fail, initiate preventive maintenance. An example of this would be examining the wear on tires and replacing the tires if needed. For a closer look at on-condition maintenance, see this month's Hot Topics.

Scheduled Preventive Maintenance:

This a maintenance type that is scheduled in advance to occur at a "hard time" regardless of the apparent condition of the equipment. It aims at restoring the initial capability of the equipment.

• Service: perform lubrication or other servicing actions on a scheduled basis (e.g. lubrication and oil change).

• Repair: repair or overhaul the equipment on a scheduled basis (e.g. overhauling an engine).

• Replace: discard the old equipment and replace it with a new one on a scheduled basis (e.g. replacing air filters at regularly scheduled intervals).

Run-to-Failure:

A deliberate decision is made to run the equipment to failure and fix it when it fails, but not to perform scheduled maintenance actions.

Design Change:

Changes can be made to the specifications of any item of the equipment. This could include: re-designing the equipment; selecting different equipment; relocating the equipment; creating redundancies; making some other one-time change to improve the intrinsic reliability/availability of the equipment; or improving something about how the equipment is used (e.g. procedures, training and/or documentation).

Choosing a Failure Management Strategy

Choosing the appropriate failure management strategy or strategies to employ for each potential failure relies on a combination of factors and considerations such as technical feasibility, judgment/experience and whether the task is worth doing. Making a good task choice starts with a good understanding of the failure behavior of the failure mode we are trying to address. This means we need to know the failure distribution model (probability density function, or pdf) that governs the failure mode. So we need to determine how much we really know about the failure behavior over time. We could use failure data from prior maintenance records or equipment testing to develop the failure distribution model, or rough estimates from engineering knowledge, or information from a supplier. This is the "R" in RCM, which is often overlooked in an RCM process. In the absence of at least a reasonable reliability estimate, implementing a task could lead to unproductive or counterproductive results.

The following figure shows a list of the common failure rate behaviors over time. Different failure behaviors call for different failure management strategies. Pattern A could benefit from preventive maintenance techniques. On-condition inspection techniques are appropriate for failure behaviors C, D and E.

There are also task selection logic diagrams that can be used to decide what failure management strategy to use. For instance, the following figure shows a portion of a logic diagram that is provided as an example in the SAE JA1012 guidelines. (The portion of the diagram presented here represents the questions to be considered for failures with effects that have been categorized as "Hidden Operational.") Similar diagrams are provided in other published RCM guidelines.

Using Simulation To Compare Actions

In addition to, or instead of, a logic diagram approach, the RCM analyst may wish to use cost- and availability-based comparisons of potential maintenance strategies when selecting and assigning maintenance tasks. Simulation techniques can help in comparing maintenance strategies. Given certain information about how the equipment will be operated, the probability of occurrence for the failure mode and the maintenance characteristics, the analyst can use simulation to estimate the cost and average availability that can be expected over the operational life of the equipment when a particular maintenance strategy is employed. The calculations can then be used to compare available maintenance strategies so that the analyst can select the most cost-effective approach that provides an acceptable level of performance. A Reliability Edge article (Volume 6, Issue 1) provides more details and examples of this task selection technique.

Default Actions

If we can not find a proactive task (on-condition inspection, preventive maintenance or failure-finding task) that is feasible and worth doing, then a default action should be taken. The appropriate default action can be decided based on the consequence of the failure. If proactive tasks cannot be found to reduce the safety or environmental risks to a tolerable level, the equipment must be redesigned or the process/system that uses the equipment must be changed. If proactive tasks cannot be found to reduce the operational consequences, the initial default decision can be to do nothing (i.e. run to failure). If the repair or the operational costs are too high, then a redesign might be needed.

Summary

This article, along with the two earlier parts, explained the steps involved in an RCM process. ReliaSoft's RCM++ software facilitates the RCM process described here and provides many useful capabilities for efficient data management and flexible reporting.

References

1. ATA MSG-3 "Operator/Manufacturer Scheduled Maintenance Development," updated in March 2003.

2. Moubray, John, Reliability-Centered Maintenance, Industrial Press, Inc., New York City, NY, 1997.

3. Nowlan, F. Stanley and Howard F. Heap, Reliability-Centered Maintenance. Issued in December, 1978.

4. SAE JA1012 "A Guide to the Reliability-Centered Maintenance (RCM) Standard," issued in January 2002.

5. Smith, Anthony, Hinchcliffe, Glenn R., RCM - Gateway to World Class Maintenance, Elsevier Inc, Burlington, MA, 2004.

Copyright 2007 ReliaSoft Corporation, ALL RIGHTS RESERVED