1. Prevent or mitigate failure occurrence
2. Detect onset of failure
3. Discover a hidden failure
4. No nothing, because of value limitations

By identifying the four factors for doing preventive maintenance, we have also set the stage for defining the four task categories from which a PM action may be specified.  These task categories, by one name or another, are universally employed in constructing a PM program, irrespective of the methodology that is used to decide what PM should be done in the program.

The four task categories are as follows:

1. Time-directed (TD): aimed directly at failure prevention or retardation.
2. Condition-directed (CD): aimed at detecting the onset of a failure or failure symptom.
3. Failure-finding (FF): aimed at discovering a hidden failure before an operational demand.
4. Run-to-failure (RTF): a deliberate decision to run to failure because the others are not possible or the economics are less favorable.

Time Directed (TD)

In the not too distant past, virtually all preventive maintenance was premised on the basis that equipment could be periodically restored to like-new condition before it was necessary to discard it for a new (or improved) item.  This premise thus dictated that equipment overhauls were about the only way to do preventive maintenance.

Today, we are slowly realizing that this is not always the correct path to pursue.  However, in many valid situations we still specify PM tasks at predetermined ("hard time") intervals with the objective of directly preventing or retarding a failure.  When such is done, we call it a time-directed task.  A TD task is still basically an overhaul action-sometimes very complete, extensive, and expensive (like rebuilding an electric motor), and sometimes very simple and cheap (like alignments and oil/filter replacements).  As a rule of thumb, whenever we have a planned intrusion into the equipment (even just to inspect it), we have in essence an overhaul-type action which is labelled a TDI (Time-Directed Intrusive) task.  Some time-directed tasks can be non-intrusive, such as simple visual inspections or minor adjustments that do not require a breach of the equipment boundary or housing.  In this case, the action is simply labelled as a TD task.

More often than not, time-directed tasks tend to be intrusive.  A simple example that everyone can picture is the changing of oil in our automobile.  Here, we intrude in the PM action by removing the drain plug (which will leak if not properly reinstalled), by injecting fresh oil (which must be of the correct type, grade, and quantity with the fill cap properly replaced), and by replacing the oil filter (which will leak if the gasket is not properly installed).  The "hard time" associated with this action is car mileage, which has been suggested by the manufacturer who has collected years of experience defining excessive engine wear as a function of oil deterioration due to contaminants and loss of viscosity.

Notice that this simple PM task, a TDI task, presents several opportunities for human error to creep into the procedure.  The keys to categorizing a task as time-directed are: (1) the task action and its periodicity are preset and will occur without any further input when the preset time occurs; (2) the action is known to directly provide failure prevention or retardation benefits; and (3) the task usually requires some form of intrusion into the equipment.

Condition-Directed (CD)

When we do not know how to directly prevent or retard equipment failure-or it is impossible to do so — the next best thing that we can hope to do is to detect its onset and predict the point in time where failure is likely to occur in the future.  We do this by measuring some parameter over time where it has been established that the parameter correlates with incipient failure conditions.  When such is done, we call it a condition-directed or CD task.  Thus, a CD task would pre-warn us to take action to avoid the full failure event.  If the warning comes soon enough, our action can most likely be taken at some favorable timing of our choice.

The CD task, like the TD task, has a periodicity for the measurements, but actual preventive actions are not taken until the incipient failure signal is given.  The CD task takes two forms: (1) we can measure a performance parameter directly (e.g., temperature, thickness) and correlate its change over time with failure onset; or (2) we can use external or ancillary means to measure equipment status for the same purpose (e.g., oil analysis or vibration monitoring).  With the CD task, all such measurements are nonintrusive.  The keys to classifying a task as CD are: (1) we can identify a measurable parameter that correlates with failure onset; (2) we can also specify a value of that parameter when action may be taken before full failure occurs; and (3) the task action is nonintrusive with respect to the equipment.

Failure-Finding (FF)

In large complex systems and facilities, there are almost always several equipment items-or possibly a whole subsystem or system-that could experience failure and, in the normal course of operation, no one would know that such failure has occurred.  We call this situation a hidden failure.  Backup systems, emergency systems, and infrequently used equipment constitute the major source of potential hidden failures.  Clearly, hidden failures are an undesirable situation since they may lead to operational surprises and could then possibly initiate an accident scenario via human error responses.  For example, an operator may go to activate a backup system or some dormant function only to find that it is not available and, in the pressure of the moment, fail to take the correct follow-up procedure.  So, if we can, we find it most beneficial to exercise a prescheduled option to check and see if all is in proper working order.  We call such an option a failure-finding task.

Let's look at a simple example-the spare tire in our automobile.  If you are like us, you don't really worry about a flat spare tire because you have AAA coverage, and are never more than 10 to 15 minutes away from an ability to get emergency road service-except for that once-a-year trip with the family into "uncharted lands" (e.g., Death Valley).  Again, if you are like us, you do check the spare before you leave — and that is a failure-finding task.

Notice that the only intent in such an action is to determine if the spare tire is in working order or not.  We are doing nothing to prevent or retard a flat tire (a TD task) or to measure its incipient failure condition (a CD task).  It is or is not in working order.  And, if it is not in working order, we fix it.  That is the essence of what a failure finding task is all about.  (Is it OK?  If not, fix it.)

Run-To-Failure (RTF)

As the name implies, we make a deliberate decision to allow an equipment to operate until it fails — and the maintenance action occurs only after the failure has occurred.  There are some limited cases where such a strategy makes common sense:

1. We can find no PM task that will do any good irrespective of how much money we might be able to spend.
2. The potential PM task that is available is too expensive.  It is less costly to fix it when it fails, and there is no safety impact at issue in the RTF decision.
3. The equipment failure, should it occur, is too low on the priority list to warrant attention within the allocated PM budget.
4. Note the distinction between FF and RTF.  With FF the failure is hidden and we do not want to be surprised by its occurrence if the failure should happen.  With RTF, we have made a deliberate decision not to be concerned about failure occurrence, be it evident or hidden, and will simply correct the failure at our time of choosing should it occur.

Printed with permission from Butterworth-Heinemann, a division of Elsevier, from RCM - Gateway to World Class Maintenance, by Anthony M. Smith, AMS Associates Inc. in California, and Glenn R. Hinchcliffe, Asset Performance Technologies Inc., Corrales, NM.  Copyright 2004.  For more information about this title and similar titles, please visit www.books.elsevier.com.