Four Task Categories To Understand In Undertaking Preventive Maintenance (2/4)

This is part 2 of the series Four Task Categories To Understand In Undertaking Preventive Maintenance

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.