MRO Magazine

Is Your Preventive Maintenance at the Right Frequency?

Mr. O’s Tips

July 7, 2021
By James Reyes-Picknell

We know that preventive maintenance (PvM), if done right, can appear to be wasteful of the items and fluids you discard. However, if done at the right frequency, the cost of that “waste” is less than the total cost of the failures that are prevented.

If the failures could result in safety or environmental risks, we make the decision on frequency on the basis of “risk,” not cost.

Let’s say you have a PvM program and you are seeing a lot of seemingly good “stuff” being thrown away. How do you know if you are doing too much PvM or not enough?

When we use reliability-centred maintenance (RCM), we calculate the task frequencies quite precisely. We will give you a way of sensing when you are doing too much or too little.

If you are doing too little PvM, then the PvM tasks are too infrequent. The area under the curve in the figure (above) will be too large and you will experience more failures than you feel you should. If you experience twice as many failures as you expected. It means you need to cut the blue area in half. Increase task frequency to achieve that. You will then be doing more PvM and you should experience fewer failures.

On the other hand, if you are doing too much PvM, then the area under the curve might be uneconomically small. The costs of doing the PvM may exceed the costs of the failures you are trying to avoid. When you compare costs, remember to include all costs, including the value of lost production.

Another clue to doing too much PvM is if you suffer “infant mortality” -type failures. These are failures that occur as a result of the change you made.

For example, you changed the oil and filter in an engine sump. When you added the new oil, you may have inadvertently also introduced some foreign contamination (dirt particles from the oil container). The new oil may now actually have more particulate than the old oil had and it could plug the filter quickly or damage the engine. The premature failure due to the dirt is an example of infant mortality.

These failures are the result of material, part, or other defects, or human errors. Even the start-up after the oil change could be botched by an inattentive operator. A simple root cause failure analysis will help you determine if your PvM interventions are actually leading to more failures.

These early premature failures are quite common. The only way to avoid them is to avoid disturbing the equipment. The adage “if it isn’t broken, don’t fix it” is quite valid. If you do your PvM too often, then you might be introducing opportunities for infant mortality failures.

A good engineer who understands the mathematics of reliability can help you get very precise in choosing task frequencies, but even without one, you can tell if you need to increase or decrease them.

James Reyes-Picknell, P.Eng. – Principal Consultant – Conscious Asset