MRO Magazine

Preventive Maintenance Can Look Wasteful


Mr. O’s Tips

July 7, 2021
By James Reyes-Picknell

Preventive maintenance (PvM) is about preventing failures that are likely to happen, but haven’t yet. Preventive activities include replacements (e.g., oil and filter changes) and restorations (e.g., calibrations), done at a fixed interval measured by time, usage, or cycles. If the PvM is effective at preventing failures, then in many cases it seems like you are “fixing what isn’t broken.” It seems wasteful in those cases because the oil being discarded may look fairly new, the component you replace may look just fine, and the calibration may be only a very minor tweak.

That’s what you should be experiencing. Here’s why. PvM addresses failures that occur with usage or age. Those failures follow a “normal distribution” (the old bell curve you know from school). The area under the curve represents the probability of a failure having occurred.

The blue area represents the probability that the failure (e.g., oil degradation beyond tolerable limits) has occurred.

The white area is the probability that it has not failed. We want to maximize the white area while getting a reasonable amount of running life.

To do that, we pick a PvM interval that gives us reasonable life while keeping the probability of it failing low enough that we can live with it.

The result is that much of what we change or replace is actually in good condition. Some will be close to failure and may show signs of that. Of course, if we have a number of these items (e.g. a fleet of vehicles with engines, oil, and filters), we don’t know which one will be ready to fail and which one will last a long time.

With PvM, we accept a small probability that the item will fail before the replacement time (i.e., blue area under the curve), and we accept that most of what we discard could have lasted longer, but we had no cost-effective way of determining which. It’s normal to think you are discarding good items and that you are doing the maintenance too frequently. The frequency should consider all the costs of a failure and balance that against the cost of the wasted items you are discarding. Those discard costs should be lower. If your PvM is based on that, then the apparent waste is actually worth it.

With CBM, we determine condition before replacing. It can be far more cost-effective.

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