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Some years back, I was helping top management set the foundation for a new way of maintaining and operating their site’s critical manufacturing equipment. On one of the plant tours with a small group of supervisors, operators, and mechanics, I heard the faint squealing of slipping belts through all of the other machinery noise. Working in and around machinery or driving a car with a worn power steering belt, we know the sound all too well. I stopped walking and asked group and asked the group to listen. They heard nothing.

As we resumed the tour, the  squeals continued. “Did you hear that?” I asked. Again, nothing, despite the fact we seemed to be  getting closer to the source of what was becoming ever louder squealing. When the group finally arrived in the room where it was the loudest, I asked, “What is that noise?” The response: “Oh, it always does that.”

What we discovered in that room was not good. At all. The exhaust-fan belts were slipping so badly that the bearing had overheated and the grease was running out. One belt of four was missing and one was too long. And rubber debris from the deteriorating belts could be seen coming through the vent holes in the belt guard. (“Oh, it always does that.”). Unfortunately, the lack of efficient exhaust was causing a product-quality problem. As a result, the machinery had to be run slower to compensate for the unbeknownst lack of sufficient exhaust.

Nearly 25 years ago our colleague John Moubray served our profession as the Reliability Centered Maintenance (RCM) leader, practitioner, teacher, and guru. John could explain the fundamentals and innerworkings of RCM like nobody else at the time. We often spoke to the same audiences, wrote in the same professional journals for years and had an ongoing personal debate on the technical aspects of RCM versus the work culture aspects of Total Productive Maintenance (TPM). We would eventually agree that it was not an “either-or” option, but rather, an “and” option: RCM and TPM synergy.

John spent a considerable time discussing how our work (our maintenance paradigm) should be redefined and shifted from “maintenance” to “physical asset management.” In that shift, he recognized the three basic types of maintenance, e.g., predictive, preventive, and corrective (PPC), as the OLD paradigm of maintenance. In his NEW paradigm, he taught four types of maintenance, e.g., predictive, preventive, corrective, and detective (PPCD).

While those musings and teachings so many years ago resonated with some, others considered the concept to be a mere play on words.  As John explained, he had looked for a word that rhymed with the three other types of maintenance and settled on “detective.” That term fit on two accounts” It rhymed, and it explained the purpose of that NEW type of maintenance.

“Detective maintenance” was an intriguing concept for us in the mid-1990s. Think about it like this: First, predictive (or condition-based) maintenance looks for the earliest signs of failures. Then, preventive maintenance tends to be a scheduled routine for replacing, rebuilding, or adjusting components. Corrective maintenance basically means fixing things that broke. Whenever John would introduce the concept of “detective maintenance,” he characterized it as a “functional check” or a “failure-finding task.”

The “functional check” part of detective maintenance in John’s teachings was to answer the question, “Is it capable of doing what it is supposed to do.” Functional checks are relatively straightforward: “Does the level alarm work?” “Do all of the control panel lights work?” “Does the steam trap work?”

Detective maintenance became a fundamental of Reliability Centered Maintenance when we learned that the RCM focus is on “functional” failures versus “catastrophic” failures: “Is the equipment (or component) doing what it is supposed to do?” But there was far more to detective maintenance.

A form of detective maintenance established a fundamental principle of TPM from the beginning as the underlying belief that “cleaning is inspecting.” Oh, the pushback in the 1980s and 1990s by traditional operations and maintenance people was huge: “We don’t have time to clean. It’s not our job. That’s why we have housekeeping around here.”

Based on my nearly four decades in maintenance and operation improvement, I can attest that cleaning machinery continues to be one of the best ways to spot problems. Buildups of oil, grease, metal shavings, and other debris often point to an early warning sign (evidence) of an imminent failure. Loose nuts and bolts, loose wiring and connectors, and loose proximity switches and sensors are easily spotted while cleaning machinery.

While working on a production-equipment-improvement project in a well-established manufacturing plant, we focused on the causes of high-dollar scrap and waste. Digging through the scrap-product bins and totes, we began separating the various scrap types into two categories: defective finished products and production waste.

The amount of defective finished products seemed to be abnormally high. So, over the next six-month period, these products were subdivided, weighed, and attributed to two different departments. We narrowed the problems down to specific machines. Then we discovered that those few machines were lacking standards and consistent training for machine operation, setup, changeover, adjustment, and quality inspection methods.

High levels of finished-product scrap, in turn, were leading to incomplete customer shipments and schedule break-ins for short re-runs of components to make up the missing pieces, all of which increased materials and labor cost and delayed production. This was a huge financial hit and bad for customer satisfaction. Accordingly, what may have started out as a detective-maintenance approach uncovered physical-asset-management opportunities that amounted to a multi-million-dollar prize.


Detective maintenance should be a fundamental physical asset management strategy, whether your plant or facility is knee-deep in RCM; whether TPM is a way of life where you work; or whether the three basic types of maintenance (PPC) are well established.

Keep in mind that anyone working around machinery of almost any type can be a “detective.” Every operator and supervisor can quickly be taught what to listen for, look at, or touch to determine if a problem could be starting to develop. And don’t forget the aspect of “cleaning to inspect.” A clean machine is a healthy machine. It’s safer, and easier to operate, maintain, setup, and adjust. The act of cleaning a machine (detective maintenance) has proven to be one of the best ways to identify problems and take corrective action BEFORE something fails.

In the meantime, I say thank you, John Moubray (1949-2004), for sharing your wisdom.TRR

Bob Williamson is a long-time contributor to the people-side of the world-class-maintenance and manufacturing body of knowledge across dozens of industry types. His background in maintenance, machine and tool design, and teaching has positioned his work with over 500 companies and plants, facilities, and equipment-oriented organizations. Contact him directly at 512-800-6031 or

Tags: reliability, availability, maintenance, RAM, skills training, workforce development, workforce issues