An in-depth look at the physics of failures is beyond the scope of this book, however, facilitators should become familiar with the general principles of them as they progress in their career. At a very high level, failures generally occur because the component in question has lost its original resistance to stress. This could be caused by normal wear where, for example, the inside diameter of a rubber seal is slowly worn by a rotating shaft until it eventually begins to leak. It also might be caused by fatigue where, for example, a quill shaft on a blower is subjected to sudden torque spikes and slowly weakens as it loses its resistance to stress. At some point, it may suddenly shear when the torque applies more stress than the now weakened shaft can bear.

As the resistance to stress decreases, the component’s performance deteriorates. This deterioration sometimes can be detected through various means, as shown in Figure 6-1, an illustration of the failure process.

In the graph, the Y axis represents the condition or performance of whatever is being examined. This could represent an entire system, an automobile for example, or something as simple and discrete as an oil seal. Starting at the upper left side, the representation at this point is whatever you are looking at is new and thus, okay.

Figure 6-1: The failure process and evidence

As age or time progresses along the X axis, there will be warnings, including some that could be detected or observed if one were to look for them. These warnings or evidence that failure is occurring are what you want to describe in your failure effects. The best practice is to describe them chronologically in the order in which they would normally occur for the failure mode under review.

Tip from Reliability Centered Maintenance: Unraveling the Mysteries by Jim Gehris

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