The P-F interval: catching failures before they happen

Machines rarely go from perfect to broken in an instant. A bearing starts to wear, then it gets a little louder, then it runs hot, then it seizes. Reliability engineers describe this with a simple curve and two points on it: P, the point where you can first detect a problem, and F, the point of functional failure.

What the window means

The time between P and F is the P-F interval. If you check the equipment often enough to catch it after P but before F, you can plan the repair at a convenient time, order the part, and avoid the breakdown. If you check less often than that window, the failure will often happen between checks.

The rule of thumb

A common rule is to inspect at least twice within the expected P-F interval, so you get more than one chance to catch the warning. If a fault typically gives you a few weeks of warning through vibration, an inspection every few months will miss it most of the time.

Earlier detection, bigger window

Different checks detect problems at different points. Vibration and oil analysis can spot wear early. Heat often shows up later. Noise you can hear and smoke you can see come very late. The earlier your detection method, the wider your window to plan the fix.

Putting it to work

Pick your critical assets and ask: what is the most likely way this fails, and what is the earliest sign we could detect? Then set your inspection interval to fit inside that window. Where you can, use readings from sensors so the check happens all the time, not just on inspection day.