industrial//maintenance//P-F interval

The P-F interval is the time between the moment a developing fault becomes detectable (point P, potential failure) and the moment the equipment fails (point F, functional failure), and it is the number that decides whether a monitoring technique is useful and how often it has to look. A bearing with a growing defect shows it first as a change in high-frequency vibration, weeks later as a rise in temperature, days later as audible noise, and finally as smoke and a seized shaft: each technique has its own point P on the same degradation curve, and the earlier P, the longer the interval it leaves to plan.

The P-F interval is the time between the moment a developing fault becomes detectable (point P, potential failure) and the moment the equipment fails (point F, functional failure), and it is the number that decides whether a monitoring technique is useful and how often it has to look. A bearing with a growing defect shows it first as a change in high-frequency vibration, weeks later as a rise in temperature, days later as audible noise, and finally as smoke and a seized shaft: each technique has its own point P on the same degradation curve, and the earlier P, the longer the interval it leaves to plan.

The interval sets the inspection rate. A route-based vibration check every month cannot protect a machine whose P-F interval is two weeks; the fault appears and runs to failure between two visits. The usual rule in reliability-centred maintenance is to inspect at intervals no longer than about half the P-F interval, so that at least one inspection falls inside it and some warning time remains after it to order parts and schedule the stop. Permanent sensors with online monitoring make the inspection interval minutes, which is why they are reserved for machines whose interval is short or whose failure is expensive.

Vibration warns before temperature, and temperature before smoke.

Choosing what to measure is choosing where point P falls; a technique whose P comes too late leaves a prognosis with nothing to decide, however accurate it is.

The interval is a property of the failure mode and the technique together, and it varies from unit to unit and with load; a published figure for a component type is a starting point to confirm on the plant's own history.

Some failures have no useful interval at all (an electronic board, a fuse, a fatigue crack that grows to fracture in minutes); for them monitoring buys nothing, and the options are preventive maintenance if wear is clear, redundancy, or living with corrective repair (maintenance).

Inside the interval, a health indicator trending toward its limit is what prognostics projects forward, and condition monitoring is the practice of keeping watch inside it.