P-F Interval Calculator for Inspection Frequency
The P-F interval is the time between the first point at which a developing failure can be detected and the point at which the equipment actually fails. This calculator helps you set an inspection interval that gives you a fair chance of catching it with time to react.
Your numbers
Time between the earliest detectable warning (P) and functional failure (F).
Planning, parts and shutdown window needed to fix the problem.
Your results
The P-F interval is rarely constant. Use a conservative value from experience or trend data, and treat these figures as a planning aid. Detection probability here assumes a failure starts at a random time and inspections are perfect.
Detection probability versus inspection interval
Sweeps Planned inspection interval from 5 to 120, holding your other inputs fixed.
Estimate P-F intervals from your failure history
Optional review of mining work order and failure data in your ERP to set inspection frequencies.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter the P-F interval for the failure mode you are monitoring, for example the days between a bearing first showing a high frequency signature and its seizure. Set the planned inspection interval and the time your team needs to respond. The result gives the rule of thumb interval, the minimum warning left after detection, the probability of detection and the yearly cost of inspections versus missed failures. Use the slider to find the interval that balances cost with risk.
- Get P-F from trend history, vendor data or experienced technicians
- Set response time realistically, including parts lead time
- Use one calculation per failure mode, not per machine
- Revisit after every real failure
The logic
If you inspect more often than the P-F interval, every developing fault must show up at least once before it fails. The classic RCM rule is to inspect at half the P-F interval, so the net warning is at least half the P-F. Where the interval exceeds the P-F, the chance of catching the fault falls roughly in proportion, which the calculator shows as P-F divided by interval. The net P-F interval (P-F minus inspection interval) is the minimum warning you get.
Reading the results
If the minimum warning is less than your response time, detection alone will not prevent unplanned downtime, since you cannot fix the problem before it fails. In that case shorten the interval, shorten the response time by stocking critical spares, or switch to continuous monitoring. The cost lines let you compare the price of more inspections against the expected loss of failures, so you can justify a route frequency to finance.
Common mistakes
A frequent error is using a P-F interval from a different technique, as ultrasound sees a bearing fault earlier than vibration and temperature later. Another is ignoring that the P-F interval varies with load and speed. Teams also forget that detection requires the right skill and instrument, so poor data quality shrinks the real P-F. Always validate with actual failures.
Frequently Asked Questions
What is the P-F interval?
It is the time between the point P, where a potential failure becomes detectable, and the point F, where the item can no longer perform its function. Condition-based tasks must be done more often than this interval to be effective.
Why inspect at half the P-F interval?
Because a fault can begin just after one inspection. With an interval of half the P-F, the next inspection finds it with at least half of the P-F still remaining, which is usually enough time to plan and execute a repair.
How do I find the P-F interval for a bearing?
Use trend data from past failures: note when a defect was first detected by a technique and when the bearing failed. Published figures exist for vibration, ultrasound and temperature but vary widely with duty, so local history is more reliable.
What if the P-F interval is shorter than I can inspect?
Then manual routes will not work for that failure mode. Consider online monitoring with sensors, or accept the failure and manage it with redundancy, spares or a redesign instead of a condition-based task.
If you want failure history from your ERP analyzed to estimate real P-F intervals and tune inspection frequency by asset class, Netray can help.
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