How can the PF interval be determined from the failure rate?
You can’t know the PF interval directly from the failure rate. The PF interval is unrelated mathematically to the failure rate. It is the lead time between the ability to detect a failure process and the actual loss of function. The PF interval can be roughly estimated from experience or from laboratory testing. The sole intent of the PF interval is to suggest a reasonable monitoring interval when starting a condition based maintenance program. Up to now, no systematic procedure has been expounded with which to use on-going experience to optimize the inspection interval based on the PF interval.
EXAKT can provide guidance to selecting an optimal inspection interval. Once a CBM program is underway having determined an initial CBM inspection interval, the LRCM and EXAKT procedures provide:
- A remaining useful life (RULE) with confidence interval, based on current age and condition of the item.
- An optimal economic maintenance decision point (based on current age and condition and business factors), and
- A quantitative process with which to measure and improve continually the CBM program’s effectiveness.
Returning to the original question, although there is no relationship between the P-F interval and the failure rate, there is one between RULE and the “conditional” failure rate. The following sequence of slides defines RULE. By monitoring RULE and its standard deviation as in Slide 5, the reliability engineer may easily judge what an appropriate inspection interval should be.
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