How long it runs.How fast it is back.
From operating time, failures and repair time: MTBF, MTTR, failure rate, availability, and the chance of a run without a failure.
How it is worked outHow reliability is worked out.
Operating time, failures and repair time give the averages; with a constant failure rate, they also give the chance of a run without a failure.
- MTBF
Operating time ÷ failures.
ISO 22400-2 lists the mean operating time between failures among its maintenance indicators.
- MTTR
Repair time ÷ failures.
- Failure rate
Failures ÷ operating time: the inverse of MTBF, shown here per 1.000 hours.
- Inherent availability
MTBF ÷ (MTBF + MTTR).
It counts failures and repairs only, not waiting for parts or planned stops.
- Reliability
e^(−length of the run ÷ MTBF): the chance of no failure during the run.
With a constant failure rate, the time between failures follows the exponential distribution (IEC 61703). When wear-out raises the failure rate with age, this figure is optimistic.
The formulas assume failures come at a constant rate. Recorded failure by failure, the maintenance history shows whether they do.
Sources
- ISO 22400-2:2014, Automation systems and integration: key performance indicators for manufacturing operations management, Part 2: definitions and descriptions. International Organization for Standardization. iso.org
- IEC 61703:2016, Mathematical expressions for reliability, availability, maintainability and maintenance support terms. International Electrotechnical Commission. webstore.iec.ch