(1) Failure Frequency
“Failure frequency” denotes the incidence of failure as a percentage of loading time (the time during which the equipment is supposed to be operating). The “frequency” part of this term was borrowed from the discipline of safety management. This index is expressed by the following formula:
(2) MTBF (Mean Time Between Failures)
MTBF is the average operating time between one failure and the next, in repairable equipment.
(3) MTTF (Mean Time to Failure)
MTTF is the average operating time from startup to failure, in the case of a component that is not repaired.
(4) Failure Severity
Like failure frequency, failure severity is also modelled on a term (in this case, “accident severity”) first used in safety management. It denotes the proportion of time for which the equipment is down due to failure, and is expressed by the following formula:
(5) MTTR (Mean Time to Repair)
MTTR, or the average time required for the breakdown maintenance of a piece of equipment, is expressed by the following formula:
(6) Availability
Availability is the proportion of time for which a repairable system or item of equipment is in a state in which it can fulfill its function, within a given period of time. Mean availability (represented by the letter A) is often calculated using the following formula:
(where ‘uptime’ is the time for which the equipment could have been working if it had not broken down).
Intrinsic Availability (At): indicates how unlikely a machine is to fail, and how quickly it can be repaired if it does fail.
At = MTBF (MTBF + MTTR), where MTBF = mean time between failures, and MTTR = mean time to repair
Achievable Availability (Aa): indicates the length of the intervals between maintenance tasks, and how quickly the maintenance tasks are done.
Aa = MTBM (MTBM + Mean), where MTBM = mean time between maintenance activities (including both preventive maintenance and breakdown maintenance) and Mean = mean maintenance time, i.e. the average time required to perform preventive maintenance and breakdown maintenance tasks