Terms Used in Reliability Calculations
The definitions and mathematical relationships often used in reliability calculation are given below.
Reliability is defined as probability that a product, a component, equipment or a system will perform its intended function for a specific period of time or life time under specified operating conditions or sum total of reliabilities of all components in prescribed operating conditions.
OR
It is also equal to one minus probability of failure
R (t) = 1 - F (t)
It is the time taken to restore the equipment to effective working condition.
- Mean Time to Repair (MTTR)
It is the time required to locate a failure and to repair (or to restore the condition of the equipment).
- Time between Failures (TBF)
The time difference between one failure to next failure
- Mean Time between Failures (MTBF)
It is the mean of the time interval between consecutive failures of equipment. Therefore,
MTBF = Total operating time /Number of failures
The total failures of the machine, i.e. the time taken after failure to working.
It is the expected value of mean time to failure, which is expressed as
MTTF = ∫ R (t) dt
Where R (t) is reliability function on t
- Time between Maintenance (TBM)
It is the time gap between two consecutive maintenances (may be planned or unplanned).
- Mean Time between Maintenance (MTBM)
It is the average of all the durations between end of one maintenance to the start of next, may it be planned or unplanned. In other words it is the average time that the equipment is utilised for the operation.
It is the number of failure per unit time. Therefore,
Failure rate (λ) = 1 / MTBF
- Probability of Failure or Unreliability
Probability of failing is equal to one minus the reliability of a system. It is also called the probability of not functioning or unreliability.
Probability of failure [F (t )] = n .( t/T)
= 1 - Reliability of the system
= 1 - e- nt
Where n = Number of elements in a system (or device),
t = Cumulative operating time, and
T = MTBF.
- Reliability Improvement Factor (RIF)
It is the ratio of probability of failure of a system to that of improved system.
RIF = Probability of failure before improvement/ Probability of failure after improvement
Maintainability is defined as the ease with which machine can be put in to operation. It is a factor of design
It is the time during which system is operating in a manner acceptable to the operator.
It is the total time during which the system is not in acceptable operating condition. Down time can further be subdivided into a number of categories.
Actual repair time is that portion of down time during which one or more technicians are working on the system to affect a repair. This time includes preparation time fault location time fault correction time and checkout for the system.
It is that portion of down time during which repair is delayed slowly because it is necessary to wait for manpower and/or for replacement part or subdivision of the system.
It is that portion of down time not included under active repair time and logistic time this includes both necessary administrative activities and unnecessary wasted time.