MTBF (Mean Time Between Failures) is the average operating time of a machine between one failure and the next, indicating how reliable the equipment is. MTTR (Mean Time To Repair) is the average time needed to repair a machine and return it to normal operation after a failure, reflecting maintenance efficiency. MTTF (Mean Time To Failure) is the average time a non-repairable component operates before it fails and must be replaced, showing the expected lifetime of the component. Together, these metrics are commonly used in maintenance and reliability management to evaluate equipment performance and improve reliability.

1. MTBF - MTTR

2. Reliability measures can be time related or cost related
Reliability
Time Measures Cost Measures
• Failure Rate
• MTBF
• MTTF
• Reliability Index
• Cost of Stores
& Spares /
unit of
production
• Cost of stores
& spares pa
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3. Reliability Measures
• Failure rate () : Refers to the frequency of failures
or the rate at which failures occur over a unit
period of time
• It is expressed as No of failures per hour
or per month, etc.
Total Operating hours
Failure rate () = Number of Breakdowns
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4. Reliability Measures
• Mean Time between Failures (MTBF) : is the
reciprocal of failure rate
• Mathematically,
Total Operating hours
MTBF = 1 =
 Number of
Breakdowns
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5. Other Reliability Measures
© ECS Limited 5
• Mean Time To Failure: Common reliability measure for
non- repairable systems like light bulbs, resistors and
transistors
MTTF = Total Operating Hours for an Equipment
Population Total number of failures
• Reliability Index : Is defined as the probability of zero
system failures during a certain time interval t
Reliability Index = R(t) = e-t

6. Maintainability
• Maintenance is performed on a system or
equipment in the event of a failure, or as a
preventive measure to preempt an expected failure
• Maintainability is a characteristic which addresses
the ease, accuracy, timeliness and economy of
maintenance actions
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7. Reliability measures can be time related or cost related
Maintainabilit
y
Time Measures Cost Measures
• MTTR (Mct)
• Mpt
• Cost of
RPM/unit
• Cost of RPM pa
• Cost per
repair
action
• Maintenance
related
environment
al clean-up
cost
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8. Maintainability Measures
• Mean Time To Repair (MTTR)
– It is also know as Mean Corrective maintenance Time (Mct)
– It is the composite value representing the arithmetic
mean of individual cycle times for any number of
individual maintenance actions over the system
utilization phase
• Mathematicall
y,
Where,
i – failure rate of the ith system element
Mcti
– corrective maintenance time of the ith system
element

i
MTTR 
(i )
(Mcti )
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9. • Build a process
flow diagram
• MTTR can be
reduced by
– Removing delays
– Simplifying tasks
– Parallel activities
MTTR can be improved by focussing on individual Mct for various
corrective action tasks
Detection
Preparation
for maintenance
Location & Isolation
Disassembly (access)
Removal of faulty item Repair
Installation of
spare/repair part
Reassembly
Alignment &
Adjustment
Checkout
or
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10. Exercise : What is the MTBF and MTTR?
24.2 hr
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7.1 hr 27.4 hr 35.3 hr 6.3 hr 46.7 hr 5.0 hr
2.1 hr 7.1 hr 4.2 hr 1.8 hr 3.5 hr 8.3 hr
Breakdowns
Operation

11. Exercise : What is the MTBF and MTTR?
• Operating time = 152 hours
• Repair time = 27 hours
• Number of failures = 6
• MTBF = 152/6 = 25.33 hours
• MTTR = 27/6 = 4.5 hours
24.2 hr
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7.1 hr 27.4 hr 35.3 hr 6.3 hr
Operation
46.7 hr 5.0 hr
Breakdowns
2.1 hr 7.1 hr 4.2 hr 1.8 hr 3.5 hr 8.3 hr