Reliability & Maintenance
Engineering
Reliability
• Reliability: The reliability of the system (a
component, a complex system, a computer program
or a human being) is defined as the probability that
the unit performs its intended function adequately
for a given period of time under the stated operating
condition and environment.
• Measures for Reliability:
– Probability of survival
– Mean time to failure
– Mean down time
– Availability
– Frequency of failure.
Metrics of Reliability:
• Failure rate(λ): It is the number of times the
system fails in a particular interval of time.
• MTBF: The mean time taken before the system’s
failure is known as Mean Time Before Failure
(MTBF), where
MTBF=1/λ
Reliability of the system R(t)= e(-λt)=et/MTBF
Introduction (Cont.)
Maintainability:
• Maintainability is the probability that a failed
equipment is restored to operable condition in a
specified time (down time) when the maintenance
is performed under stated condition.
• Repair Rate (µ): It is the rate at which the system
is repaired to its operating condition.
• MTTR: It is the average time taken by system to
repair to its operating state.
MTTR=1/µ
Introduction (Cont.)
Availability:
• Availability is defined as the probability that
the unit is performing its intended function at a
particular given time when used under an
stated operating condition.
• It is proportion of time during which during
which an equipment is available for use.
𝑈𝑝−𝑡𝑖𝑚𝑒 µ
𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑖𝑙𝑡𝑦 = =
𝑈𝑝−𝑡𝑖𝑚𝑒+𝐷𝑜𝑤𝑛 𝑇𝑖𝑚𝑒 µ+𝜆
Failure & Failure Mode
A failure is the partial or total loss or change in the
properties of a device in such a way that its
functioning is seriously impeded or completely
stopped.
Failures can be grouped in the following categories:
• Initial Failure or Infant Mortality.
• Wear-out Failure
• Random or catastrophic failure.
Types of Failure
• Initial Failure or Infant Mortality
– These defects are due to manufacturing defects,
such as weak parts, poor insulation, bad assembly,
poor fits.
– Large number of failure likely to occur during this
period.
– Also known as debugging or burn –in period.
– In this period, failure rate decreases with time.
Types of Failure
• Wear-out failure
– These failures occur due to deterioration of the
machine parts.
– Gradual change in the parameter become beyond
the admissible limit.
– The changes are irreversible in nature.
– In this period, failure rate increases
Types of Failure
• Random or catastrophic failure
– These failures occur due to the sharp change in the
parameters determining the performance of the
system.
– These may occur due to change in environmental
condition.
– Also known as the random failures or catastrophic
failure, as these failures are difficult to detect.
– In this period, failure rate almost remain constant.
Bathtub Curve
Causes of Failures
• Poor Design
• Wrong Manufacturing Techniques
• Lack of knowledge and expertise
• Complexity of equipment
• Poor maintenance policy
• Organizational rigidity
• Human error
