Project & Quality Management
Quality Management
Reliability
Reliability Management
Why is it needed?
• Reliable operation of critical equipment
• Planning of maintenance activities
• Improved ‘quality’ of an item
Reliability Management
Reliability management is concerned with
performance and conformance over the
expected life of the product
“the probability that a product or a
piece of equipment
performs its intended function for a stated period of time
under specified operation conditions’”
Definition of Reliability
The definition has four important elements:
• Probability
• Time
• Performance
• Operating conditions
Definition of Reliability
Probability
• A value between 0 and 1
• Precise meaning
e.g. probability of 0.97 means that 97 of 100
items will still be working at stated time
under stated conditions
Definition of Reliability
Performance
• Some criterion to define when product has
failed
e.g. bearing clearances in an engine or amount
of emissions from a car
Definition of Reliability
Operating conditions
• These describe the operating conditions that
correspond to the stated product life. e.g. for a
car engine this might mean
→ Speed
→ Loading
→ Effects of an expected amount of
misuse such as over-revving and stalling.
Reliability Measurement
This is based on the Failure Rate
i.e.
Items Failed
Failure rate 
Total Operating Time
Some products are scrapped when they fail
e.g. hairdryer
Others are repaired e.g. washing machine.
Failure rate over the life of a product
The failure rate is expected to vary over the life
of a product – ‘Bathtub Curve’
A
D
C
Failure Rate
B
Time
Bathtub Curve
A-B Early Failure
• ‘Teething’ problems. Caused by design/material
flaws
B-C Constant Failure
• Lower than initial failure rate and more or less
constant until end of life
C-D End of life failure
• Failure rate rises again due to components
reaching end of life
Calculating Failure Rate
Simplifying Assumption
• Exponential distribution of failure rate is
assumed. This means that the failure rate
remains constant over life of product
Failure Rate
• Bathtub curve becomes a straight line
Time
Calculating Failure Rate
Failure rate
Items Failed

Total Operating Time
usually expressed by the Greek letter lambda ()
The probability of a product surviving until time
(t) is given by the following function:
Reliability at time (t) =
e is the exponential function
e
 t
Procedure
To establish reliability of an item:
• Conduct a series of tests until a number of
them fail.
• Calculate failure rate (Lambda).
• Calculate reliability for a given time using
Reliability at time (t) = e-t
Example
Trial data shows that 105 items failed during
a test with a total operating time of 1 million
hours. (For all items i.e. both failed and
passed).
105
4
 1.05 x10
The failure rate  
1000000
Example
Find the reliability of the product after 1000
hours i.e. (t) =1000
Reliability at 1000 hours:
R(1000)
e
e
 (1.05 x104 x1000)
 t
= 0.9
Therefore the item has a 90% chance of
surviving for 1000 hours