project & quality management quality management reliability
TRANSCRIPT
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.TimeOperatingTotal
FailedItemsrateFailure
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’
Time
Fa
ilu
re R
ate
A
C
D
B
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
Simplifying Assumption
• Exponential distribution of failure rate is assumed. This means that the failure rate remains constant over life of product
• Bathtub curve becomes a straight line
Calculating Failure Rate
Time
Fa
ilu
re R
ate
Calculating Failure Rate
Failure rate TimeOperatingTotal
FailedItems
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
te
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).
The failure rate 41005.1
1000000
105 x
Example
Find the reliability of the product after 1000 hours i.e. (t) =1000
Reliability at 1000 hours:
R(1000) = 0.9
te
)10001005.1( 4 xxe
Therefore the item has a 90% chance of surviving for 1000 hours