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The design for maintainability should be considered early when flexibility is high
and design change costs are low. The design flexibility is the greatest in the conceptual
stage of the product, and design change costs are the lowest as show in Figure 14.3. 22
Maintainability features should be considered as early as possible in the DFSS de-
sign process. Maintainability may increase the cost during the design phase, but
it should reduce the end users maintenance costs throughout the product's life.
Table 14.6 23 lists typical design for maintainability features used in the product
development stage and the benefits these features provide to the designer and the
customer.
A system design that has the maintainability feature can reduce or eliminate
maintenance costs, reduce downtime, and improve safety.
APPENDIX 14.A
Reliability engineering is an engineering field that deals with the study of reliability,
of the ability of a system or component to perform its required functions under stated
conditions for a specified period of time. It often is reported in terms of a probability.
Mathematically reliability R(t) is the probability that a system will be successful in
the interval from time 0 to time t:
R ( t )
=
P ( t
>
T )
=
f ( t ) dt
,
t
0
(14.A.1)
T
where T is a random variable denoting the time-to-failure or failure time, f(t) is the
failure probability density function, and t is the length of time (which is assumed to
start from time zero).
The unreliability F(t) , a measure of failure, is defined as the probability that the
system will fail by time t .
F ( t )
=
P ( t
T )
,
t
0
(14.A.2)
In other words, F(t) is the failure distribution function. The following relationship
applies to reliability in general. Reliability R(t) , is related to failure probability F(t)
by:
R ( t )
=
1
F ( t )
(14.A.3)
We note the following four key elements of reliability (Reliability Engineering,
2010):
1. Reliability is a probability. This means that failure is regarded as a random
phenomenon; it is a recurring event, and we do not express any information
22 See http://www.theriac.org/DeskReference/viewDocument.php?id
=
222.
23 See http://www.theriac.org/DeskReference/viewDocument.php?id
=
222.
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