Reliability
195
Equation (4.17) has been plotted in Fig. 4.3 to show how reliability is related
to age and MTTF. The figure shows that:
1. Accurate MTTF estimates are necessary to define the reliability of
relatively unreliable devices.
2. Age or operating time is an important parameter in determining
the reliability of devices with poor reliability.
3. Accurate MTTF estimates are less important for highly-reliable
devices.
It is also important to note that the reliability for the exponential model
at t = MTTF is only 0.368 (and not 0.5), and, the failure Q(t)=0.632.
In complex equipment where wear out failure is significant, if the aging
characteristics of different parts varies, then the failure pattern of the components as reflected in the reliability of the total system often appears
as a series of random or chance events. Hence, the reliability of a complicated system may appear to be an exponential function, even if the individual failure characteristics
of the components are not of the
exponential type. Cumulative failure data for a marine power plant Fig.
1,0
0.8
\ \
0.5 \
\ "K
\
0.2
~
0.1
0 1
3
4 5 6 7 8 9 10
11
12
Time, Thousands of hours
Figure 4.3. Constant reliability as a function of time.