Also, the following result can be obtained.
134 CHAPTER 4 CONTINUOUS RANDOM VARIABLES AND PROBABILITY DISTRIBUTIONS
If Xhas a Weibull distribution with parameters and ,
(4-24)
E 1 x 2 a 1
1
b and 2 V 1 x 2 ^2 a 1
2
b^2 c^ a 1
1
bd
2
EXAMPLE 4-25 The time to failure (in hours) of a bearing in a mechanical shaft is satisfactorily modeled as a
Weibull random variable with Determine the mean time until
failure.
From the expression for the mean,
Determine the probability that a bearing lasts at least 6000 hours. Now
Consequently, only 33.4% of all bearings last at least 6000 hours.
P 1 x 60002 1 F 160002 expca
6000
5000
b
(^1) 2
de1.0950.334
E 1 X 2 5000 (^31
11) 0.5 24 5000 334 5000 2 !10,000 hours
(^1) 2, and 5000 hours.
Figure 4-27 Weibull probability density functions
for selected values of and .
0
0.0
0.2
0.4
0.6
0.8
1.0
3691215
x
f (x)
1
3.4
4.5
1
2
6.2
δ β
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