III. Reliability in Design and Development
A. Reliability design techniques
12. Reliability apportionment (allocation) techniques (Analyze)
Use these techniques to specify subsystem and component reliability requirements.
Breaking down system reliability requirements provides necessary guidance across the team.
Additional References
Reliability Allocations (article)
Reliability Apportionment (article)
Quick Quiz
1-46. A system made up of four series components has a design reliability set to .97. If three of the
components have reliabilities apportioned to them of 0.992, 0.994, and 0.991, what should the
reliability apportionment for the fourth component be?
(A) 0.970
(B) 0.990
(C) 0.993
(D) 0.997
(C) 0.993
The formula for a series system and a little algebra is all that is needed here. The formula to determine system reliability given a series system is
$$ {{R}_{sys}}={{R}_{1}}\times {{R}_{2}}\times {{R}_{3}}\times {{R}_{4}}$$
Since we’re given Rsys and three of the four other reliability values, solve for the missing value, say R4
$$ {{R}_{4}}=\frac{{{R}_{1}}\times {{R}_{2}}\times {{R}_{3}}}{{{R}_{sys}}}$$
Plugin the values for the three component reliability values and divide the system reliability to find the minimum reliability value of the last component.
$$ {{R}_{4}}=\frac{0.992\times 0.994\times 0.991}{0.97}=0.993$$
Leave a Reply