III. Reliability in Design and Development
A. Reliability design techniques
6. Tolerance and worst-case analysis (Understand)
Describe how tolerance and worst-case analyses 9e.g., root of sum of shares, extreme value) can be used to characterize variation that affects reliability.
Work with your engineering team to establish meaningful tolerances.
Additional References
Purpose of Tolerances (article)
Worst Case Tolerance Analysis (article)
Root Sum Squared Tolerance Analysis Method (article)
Reliability and Root Sum Squared Tolerances (article)
Reliability and Monte Carlo Determined Tolerances (article)
Why do Tolerance Analysis (article)
Quick Quiz
1-50. What would a reliability professional be expected to do in a “worst-case” design analysis?
(A) Analyze the worst rejects.
(B) Analyze only those products failing to meet specification requirements.
(C) Assume that all subassembly tolerances are at their maximum limit.
(D) Determine whether product requirements can be met with subassemblies assumed at their worst combination of tolerances.
(D) Determine whether product requirements can be met with subassemblies assumed at their worst combination of tolerances.
The process to analyze a design by worst cases analysis is defined by setting the individual component values at not the maximum values, rather the values which create the least desirable situation. Worst case tolerance analysis is a conservative method to evaluate the impact of tolerances on product performance.
Worst here does not imply failures or rejects, instead looking at failures or rejects is part of a failure analysis process or potential improvement project.
1-95. An engineer is using an extreme worst-case method of variability analysis and finds that the indicated performance characteristics values fall within specifications. What can be concluded?
(A) The probability of occurrence will be unknown.
(B) The system can withstand high component parameter drift.
(C) A low parameter tolerance limit is needed.
(D) A high parameter tolerance limit is needed.
(B) The system can withstand high component parameter drift.
Component parameter values may change over time, beyond the process related variability. A design the functions correctly even when using the extreme worst set of component parameters indicates a robust design with ample margin for the nominal set of components. In general, the components that have parameter drift may do so over wider ranges than a design unable to demonstrate functionality using this method.
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