Improving Reliability by Reducing Tolerance Stack-Up Failures

摘要

Excalibur is the Army's 155-mm guided bullet for howitzers. Once fielded, Excalibur is expected to have a reliability of 90%. This paper describes a method to predict the reliability of the locking mechanism in the fuze safe and arm device. Hand calculations showed that a locking failure could occur if several dimensions were machined close to a tolerance limit in the same device. The probability of a locking failure was estimated using Monte Carlo simulations and a First Order Probability method. The governing equation was a function of about a dozen other equations and 21 variables. A probability distribution function described each variable's nominal dimension and tolerance. Both Monte Carlo simulations and First Order Probability methods predicted fewer than 35 failures in a million devices. Output also included a ranking of dimensions and tolerances and their relative effect on reliability.