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MIL-HDBK-338B §5.3.8.1: number of spares for a 95 % confidence of no stock-out

ToolSystem Reliability
SourceU.S. Department of Defense (1998). Electronic Reliability Design Handbook. MIL-HDBK-338B, 1 October 1998. Section 5.3.8.1, equation 5.58.
LevelUndergraduate

Problem

A part with a failure rate of 0.001 per hour is to be supported for 500 hours of operation. How many spares give at least a 95 % probability that a spare is available when needed?

Inputs to Enter

  • Component: A exp 0.001; structure A
  • Spares provisioning on: spares period 500 hours, confidence 95 %
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Answers

  • Expected demand λt = 0.5; P(0 spares) = e^−0.5 = 0.607, 1 spare 0.9098, 2 spares 0.986 (0.9856)
  • Two spares are needed for 95 %; 99 % would need 3

Notes

Set the spares period to 500 and keep the 95 % confidence; the default period of 8,760 hours is a year.

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