Base stock level determination for “insurance type” spares

“Insurance type” spares are spares for critical parts which are essential for the operation of the system in which they serve, and which have an appreciable probability of not needing replacement during the lifetime of the system. Owing to their nature, the (S‐1, S) inventory control model provides an appropriate replenishment policy for this class of spares, where S is the maximum number of spares in inventory. Presents a graphical aid for systems containing a finite number of machines representing sources of part failure. For a given number of machines, the graphs indicate the range of the ratio (mean re‐supply lead‐time/mean failure‐free operating time) for which a minimum S is required in order to satisfy the reliability constraint: Pr[a spare is available at a machine stoppage due to part failure] ≥ α for α = 0.90, 0.95 and 0.99.