A replacement policy maximizing MTTF of a system with several spare units

A replacement policy is considered that maximizes mean time-to-failure (MTTF) of a system with N spare units. The optimum replacement time of a system with k spares (k=1, 2, ..., N) is derived successively from MTTF with k-1 spares by induction. The maximum MTTF is approximately given by a reciprocal of the hazard rate. >