What the Spares Store Is Worth, and What It Is Actually Protecting

How to read this sheet

Coverage here is the ability to serve failures inside one replenishment window: the shelf absorbs what it can hold, and the rest wait for supply. It is a mean, not a distribution, so it will not tell you the chance of a bad year - a part failing once every eight years with a four month lead time is a risk this sheet prices at a twelfth of a failure and reality prices at nothing or at everything. Where a single part carries the whole plant, size it on the consequence rather than on the average, and use the per-part reorder calculator for the probability. Failure rates from a maintenance history are only as good as the history: a plant that records breakdowns as machine hours lost and not as parts consumed cannot fill this table honestly, and guessing the rates makes the ranking arbitrary rather than approximate. The emergency route is assumed always available at the stated lead time and premium, which is exactly what fails during a monsoon, a customs strike or an allocation shortage - a part whose exposure is small only because air freight is cheap is a part whose exposure depends on the airport. Obsolescence is inside the carrying percentage and nowhere else, so a machine due for replacement makes its spares worth less than this sheet says. Finally, the last column ranks by what one more unit saves net of carrying it, which is the right question for the next purchase order and the wrong one for the first: a part held at zero that should be held at two shows only the first unit's saving here, and the second is worth running again.