Deciding which spares are genuinely critical

A critical spare is a part you hold because you cannot afford to wait for it, not because it is expensive or because it fails often. Those are different lists, and confusing them is why storerooms fill up with the wrong stock.

Three questions per part

• •What happens while we wait: production stopped, building unusable, safety system out, or just inconvenient?
• •How long is the real lead time, including the days before anybody places the order?
• •How many machines use this part, and would we cannibalise one to fix another?

Anything with a serious consequence and a lead time longer than you can tolerate is critical. Everything else is ordinary stock or a purchase when needed.

Get the lead time right

Catalogue lead times are optimistic and they exclude your own process. The number you need is from the moment the machine stops to the part being in a technician’s hand: diagnosis, finding the part number, raising the order, approval, supplier despatch, delivery, and goods in.

In most organisations the internal steps are a larger share of that total than the supplier’s. That is worth knowing, because it can be fixed without buying any stock at all.

Do the arithmetic on the expensive ones

For a costly spare, compare the annual cost of holding it, roughly a fifth to a quarter of its value once you count space, capital and obsolescence, against the probability of needing it multiplied by the cost of the downtime. That single line turns an argument with finance into a calculation.

Check the shared parts and the single points

Two identical machines mean you can borrow from one in an emergency, which changes the answer. One machine of a kind, with an obsolete part and a supplier who no longer makes it, is the case where holding a spare is not optional whatever the cost.

Rotables need their own treatment

A part you swap out, repair and return to stock is not consumed. It needs its own record: where it is, whether it has been repaired, how many cycles it has done. Treated as ordinary stock it disappears, and you buy new ones while three repaired units sit in a corner.

Review it after every long outage

The best time to revise the critical spares list is immediately after a failure where you waited for a part. Ask what would have saved the time, add it, and take something else off. A list revised from experience beats one built from a spreadsheet in a single afternoon.