Spare-parts management balances two opposing costs: the cost of carrying stock, and the cost of a machine standing idle while a part is sourced. Getting the balance right is an operating discipline, not a purchasing task.
Start with criticality
Classify every part by the consequence of its failure — safety, production impact and lead time. A cheap part with a long lead time that stops a line is more critical than an expensive part that is easy to replace. Stock the critical few; manage the rest.
Set stocking levels from data
Minimum stock covers lead time plus a safety margin sized to demand variability; maximum covers the reorder interval plus buffer. Review against actual consumption rather than keeping last year’s numbers.
Plan for obsolescence
Identify parts at risk of discontinuation and qualify a replacement or cross-reference before the last unit is consumed. This is the single highest-value habit in spare-parts management.
Cross-reference with discipline
An equivalent part must match function — ratings, materials, interfaces — not just dimensions. Document each qualified alternate with its verification and residual risk.
Measure
- Service level (requests filled from stock).
- Dead-stock value and turnover.
- Emergency-buy premium.
- Obsolescence exposure.
The five numbers worth measuring
Spare parts management is a discipline that can be measured, and the measures are unflattering when things are going badly in a way that stock levels alone never reveal. Five indicators cover most of what matters.
| Measure | How to calculate it | What a bad number tells you |
|---|---|---|
| Stockout rate on critical items | Number of occasions a critical item was unavailable when needed, per year | Either the criticality list is wrong or the min/max levels are set below actual consumption |
| Stock turn by class | Value issued divided by average stock value over the period | Low turnover on a large inventory value means capital is sitting still |
| Proportion of stock that has not moved in 24 months | Value of unmoved items divided by total stock value | The clearest single indicator that stocking decisions are being made by habit |
| Unplanned work orders caused by parts availability | Count of corrective work orders where waiting for a part extended the repair | Links the parts organisation to production performance in a way management understands |
| Obsolescence exposure | Value of stock held against parts whose makers have announced end-of-life | Tells you which decisions have a deadline attached |
Review them annually, not monthly. Spare parts data moves slowly, and a monthly review of small numbers produces noise rather than insight.
A min/max calculation worked through
Levels are usually set by feel and then never revised. The calculation is not difficult and produces a defensible number:
- Establish consumption per period from actual issues, not from the maker’s recommended replacement interval. Take the last two or three years if the data exists.
- Establish the real replenishment lead time. This is the supplier’s lead time plus the time it takes you internally to raise and approve the order. The internal part is frequently longer than the external part and is almost always left out.
- Set the reorder point at consumption during the lead time, plus a safety allowance sized to the criticality of the item. High-consequence items get a larger allowance; low-consequence items get none.
- Set the maximum at the reorder point plus one sensible order quantity, not at a year’s worth of use. The maximum exists to stop orders being placed in uneconomic fractions, not to build a warehouse.
Then apply the test that catches most errors: if the level has never triggered in three years, it is either set too high or the item does not need to be stocked at all. Both conclusions save money.
Making the discipline survive staff turnover
Spare parts management in most plants depends on one or two people’s memory. When they leave, the knowledge leaves with them, and the symptoms appear as duplicate stock, missed obsolescence deadlines and parts that nobody can identify.
The countermeasures are unglamorous and effective:
- Record the criticality rationale, not just the score. “Score 375 because a failure stops the whole extrusion line” survives; a bare number does not.
- Record cross-references with a confidence level and a date. Who confirmed the equivalence, and on what evidence.
- Store the specification package with the item — dimensions, material, markings and photographs. That package is what allows a replacement to be found years later.
- Review the register annually and timestamp the review, so the next person knows how current the information is.
Questions we are asked about spare parts management
Where should a plant start? With the criticality list. Every other decision — what to stock, what to source ahead, what to leave to the supply chain — follows from it. Starting with stock levels means optimising the wrong list.
How much stock is too much? When the value of items that have not moved in two years exceeds the annual cost of expedited freight, the balance has tipped. That comparison makes the argument better than a percentage target does.
Is it worth holding spares for obsolete equipment? For equipment you intend to keep running, yes — and the quantity is a calculation, not a guess. For equipment scheduled for replacement, holding anything beyond the next expected failure is capital with no return.
Should the maintenance team or purchasing own spares? Maintenance owns the criticality and the specification; purchasing owns the commercial relationship and the ordering. Where one side owns both, either the technical detail or the commercial discipline gets neglected.
References we have recently quoted in this category
Need a specific reference? Send the exact number to [email protected] with quantity and target date — we reply with sourcing options, equivalent/replacement candidates, condition and lead time.
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