Critical spares: deciding what to stock, and what to accept the risk on
The four classes of spare and why each is held, the questions that make up the stocking decision, the alternatives worth checking before a spare is bought, the register that ties every spare to the equipment and the failure it serves, what holding a spare properly involves, and how the holdings are reviewed. With a free register template.
Key takeaways
- A critical spare is one the plant cannot wait for: the failure it restores from would cost more during the lead time than the spare costs to hold.
- Spares fall into four classes by why they are held: insurance, critical, operational and consumable. The class decides who makes the decision and how it is reviewed.
- The decision is a comparison, made per spare: the cost of waiting over the lead time, weighted by the likelihood of needing it, against the whole cost of holding it, with the alternatives considered first.
- A spare with no failure mode behind it is a guess. The register ties every spare to the equipment it fits, the failure it restores from and the criticality of that equipment.
- A spare that is held wrongly is not a spare. Storage, preservation, rotation and a review date are part of the decision, not an afterthought.
A store is where two opposite failures meet. In one aisle sits a spare that was bought for a machine scrapped years ago, held because nobody dares write it off. In the next sits an empty bin for the gearbox that has stopped the plant twice, because the part is expensive and the last person to ask was told to make do. Both failures have the same cause, which is that nobody made a decision, per spare, with the failure mode, the lead time and the consequence in front of them.
This guide sets out the four classes of spare and why each is held, the questions that make up the stocking decision, the alternatives to holding a spare that are worth checking first, how the register ties every spare to the equipment and the failure it serves, what storing a spare properly involves, how the holdings are reviewed, and the mistakes that are common enough to plan against. A register template comes with it.
Free download
Critical spares register template
A register with the failure mode, lead time, downtime cost, likelihood and alternative against every candidate spare, the decision and the holding it produced, a review date, and four worked rows on the crusher branch used across this site.
Four classes of spare
Spares are classed by why they are held, because the reason decides who makes the decision and how often it is reviewed. The store manages consumables on its own rules. The planner manages operational spares from the maintenance plan and the usage history. Critical spares are decided by reliability engineering from the criticality assessment and the failure modes. Insurance spares are a whole of life decision for the asset owner, because the holding cost runs as long as the plant does.
| Class | Why it is held | Example | Decided by |
|---|---|---|---|
| Insurance spare | A failure is unlikely, the lead time is long and the consequence of waiting is severe. Held for years, perhaps never used | A mill girth gear, a main transformer, a crusher main shaft | Consequence over the lead time against the cost of holding, as a whole of life decision |
| Critical spare | Needed to restore a critical function inside the time the plant can tolerate, and not available inside that time otherwise | A conveyor drive gearbox, a VSD for a critical pump, a PLC processor | Criticality of the equipment, the failure mode and the lead time against the tolerable downtime |
| Operational spare | Consumed by the maintenance program at a known rate, stocked on usage with a minimum and a reorder point | Idlers, screen panels, pump wear rings, filters | Usage history and the maintenance plan intervals |
| Consumable | Low value, high turnover, stocked by the store on its own rules | Fasteners, lubricants, gaskets, cable ties | The store's min-max, reviewed on usage |
The decision
The decision is a comparison. On one side, the cost of waiting: what the plant loses per day without the equipment, from the cost of downtime, multiplied by the days the wait would last without the spare, which is the lead time plus transport plus the repair, weighted by how likely the failure is inside that lead time. On the other, the cost of holding: purchase, storage, preservation, insurance and the risk of obsolescence, over the years the part will sit. Where the first clearly exceeds the second, the spare is held. Where it clearly does not, it is not. In between sit the alternatives.
The grid gives the shape of the answer. The exact lines are drawn by the site, from its own downtime costs and its own appetite for risk, and the questions below are what fill the grid in for each candidate. The one question that must be answered first is which failure mode the spare is for, because a spare with no failure mode behind it is a guess dressed as prudence, and the FMECA or the maintenance plan is where the answer lives.
Consequence of waiting ↓
Rare
Possible
Likely
Severe
Insurance spare
Held for years against a day the plant cannot afford
Critical spare
Stocked, or a repairable pool with one on the shelf
Critical spare
Stocked with a minimum, and the failure mode sent for review
Moderate
Accept the risk
Or a consignment or sister-site agreement if one is cheap
Consignment or shared
Vendor-held stock, a shared holding, a local repairer
Operational spare
Stocked on usage with a reorder point
Minor
Do not stock
Order on failure
Do not stock
Order on failure, or share across sites
Operational spare
Store min-max from usage
Likelihood of needing it inside the lead time →
| Question | Answered from |
|---|---|
| Which failure mode is this spare for? | The FMECA or the maintenance plan. A spare with no failure mode is a guess |
| What does the plant lose while it waits, per day? | The cost of downtime for the equipment, from its criticality and its function |
| How long is the wait without the spare? | Supplier lead time, plus transport, plus the repair time once it arrives |
| How likely is the failure inside that lead time? | Failure history, the Weibull life of the part, or the vendor's figure as a last resort |
| Could it be made, repaired or borrowed faster? | Repairable items, a rotable pool, a local workshop, a sister site's holding, a vendor consignment |
| What does holding it cost? | Purchase, storage, preservation, insurance and obsolescence, over the years it will sit |
| Does it fit more than one item? | Interchangeability across equipment raises the value of one spare and lowers the holding |
The alternatives to holding it
Before a spare is bought, the alternatives are checked, because most of them are cheaper and several are better. A repairable item can be held as a rotable pool: one on the shelf, one in service, the failed one repaired on exchange, which halves the holding for a pump or a gearbox. A vendor may hold consignment stock on site, owned by them until used. A sister site or a neighbouring operation may hold the same part, with an agreement to share. A local workshop may be able to make or repair a part faster than the supplier can ship one. And a part that is interchangeable across several items of equipment is worth more as one spare than the same part held for one machine.
Each alternative has a condition that makes it real. A rotable pool needs a repair turnaround shorter than the next expected failure. A consignment needs a contract that says the part will be there. A sharing agreement needs the other site to tell you before they use it. A local repair needs the drawings and the material spec, which is a handover question. The alternative is recorded on the register with its condition, so that the next reviewer knows what the decision rested on.
The cheapest critical spare is the one shared, repaired or made faster than it can be shipped. Check that before the purchase order.
The register
The register is the record of every stocking decision and its basis, and the columns in the template are the ones that make the decision reviewable later. The where-used column ties the spare to the equipment through the bill of materials, the failure mode column ties it to the strategy, and the criticality column to the assessment. Without those three, the register is a stock list, and a stock list cannot say whether a spare should still be held when the plant changes.
The register lives beside the CMMS, or in it where the system has the fields, and the store's stock codes are the join. A spare on the register that the store does not know, or a store holding that the register does not explain, is a line to resolve at the review.
| Column | What goes in it |
|---|---|
| Part | Description as the store and the bill of materials know it |
| Stock code | The store's number |
| Where used | The equipment tags it fits, from the bill of materials |
| Equipment criticality | The highest criticality among the items it fits |
| Failure mode | The failure this spare restores from |
| Class | As it says |
| Installed quantity | How many are in service across the items it fits |
| Lead time (weeks) | Order to on site, including transport |
| Downtime cost per day | From the cost of downtime for the equipment |
| Likelihood in lead time | As it says |
| Alternative | Repair, rotable, borrow, local make, consignment, or none |
| Decision | As it says |
| Quantity held | As it says |
| Minimum | The reorder point |
| Unit cost | As it says |
| Storage requirements | Shelf life, rotation, climate, preservation |
| Supplier | As it says |
| Interchangeable with | Other parts or stock codes that fit |
| Last issued | As it says |
| Review date | When the decision is next checked against usage and failures |
| Decided by | As it says |
Holding it properly
A spare that has sat on the shelf for five years is only a spare if it still works, and many do not. Bearings seized in their packaging by condensation, elastomer seals perished, motors with brinelled bearings because they were never rotated, electronics past their capacitor life, a shaft rusted under a cover that leaked. Storage is part of the decision: the climate it needs, the shelf life, the rotation interval for anything with a bearing, the preservation for anything with a machined face, and the inspection that proves it is still fit.
Insurance spares deserve the most care, because they wait the longest and the plant has the least tolerance for finding them unfit on the day. A main shaft or a transformer is inspected on a schedule, its preservation renewed, and the record kept with the register. The cost of that care is part of the holding cost in the decision, which is one more reason the decision is a whole of life one.
Reviewing the holdings
Every line carries a review date, and the review reads the register against the year's usage and failure history. Spares issued often are checked against their minimum. Spares never issued are checked against their basis, and an insurance spare that has never moved is right if the basis still holds and wrong if the equipment has gone. Lead times are confirmed with suppliers, because a part that was twelve weeks away may now be two or forty. Vendor obsolescence notices reopen every line they touch, and a control system or a drive going out of support is the commonest reason a plant discovers it needs a spare it cannot buy.
The review also reads the other way. A failure that stopped the plant because a part was not held is a decision to revisit, and a failure that used a critical spare is a question for the maintenance strategy, because a spare restores the function and does nothing about the failure. Root cause analysis on the repeaters is how the spares bill comes down for good.
Where it goes wrong
- Spares bought with the equipment, on the vendor's recommended list, with no failure mode or criticality behind any of them.
- A critical pump with no spare because the last request was refused on price, with no downtime cost beside it.
- Insurance spares written off by a stocktake that read only the issue history.
- A bill of materials never built, so nobody knows which spares fit which equipment.
- Motors never rotated, bearings stored damp, seals past their life, so the spare fails on fitting.
- Lead times never confirmed, so the register says twelve weeks and the supplier says forty.
- Obsolescence discovered when the last spare is fitted.
- No review date, so every one of the above is nobody's job.
Common questions
What is a critical spare?
A spare part held because the equipment it fits is critical to the plant and the part could not be obtained inside the time the plant could tolerate being without that equipment. It is defined by consequence and lead time together: a cheap part with a sixteen-week lead time on a critical pump is a critical spare, and an expensive part available tomorrow is not.
What is the difference between a critical spare and an insurance spare?
An insurance spare is a critical spare at the extreme: the failure is unlikely, the lead time is very long, the consequence is severe, and the part may sit for years and never be used. A mill girth gear or a main transformer is the usual example. The decision is made as a whole of life one, often by the asset owner rather than the maintenance department, because the holding cost runs for the life of the plant.
How do you decide which spares to stock?
By asking, for each candidate, which failure mode it restores from, what the plant loses per day while it waits, how long the wait is without the spare, how likely the failure is inside that lead time, whether the part could be repaired, borrowed or obtained faster another way, and what holding it costs over the years it will sit. The stocking decision follows from the comparison, and it is recorded with its basis so it can be reviewed when the lead time, the usage or the criticality changes.
What is spare parts management?
The practice of deciding which parts to hold, in what quantity, where and how, and of keeping those decisions current as equipment, suppliers and usage change. It joins the maintenance strategy, which says which failures are managed and how, to the store, which holds the parts, through the bill of materials and a register of the critical holdings. IEC 60300-3-14 covers it within the broader subject of supportability.
How often should critical spares be reviewed?
Each decision carries a review date, typically a year out, and is also reopened by an event: a change in lead time or supplier, a failure that used the spare, a change in the equipment's criticality, or obsolescence notice from the vendor. The annual review reads the register against usage and failure history and asks of each line whether the basis still holds. Spares that have not moved in years are not automatically wrong, if they are insurance spares with the basis recorded.
What should a critical spares register include?
For each spare: the part and its stock code, the equipment it fits, the criticality of that equipment, the failure mode it restores from, its class, the quantity installed, the lead time, the downtime cost per day, the likelihood of needing it inside the lead time, the alternatives considered, the decision and the quantity and minimum it produced, the unit cost, the storage and preservation requirements, the supplier, what it is interchangeable with, when it was last issued, the review date and who decided. The template with this guide has those columns.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- IEC 60300-3-14 Dependability management: application guide, supportability and support (IEC)
- ISO 14224:2016 Collection and exchange of reliability and maintenance data for equipment (ISO)
- ISO 55001:2024 Asset management system requirements (ISO)
- Best practices, metrics and guidelines for maintenance and reliability (SMRP)
- SAE JA1011 Evaluation criteria for reliability-centered maintenance processes (SAE International)
Related case studies and tools
- Cost of Downtime Calculator (tool)
- Criticality Matrix Calculator (tool)
- Reliability engineering and asset strategy program (case study)
Related reading
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