Why spare parts do not behave like normal stock
Ordinary inventory is held because you expect to sell it. Spare parts are held because you expect not to need them, and would be badly hurt on the occasion you do. That reverses almost every rule of thumb: high turnover is not a goal, a part sitting untouched for three years may be doing its job perfectly, and the cost of a stockout is measured in downtime rather than a lost sale.
The consequence is that consumption-based replenishment does not work. A part used twice in five years has an average demand indistinguishable from zero, so any reorder point derived from usage history will tell you to hold nothing, right up until the day the machine stops and the part is on a six week lead time from Germany.
It also means the value of a spare is not its purchase price. A gasket costing eleven euros that idles a packing line for a full shift is, in the only sense that matters, an expensive part. Ranking a spares storeroom by unit cost produces a list that is almost exactly backwards.
Classify by criticality before anything else
Start by asking, for each part, what happens if it fails and you do not have one. Three broad answers are enough: production or service stops entirely, something is degraded but work continues, or it is an inconvenience. That single question sorts a storeroom faster than any spreadsheet analysis, and it is a question the maintenance team can answer without data.
Cross that with lead time, because criticality only becomes expensive when replacement is slow. A critical part available from a local distributor by tomorrow morning does not need to sit on your shelf. A non-critical part with a fourteen week lead time might, if losing it for a quarter is genuinely unacceptable.
Value comes third, and only to decide how much care each tier gets. ABC analysis is still useful for setting count frequency and approval thresholds, but running it before criticality produces a storeroom optimised for the accountant and useless to the person holding a broken machine.
- Consequence of failure: stops work, degrades work, or inconvenience
- Replacement lead time: hours, days, or months
- Value: sets count frequency and approval limits, not stocking decisions
- Shared use: a part fitted to four machines is more worth holding than one fitted to a single machine
Setting stocking rules for parts that barely move
For critical, long-lead parts the rule is usually one on the shelf, replaced immediately on use. The quantity is not calculated, it is decided: you are buying the ability to recover from one failure while a replacement is on order, and holding two is a judgment about how likely a second failure is before the first replacement lands.
For parts fitted to several machines, size the minimum against the number of installations and the failure pattern rather than against usage. Four identical pumps across the site with a history of roughly one failure a year justifies a different minimum from a single pump that has never failed.
For genuine consumables such as filters, belts and lubricants, normal min and max levels work fine because consumption is regular and predictable. These are the parts where a reorder point calculated from usage is meaningful, and separating them from true spares is what keeps the rest of the policy honest.
Write the reasoning down next to the part. A minimum quantity with no explanation gets cut in the next cost review by someone who has no way of knowing it was set after a two day outage in 2024.
Let the maintenance log drive the parts list
The only reliable demand history for spares is the record of what maintenance actually consumed. If services are logged with the parts used, that log answers three questions no supplier catalogue can: which parts are genuinely being fitted, how often, and on which assets.
It also finds the dead stock. A storeroom that has never issued a part in four years is holding either an excellent insurance policy or a mistake, and the difference is whether the equipment it fits is still in service. Retiring a machine without reviewing its spares is the most common way an MRO storeroom accumulates money it will never see again.
Reading the log the other way round is just as useful. An asset generating a steady stream of corrective work and parts consumption is telling you something about replacement, and that argument is far easier to make with two years of parts cost against the machine than with a general sense that it keeps breaking.
Storeroom discipline that survives a busy week
Parts have to be issued, not taken. The single most common failure in spares management is a technician grabbing what they need at seven in the morning with every intention of writing it down later. Scanning the part out on a phone against the asset or work order takes seconds and is the only version of this that survives contact with a breakdown.
Count the critical tier more often than everything else. A short cycle count on the parts that stop production, done monthly, catches a missing spare while it can still be reordered calmly rather than during the outage it was bought for.
Keep spares physically and logically separate from sellable stock. Parts held for your own equipment should not appear in what you can promise a customer, and equipment should not be repaired out of stock earmarked for an order.
In StockFlow, spare parts can be tracked as stock with their own locations, minimum levels and low-stock alerts, while the equipment they belong to lives in the Assets module with check-in and check-out, maintenance status and readiness reporting. The Assets module is included on Professional and above, or available as a $4/month add-on.
- Issue parts by scanning against the asset or work order, not from memory
- Cycle count the critical tier monthly, the rest far less often
- Keep spares out of the pool of stock you can sell or promise
- Review spares whenever an asset is retired, not a year later
Know the critical spare is there before the machine stops
Track spare parts with their own locations and minimum levels, issue them against the asset that consumed them, and get alerted before the shelf is empty. Free to start.
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