Maintenance stores in many factories have two problems at once: shelves full of spares nobody has used in years, and the one part that's actually needed is missing when a critical machine breaks down. Both come from the same root cause — spares stocked by habit rather than by analysis. Criticality analysis gives a simple, defensible way to decide what to stock and how much.
Why maintenance spares are different from production materials
Production materials have steady, predictable demand driven by the production plan. Spares have irregular demand driven by failures. A bearing may be needed twice a year — or not at all for three years. Standard inventory formulas based on average consumption don't work well. The question isn't "how much do we use?" but "what happens if we don't have it?".
Step 1: Rank machine criticality
Start with the machines. Classify each by the consequence of its failure:
- A — critical: failure stops a line or the plant, causes safety or environmental risk, or has no backup.
- B — important: failure reduces output or quality, but there's a workaround.
- C — non-critical: failure has little immediate impact.
Step 2: Identify spares for critical machines
For each A and B machine, list the components that fail or wear: bearings, seals, belts, motors, drives, sensors, valves, PLC cards, contactors. Manufacturer recommended spares lists and your breakdown history are the best sources.
Step 3: Assess each spare
For each spare, consider:
- Machine criticality — from step 1.
- Failure likelihood — from history or experience.
- Lead time — local stock, days, weeks or months (imported parts).
- Cost — unit price and holding cost.
- Alternatives — can another part, a repair or a common spare substitute?
Step 4: Decide the stocking policy
- Stock as critical spare — high impact, long lead time: keep minimum stock regardless of usage.
- Stock by consumption — frequently used parts: set reorder levels from usage.
- Buy on demand — low impact or quick availability: don't stock.
- Vendor-managed or consignment — for common items local suppliers can deliver quickly.
Step 5: Set quantities
For critical spares, quantity is usually small — often one — based on how many identical machines use it and how often it fails. For consumption-based spares, use reorder levels. See safety stock and reorder levels.
Standardise and reduce variety
Factories often stock many variants of the same part because machines were bought from different suppliers. Standardising on common bearings, motors, sensors and contactors where possible reduces the number of items and lets one spare cover several machines.
Repairable spares
Motors, drives, pumps and PCBs are often repaired rather than replaced. Track repairable items as they move: in stock, installed, sent for repair (vendor and date), returned. Recording repair cost and frequency helps decide when replacement is better than another repair.
Linking spares to machines
When spares are linked to the machines they fit, technicians opening a breakdown job see compatible spares and current stock immediately. Consumption per machine becomes visible, revealing machines that consume unusually many spares — candidates for root-cause analysis or overhaul.
Dead stock and obsolete spares
Review spares with no movement in two to three years, and spares for machines that have been retired. Decide whether to keep, return, sell or scrap them. Freeing capital and space is a quick win once data is available.
Store organisation
Clear bin locations, labels with part numbers and descriptions, separate areas for critical spares, and preservation for sensitive items (bearings, electronics, rubber parts) prevent loss and deterioration. Issue only against maintenance work orders so every movement is traceable.
Common mistakes
- Stocking based on what the last maintenance manager preferred.
- No list linking spares to machines.
- Critical imported spares not stocked until the first failure.
- Repairable items lost at vendors.
- Spares issued without work orders, so consumption is unknown.
KPIs to track
- Breakdown hours waiting for spares.
- Stock-outs of critical spares.
- Spares inventory value and turnover.
- Dead stock value.
- Spares cost per machine.
How software helps
A CMMS with spares management links spares to machines, flags critical items, alerts at minimum levels, issues spares against work orders, tracks repairables and reports consumption and dead stock. See spare parts management software.
Talk to us about organising your maintenance stores.
Working with suppliers on spares
For common items, local suppliers may agree to hold stock on your behalf or deliver within hours, letting you reduce your own inventory safely. For imported critical spares, discuss lead times and stocking options with the machine manufacturer, and consider sharing critical spares with sister plants where machines are identical.
Frequently asked questions
How do we decide if a spare is critical?
Ask what happens if it fails and isn't in stock: does production stop, is safety affected, and how long would it take to get? High impact plus long lead time means critical.
Should we stock spares for every machine?
No. Stock spares for critical machines and parts with long lead times or frequent failures. Many low-impact spares can be bought when needed.
What do we do with obsolete spares?
Review spares for retired or replaced machines regularly; sell, return, scrap or transfer them to free cash and space.
How do we track repairable spares?
Record each unit sent for repair, its vendor, expected return date and repair cost, and bring it back to stock as a repaired item when returned.
Can maintenance software alert us when critical spares are low?
Yes. Minimum levels per critical spare trigger alerts or purchase indents automatically.
How many critical spares should a typical factory keep?
It depends entirely on machines, lead times and risk. Start with the critical machines, list the components whose failure would stop them for longest, and stock those first.
