Spare Parts Inventory: How Much Is Too Much?

Spare parts inventory is one of those topics where every factory thinks it has the right level until something breaks and the part isn't there. Then over-correction kicks in: order three of everything, never run short again. Eighteen months later you have ₹15 lakh of bearings, belts, and seals sitting on a shelf, most of which will expire or become obsolete before they're used.

The right level isn't a number. It's a system that distinguishes between three classes of spares and stocks each one accordingly.

Class A: Production-Stopping, Long Lead Time

These are the parts whose absence stops the machine and which take more than two weeks to replace. Critical bearings on a key press. Specialty seals on a hydraulic system. Stock at least one spare, sometimes two for highly critical equipment. Cost of holding: high. Cost of not holding: much higher.

Class B: Production-Stopping, Short Lead Time

Parts whose absence stops the machine but which are available locally within 2–4 days. Standard bearings, common belts, generic filters. Stock minimal — one of each, ordered for replacement the day after use. The local lead time is your buffer; don't double-buffer with inventory.

Class C: Non-Stopping, Any Lead Time

Gauges, indicator lamps, decorative panels, secondary instrumentation. The machine runs without them. Stock zero. Order as needed. The cost of holding these is real; the cost of not having them is cosmetic.

Classify Once, Review Quarterly

Do the classification once with your maintenance lead. It takes a half-day for a 15-machine factory. Review quarterly — equipment ages, supplier relationships change, criticality shifts. Without review, classifications drift and the system stops matching reality.

The Single Question for Every Spare

"If this part fails right now, what's the cost of waiting for a replacement vs. the cost of holding one in stock?" If holding cost < waiting cost, stock it. If not, don't. The arithmetic is usually obvious once you actually compute both sides.

The Number to Watch

Spare parts inventory as a percentage of annualized maintenance spend. For a healthy SME factory, this lands around 15–25%. Below 15% and you're probably under-stocked on critical Class A items. Above 30% and you're hoarding Class C items that shouldn't be on the shelf at all. Track this number quarterly — it's the simplest single indicator that your spares policy is working.