Maintenance, Repair and Overhaul

We Keep Running Out of Spare Parts

Spares are hard to stock: many items, low and lumpy usage, long lead times, and a stockout that stops a machine or a repair. Stocking everything fills the shelves and still misses, and stocking by feel leaves gaps. Rank items by what a stockout stops, then set levels from usage and lead time.

What Is Usually Going On

  • Min/max levels were set years ago and do not match current usage or lead time.
  • Every spare gets the same rule, whether a stockout stops a machine, a vehicle, an aircraft or nothing.
  • Supplier and repair vendor lead times are longer and less reliable than the system says.
  • Repairable parts (rotables) are not tracked through repair, so the pool runs short when units are out.
  • Parts are taken without a transaction, so on-hand counts are wrong.

How to Fix It

  1. Rank by criticalityFor each spare: what does a stockout stop, and is there a substitute? Critical, important and routine items get different service levels.
  2. Get real usage and lead timeUsage per month over two or more years, and actual lead times from receipts. Note how lumpy the usage is.
  3. Set safety stock for steady itemsSafety stock = z x demand variation x √lead time. Reorder point = usage over the lead time + safety stock.
  4. Treat slow movers separatelyFor items used a few times a year the normal formula does not fit. Set critical ones to a minimum of one or two based on lead time and what a stockout stops.
  5. Size the rotable poolPool = removals per week x repair turn time in weeks, plus a margin for variation (Little's Law). A shorter repair turn means a smaller pool.
  6. Fix the recordsCycle count critical spares and require a transaction for every issue.

Start Free

Guides and web tools on this site. No account needed.

Guide
Inventory Management for Manufacturing
Guide
Preventive & Predictive Maintenance
Guide
Supplier Management for Manufacturing
Calculator
Safety Stock Calculator
Calculator
WIP and Little's Law Calculator

When It Moves to the Floor

The same work as an offline file you keep: saved scenarios, print and export, no internet needed on the floor.

Common Questions

Does the safety stock formula work for spares?

For spares with steady monthly usage, yes. For items used a few times a year it does not fit well; set those by criticality and lead time and review them by hand.

What service level should spares get?

Set it by what a stockout stops. A spare that grounds a vehicle or stops a constraint machine gets a high level; a routine consumable can run lower.

What is a rotable?

A repairable part that cycles between installed, in repair and on the shelf. The pool has to cover the units in repair, so repair turn time drives how many you need.

How does preventive maintenance help?

Planned work makes usage predictable. Parts for scheduled tasks can be kitted ahead instead of held as safety stock.