11.4 MRO Inventory and Maintenance Planning Inputs

A production component gets its demand from a bill of material explosion. A maintenance, repair, and operating (MRO) item gets its demand from a machine breaking — or from a preventive maintenance calendar. That one difference breaks most of the inventory logic from Chapter 10, and ECM 9.0 tests whether you know why. Section 8.2 covered MRO purchasing; this section covers the reliability inputs, stocking arithmetic, and handling rules underneath it.

Why MRO Behaves Unlike Production Inventory

The stockout row is the exam point. A $180 bearing that idles a bottleneck line for 14 hours at $6,200 per hour of contribution costs 14 × $6,200 = $86,800 in lost throughput to avoid $180 of exposure. MRO stocking is consequence-driven, not volume-driven, so ranking spares by turns or dollar usage sends the wrong signal.

Reliability Inputs: MTBF, MTTR, and MTTF

Three measures drive MRO planning, and the exam separates them precisely.

• Mean time between failures (MTBF) — average operating time between consecutive failures of a repairable item: total operating time ÷ number of failures.
• Mean time to repair (MTTR) — average elapsed time to restore a failed item to operating condition, covering diagnosis, parts retrieval, repair, test, and return to service.
• Mean time to failure (MTTF) — average operating time until failure for a non-repairable item, one that is discarded and replaced rather than restored.

The discriminator: MTBF applies to repairable items; MTTF applies to non-repairable ones. The bearing you throw away has an MTTF; the gearbox you rebuild has an MTBF.

Availability

Availability = MTBF ÷ (MTBF + MTTR)

Take a pump with MTBF of 450 hours and MTTR of 6 hours:

Availability = 450 ÷ (450 + 6) = 450 ÷ 456 = 0.9868, or 98.68%

Now find the materials lever. Four of those 6 MTTR hours are spent waiting for a seal kit shipped from the supplier. Stock the kit on site and MTTR falls to 2 hours:

Availability = 450 ÷ (450 + 2) = 450 ÷ 452 = 0.9956, or 99.56%

The gain is 0.88 percentage points. Across a 6,000-hour operating year that is 0.0088 × 6,000 ≈ 53 hours of recovered uptime. Know which lever belongs to whom: maintenance and engineering raise MTBF through preventive maintenance, lubrication, and design; materials management shrinks MTTR by having the part on the shelf. Answering an MTTR question with "run more preventive maintenance" misses the point.

A Worked Spare-Parts Stocking Decision

Twelve identical pumps each run 6,000 hours per year. Seal-kit MTBF is 8,000 operating hours. Replenishment lead time is 10 weeks.

1. Fleet operating hours per year: 12 × 6,000 = 72,000 hours
2. Expected annual failures: 72,000 ÷ 8,000 = 9 per year
3. Expected failures during lead time: 9 × (10 ÷ 52) = 1.73 units

Stocking exactly 2 covers the average and nothing more; with lumpy arrivals, three or more failures inside a 10-week window is not rare. Set the level from consequence: a kit costs $900, so carrying cost at 25 percent is $225 per unit per year, while one unprotected failure costs a 40-hour emergency window at $6,200 per hour — $248,000. You carry 3 or 4. You do not pick a generic 95 percent fill rate and stop thinking.

The slow-moving critical spare

A $140,000 gearbox has an expected life near 15 years — annual failure probability about 1 ÷ 15 = 6.7% — and a 40-week replacement lead time. Usage logic says never stock it. Consequence logic says expected annual downtime cost avoided = 0.067 × $2,000,000 = $134,000, against carrying cost of 0.25 × $140,000 = $35,000. Stock it, and label it an insurance spare so no inventory-reduction campaign deletes it.

Criticality-Based Classification

ABC by annual dollar usage ranks that gearbox last. Near-zero usage puts it in class C, where it receives the loosest control, the least frequent count, and the first cut when someone is told to reduce inventory dollars. That is exactly backwards, and it is a named trap.

Classify MRO by criticality — consequence of failure, redundancy, and lead time — and use dollar ABC only inside the consumable tail.

Criticality coding needs asset spare-parts lists and where-used links, so a part serving four assets inherits the criticality of the most critical one.

Special Handling Requirements

MRO stores hold what production stores usually do not: solvents, compressed gases, batteries, adhesives, calibration standards, and protective equipment. That is why ECM 9.0 pulls compliance with regulations, environmental standards, materials-handling protocols, personal protective equipment (PPE), and safety into the inventory domain.

• Hazardous materials — segregate incompatible classes (oxidizers away from flammables), use ventilated flammable-liquid cabinets and secondary containment, respect fire-code quantity ceilings per storage area, keep spill kits at the point of storage, label under hazard communication rules, and restrict issue to trained personnel. A safety data sheet (SDS) must be available for every chemical held.
• Shelf-life-controlled stock — adhesives, elastomer seals, batteries, welding consumables, and calibration fluids expire. Carry an expiry or re-test date in the item master, issue first-expired, first-out (FEFO), and block expired stock from issue. Expired material is not on-hand supply; if the system still counts it, both MRP and the maintenance kit list are lying.
• PPE is simultaneously a control and a stocked consumable. Gloves, respirator cartridges, and face shields carry sizes and expiry dates, and their stockout consequence is that work stops.
• Storage environment — temperature and humidity control, electrostatic-discharge protected areas for electronic spares, and secured cages for high-value or controlled items.
• Documentation that follows the material — SDS, certificate of conformance or analysis, material test report, calibration certificate, hazardous-materials shipping papers, and proof of compliant disposal for expired hazardous stock.

Trap: "consolidate every stockroom into the new random-location racking to raise cube utilization." Special-handling items sit in legally constrained locations. Segregation, containment, and FEFO rules override slotting optimization every time.