Stocking Strategy for MRO Parts: What to Keep on the Shelf vs. What to Just Order
Every plant manager has their own version of this story. A conveyor goes down at 2 a.m., the shift supervisor calls, and the one bearing that would've fixed it in 20 minutes is sitting in a warehouse in Ohio. Six hours later, you may have lost an entire shift.
Flip it around and the problem is just as real: a stockroom full of parts ordered three years ago "just in case," half of them for machines you don't even run anymore. We view this as cash on a shelf, potentially accomplishing little to nothing.
Getting this right means living between those two failure modes, here's our framework we've put together below.
The first question to ask
Before any part earns shelf space, ask one thing: what happens if this isn't here when something breaks?
That puts every part into one of three buckets: critical (failure stops production or damages other equipment), important (failure slows things down but the facility keeps running), or non-critical (inconvenient, not consequential).
Critical parts almost always belong on the shelf. Non-critical parts almost never do. Important parts are where the real judgment calls live.
Then look at lead time
Criticality alone doesn't settle it. A critical part available in two days from a local distributor is a completely different decision than a critical part sitting on a six-week factory order.
Map both together:
| Short lead time | Long lead time | |
|---|---|---|
| Critical | Keep 1–2 on shelf | Keep on shelf with min/max |
| Important | Order on-demand, monitor | Keep safety stock |
| Non-critical | Order on-demand | Order on-demand |
This matrix won't cover every situation, but it could help cover most of them.
What to stock
High-consumption consumables are the easy call. V-belts, filters, seals, gaskets, lubricants, things you go through whether or not anything breaks, would fall under this. If you burn through six belts a quarter, keep six on the shelf. Your usage history has the answer; you just have to look at it.
Common failure points on critical equipment come next. Bearings are the obvious example. If you have ten motors running the same shaft bearing and bearings are your top failure mode, that bearing lives in your stockroom. Pull your maintenance logs, find what you've replaced most over the last two or three years. That's your stocking list.
Long-lead-time parts on critical machines deserve attention even if they don't fail often. A ten-week lead time on a critical drive component is real operational risk. One unit on the shelf is cheap insurance.
Single-source and obsolete parts get stocked, full stop. Proprietary, discontinued, or available from exactly one supplier: carrying that inventory almost always costs less than scrambling for it.
What not to stock
Low-failure parts on non-critical equipment are almost always dead weight. If something has failed once in five years on a machine that doesn't touch your main line, order it when you need it.
Large gearboxes, specialty motors, and custom fabrications can run thousands of dollars each. If failure history is thin and lead times are workable, the carrying cost probably isn't justified. A blanket order or reserved-stock agreement with your distributor is usually the smarter move.
Parts with short shelf lives are a trap. Some seals, belts, and lubricants degrade in storage. Buying more than you'll use before they expire is spending money twice.
ABC analysis if you want a formal system
For categorizing an entire parts list, an ABC analysis is straightforward:
- A items: high-value or high-criticality: tight control, regular counts, exact quantities. Usually 10–20% of your SKUs but 70–80% of inventory cost.
- B items: moderate value and criticality, reviewed twice a year with reasonable safety stock.
- C items: low value, low criticality, or high-turnover consumables. Less oversight; buy in bulk.
Review A items quarterly. B items twice a year. C items annually, or when you do a full stockroom walkthrough.
The habits that break this
The framework isn't complicated. The habits that work against it are.
Stocking by memory instead of data is the most common one. "We've always kept those on the shelf" is not a strategy. Maintenance logs tell you failure frequency, we recommend you to use them as they can end up showcasing certain products/brands that may fail quicker than others.
Carrying costs are easy to underestimate. Storage, obsolescence, damage, capital tied up, that typically runs 20–30% of inventory value per year. The expensive bearing you're holding in reserve for a machine you rarely run may cost more to keep than it would to just order it when the time comes.
No min/max system means you eventually find out you have zero of something critical at exactly the wrong moment. Most CMMS and ERP platforms have this built in. Set the thresholds.
Any time equipment is added, decommissioned, or replaced, your stocking strategy needs a review. Parts for machines you no longer run are just expensive clutter.
Thanks for reading!
- The MRO-PT Team