Critical spares analysis: prioritize parts by risk

Critical spares analysis ranks replacement parts for inventory review using owner-approved criticality, usable stock, supplier lead times, usage, and downtime exposure. Start with inventory and purchasing exports, then add assumptions approved by maintenance and finance. The result is a purchasing discussion with traceable inputs.

Separate recorded facts from owner assumptions

Inventory supplies part IDs, site, usable stock, and unit cost. Issue history supplies quantities consumed over a defined observation period. Purchasing supplies receipt dates, current quoted lead times, open orders, and supplier identifiers.

Maintenance supplies the equipment dependency, approved substitutes, repair options, and required minimum stock. Safety owners flag parts requiring mandatory review. Finance approves the hourly business exposure, including what costs or lost contribution it represents. Keep assumption dates and owners in the working file.

The EPA's total productive maintenance guidance identifies downtime, labor, spare parts, and lost production opportunity among the costs of unexpected breakdowns. It does not make a part's cost equal to the cost of its absence. A cheap component can deserve urgent review if its absence prevents an essential operation.

For this method, conditional downtime exposure means the owner-estimated cost of an event with no usable spare available. It is neither annual expected loss nor a forecast. No failure probability is implied.

Assemble one row per part and site

An illustrative US manufacturer reviews four parts at one plant. Usable stock excludes quarantined, damaged, reserved, or incompatible units. Unit costs are in USD. The usage window is 365 days, and lead time uses calendar days.

Put these inputs in columns A:I in Google Sheets:

The valve's mandatory flag and all minimums are supplied by the responsible owners. This article does not establish engineering requirements. Downtime hours reflect their assessed uncovered-event scenario, including practical alternatives; they are not automatically equal to supplier lead time.

Calculate stock, usage, and exposure

Add columns J:O, entering these formulas in row 2 and filling through row 5:

J2  Stock value: =B2*C2
K2  Average lead-time usage: =D2/365*E2
L2  Conditional downtime exposure: =F2*G2
M2  Gap to approved minimum: =MAX(0,H2-B2)
N2  Cost to close gap: =M2*C2
O2  Review label: =IF(I2="Yes","Mandatory review",IF(M2>0,"Stock gap","Within minimum"))

Average lead-time usage simply spreads recorded annual issues across calendar days. It is a descriptive baseline, not a recommended reorder point for rarely used parts. Leave missing lead times or exposure assumptions blank and request an owner answer before calculating them.

For raw issue exports with several rows per part, Google documents SUMIFS for totaling values that meet multiple conditions. For example, use matching part, site, and observation-period criteria before placing the approved issue total in column D. Covirage is not affiliated with Google.

Lead-time usage is rounded to two decimals, while purchasing gaps are whole units. For the drive, 1 / 365 × 90 = 0.25 units of average usage, but the owner-approved minimum remains two units. A historical average cannot overrule that requirement.

Turn the table into a review order

Review mandatory flags first, then gaps against approved minimums. Within the remaining review list, compare conditional exposure and supply constraints. Avoid a single weighted score that lets a low purchase price offset a safety requirement.

The valve goes to its responsible owner first: it has a mandatory flag, no usable stock, and a gap of one unit. Its conditional exposure is 24 × 1,500 = $36,000. Zero recorded issues do not remove the review obligation.

The drive follows with a gap of one unit and conditional exposure of 72 × 2,000 = $144,000. The seal and filter meet their supplied minimums. Their greater usage can still justify checking replenishment timing, but their exposure assumptions do not create additional purchase requirements by themselves.

The displayed $5,200 is the cost to close both recorded gaps at the supplied unit costs: $4,000 for the drive plus $1,200 for the valve. Before purchasing, check usable incoming orders, reservations, approved equivalents, current quotes, and pack sizes. An open order arriving too late does not protect an earlier requirement.

Use supplier lead-time variability to inspect the delivery record and inventory aging by site to review excess stock separately. A slow-moving spare should not be removed simply because its usage is low.

Prove the inventory and spending totals

Reconcile stock value to the same inventory extract: 4,000 + 1,500 + 500 + 0 = $6,000. This is the listed parts' value, not the plant's entire spare-parts balance. Confirm that every included part-site pair appears once.

Match the 365-day issue totals back to the issue register, checking returns and reversals. Match lead times to the dated purchasing source and distinguish a supplier's current quote from historical order-to-receipt performance. Never treat a missing receipt date as a completed delivery.

Do not sum the conditional exposures into an expected annual loss. The scenarios can involve the same production line and overlapping lost output. A combined business exposure needs a separate, owner-approved scenario.

Where it goes wrong

• Treating all on-hand units as usable. Quarantine and equipment compatibility can make a recorded spare unavailable when needed.
• Using frequency as criticality. Frequent issues measure consumption; rare issues do not prove low consequences.
• Inventing downtime assumptions. Procurement should obtain owner-approved dependencies and financial inputs instead of inferring them from part descriptions.
• Ignoring current supply commitments. Approved minimum gaps need a time-specific check of incoming stock, reservations, and supplier constraints before purchase authorization.

For the separate question of working capital, inventory turnover measures movement relative to inventory value; it does not establish which protective spare can safely be removed.

Do this on your own file in Covirage

Bring a sample of your own spare-parts inventory, issue history, purchasing export, and owner-approved criticality and downtime assumptions. Covirage maps the columns, and its tools compute and check the figures; the external AI model explains the comparison with every figure traced to its rows. Create a free Covirage account, which takes about a minute, then set up your business and bring your files.

See self-service analytics in Covirage for the supporting business application.