Reducing unplanned downtime with a critical spares plan

Many unplanned stoppages last longer than necessary for a simple reason: the failed part is not in stock, and nobody knows quickly where to get it. A critical spares plan addresses this by deciding in advance which parts must be available on site, which can be sourced when needed, and what to do for parts that are hard to find. This article explains how to build such a plan step by step.

What makes a spare part critical

A part is critical when its failure stops or seriously limits production, affects safety or the environment, and cannot be replaced quickly. Criticality depends on 3 factors:

1. Consequence of failure: what happens to production, safety, quality and delivery commitments when the part fails.
2. Likelihood of failure: how often the part fails or wears out in your operation.
3. Time to replace: how long it takes to obtain and install the part, including sourcing, transport and customs.

A cheap part can be highly critical. A small sensor or a specific fuse can stop a line just as effectively as a large gearbox.

Step 1: Rank your equipment

Start at the machine level rather than with individual parts. List your production assets and rate each one for the consequence of a stoppage. A simple scale is enough, for example:

• A: stops the plant or a bottleneck line, or creates a safety or environmental risk.
• B: stops a line, but production can continue elsewhere or catch up later.
• C: limited impact, a redundant unit or workaround exists.

Focus the detailed work on class A equipment first.

Step 2: Identify the critical parts of critical machines

For each class A machine, go through the bill of materials and the maintenance history. Ask for each assembly:

• Has this part failed before, here or on similar machines?
• Which parts are known wear items (bearings, seals, belts, filters, contactors)?
• Which parts are obsolete or only available from a single source?
• How long did it take to obtain this part the last time it was needed?

Maintenance technicians and operators are a valuable source here. They often know which parts cause trouble, even if it is not recorded in the maintenance system.

Tip: Pay special attention to electronics and drives on older machines. They tend to fail without warning and are often the hardest parts to source.

Step 3: Decide how to cover each part

Not every critical part has to be on the shelf. Typical options are:

For obsolete parts, consider a last time buy, a repairable spare, or a planned upgrade. Our article on sourcing obsolete industrial parts explains these options in detail.

Step 4: Set stock levels

For each part you decide to stock, define a minimum quantity and a reorder point. Base this on how many identical parts are installed, how often they fail and how long replenishment takes. For most critical spares in a single plant, the minimum is often 1 unit. Avoid stocking far more than needed, because tied up capital and ageing stock are real costs. The article on spare parts inventory strategy covers stock levels in more depth.

Step 5: Make sure the spare actually works

A spare that fails when installed is worse than no spare, because it creates false confidence. Good practice includes:

• Correct storage: dry, clean and temperature stable conditions, original packaging, protection from vibration.
• Rotating equipment: follow the manufacturer's guidance for stored motors and pumps, which may include turning the shaft periodically.
• Electronics: some electronic modules contain electrolytic capacitors or batteries that age in storage; check the manufacturer's recommendations.
• Elastomers and seals: rubber parts have limited shelf life; record the date of manufacture.
• Version control: make sure the spare has the same hardware and firmware version as the installed part, or that the difference is known.
• Testing: test critical spares on a bench or by swapping them in during planned maintenance.

Step 6: Prepare the information, not only the parts

When a critical part fails and no spare is available, the speed of sourcing depends on information. For each critical part, keep a record with:

• Manufacturer, full part number and description.
• Photos of the part and its nameplate.
• Known alternatives and cross-references.
• Approved suppliers and the last purchase details.

Having this ready allows you to request a part immediately with complete data. If the original manufacturer can no longer supply it, a sourcing agency such as LiteHard can search distributors, surplus and stock holders worldwide.

Step 7: Review regularly

A spares plan is not a one time project. Review it at least once a year, and after every significant breakdown. Update criticality when production changes, remove spares for machines that were scrapped, and add new obsolescence risks as manufacturers announce discontinuations.

Key takeaways

• A part is critical when its failure has high consequences and it cannot be replaced quickly.
• Rank equipment first, then identify critical parts on the most important machines.
• Choose how to cover each part: stock, supplier stock, shared stock, repair pool or redesign.
• Store, version check and test spares so they work when needed.
• Keep complete part information ready and review the plan regularly.