Critical Spares When Supply Chains Are Disrupted: What to Hold and What to Risk

If you have tried to replace a drive, a gearbox or a specified motor this year, you will know the problem already. Parts that used to arrive the following week can now come with a lead time measured in months, and the quote arrives with a caveat about shipping.

The data says the same. S&P Global's UK Manufacturing PMI for September found that average vendor delivery times "increased markedly, with supplier performance deteriorating at its fastest rate since June", and put it down to port congestion, shipping delays, geopolitical tensions and the raw-material shortages that follow. Container capacity through the Suez Canal in the first half of October was just over half what it was in the same weeks of 2023, and the conflict in the Gulf has disrupted shipping through the Strait of Hormuz for most of the year.

The instinct is to hold more stock. Across the board, that is the expensive answer, and with costs where they are this year few manufacturers can afford it. The better question is narrower: which parts, on which machines, earn their place on the shelf, and what does the answer depend on?

Why last year's spares list is out of date

Most stores lists rest on an unspoken assumption: if something fails, another one can be had quickly. For a lot of parts that is still true. For the ones where it no longer holds, the risk has changed without a single line on the list changing.

The risk a spare covers is, roughly, what a day without the machine costs multiplied by the number of days you would wait for the part. Double the wait and you have doubled the exposure, even though the machine, the part and its failure rate are exactly what they were last year.

So a list that was right in a stable year can be wrong in a disrupted one. Any review has to start from today's lead times, not the ones recorded in the purchasing system when the part was last bought.

Start with the machine, not the stores list

Spares reviews stall when they start from the part catalogue. A stores system can hold thousands of lines, and most of them do not matter much. Start instead from the machines whose failure would hurt most, and work down to the parts that would stop them.

A machine is critical if any of these is true:

• It stops the line. A bottleneck with no way round it.
• There is no standby. No duty and standby pair, no second line, no way to push another machine harder for a week.
• Failure has safety or environmental consequences, not only production ones.
• It is slow or expensive to repair. Large, specialised, or dependent on a single supplier.
• It has a history. The same machine keeps reappearing on the work orders.

Most sites can name their top five without a spreadsheet. Write them down anyway, with the reason next to each. The reason is what you will need when someone asks why that stock is sitting on the shelf.

A critical spare is a replacement for a part whose failure would stop a critical machine, and which you could not get from a supplier quickly enough. Those parts are what the rest of this article is about.

Three numbers for every critical part

For each critical machine, list the parts whose failure would stop it. Not every part, only the ones that would take the machine down. Then put three numbers against each one.

1. The lead time today

Not the figure in the system, and not what it was last year. Ask the supplier for a current quote, including the route if the part comes from overseas. Where a UK repairer can rebuild the part, or an exchange unit is available, get that figure as well; it can be the shorter path by some distance.

2. What a day without the machine costs

Lost output, idle labour, the energy and overheads that carry on regardless, expediting, overtime to catch up, and any penalty for a missed delivery. If you have never put a figure on it, our guide to the true cost of unplanned downtime walks through each part of the sum, and the downtime cost calculator will give you a first number in a few minutes.

3. How much warning you would get

This is the number most spares reviews leave out. Some failures develop over weeks and can be seen coming, provided something is measuring the right thing in the right place. Others arrive with little or no warning. Electronic parts such as drives and controllers tend to fall into the second group. Wear in bearings, gears and couplings often falls into the first, if it is being monitored.

The warning is specific to the failure mode, the machine and what is being measured. There is no single figure for "a bearing", which is the point our article on the P-F curve makes in more detail. For this exercise a rough, honest estimate is enough: none, days, or weeks.

Four decisions: hold, order on warning, repair or exchange, risk it

With those three numbers in front of you, almost every critical part lands in one of four places.

Two habits make the table work. Record the decision against each part with the three numbers that justified it, so the next review can see the reasoning rather than repeat it. And agree the trigger for "order on warning" before it is needed: who places the order, on what signal, from whose budget. A trigger that has to be argued about after the alert arrives is not a trigger.

Where condition monitoring changes the answer

Monitoring moves parts between those rows. If a failure mode can be seen coming early enough, a part you would otherwise hold can be ordered when the warning arrives, and the money stays in the business instead of on a shelf.

The catch this year is arithmetic. Ordering on warning only works while the warning is longer than the lead time, and lead times have just got longer. Some parts that qualified comfortably last year no longer do. It is better to be honest about that than to assume a machine's spares are covered because the machine is monitored.

What monitoring does in every case is buy time to act: to expedite an order, line up a repairer, bring a planned stop forward, or run the machine more gently until the part arrives. That time matters even for parts you hold, because a spare on the shelf still has to be fitted, and fitting it in a planned window costs far less than fitting it in the middle of a night shift.

Monitoring is also only as good as its coverage. A sensor on the motor says nothing about the gearbox downstream of it. When you map parts to decisions, map them to what is actually being measured, not to the machine as a whole. Our guide to vibration analysis explains how a measurement turns into a decision.

The cheapest spare is the one you stop needing

A part that fails every few months is not really a spares problem. It is a reliability problem showing up in the stores. Repeat failures from misalignment, poor lubrication, looseness or the way a machine is run consume spares at a rate no stock policy can keep up with, and this year every one of them comes with a longer wait.

Your work orders and parts history will usually show where it is happening: the same part number issued to the same machine, again and again. Finding the root cause and engineering it out does more for your spares exposure than any amount of extra stock, because it removes the demand instead of buffering it. That is the argument for closing the loop rather than stopping at the alert.

Make sure the stock you hold is usable

A spare only helps if it works when you need it, and stores have failure modes of their own:

• Storage. Bearings, seals and electrical parts deteriorate in damp, dirty or hot stores. Motors and gearboxes in long-term storage need the care their manufacturers specify.
• Shelf life. Seals, belts, lubricants and some electronic parts age on the shelf. Issue the oldest first.
• Identity. A spare nobody can find, or that turns out to be the wrong revision, is not a spare. Label it against the machine it protects as well as the part number.
• Documentation. Keep fitting instructions, settings and any parameter or configuration files with the part, especially for drives and controllers.
• Ownership. Someone should be responsible for each critical spare, including knowing when it has been borrowed for another job.

Review it while lead times are moving

In a stable year an annual spares review is plenty. This is not a stable year. While lead times keep moving, review the critical spares list every quarter, and whenever a supplier warns of a price rise, a discontinuation or a change in lead time. Keep the review short: the critical machines, their critical parts, the three numbers and the decision. If the numbers have not moved, nothing needs to change.

Obsolescence deserves its own line on the list. Drives, PLCs and controllers reach the end of their life on the manufacturer's timetable, not yours, and a disrupted year is a bad time to discover that the replacement for a critical controller needs re-engineering rather than a straight swap.

Where AWI fits

The three numbers come from information most sites already hold, though rarely in one place: downtime history, work orders, parts history and what the machines themselves are reporting. AWI Analytics brings those together, so the questions in this article (which machines stop the line, which parts keep coming back, how much warning a developing fault is giving) can be answered from your own records rather than from memory.

Where a critical machine keeps consuming the same parts, AWI Labs puts engineers on site to find the root cause and manage the work to engineer it out, so the same fault doesn't come back. It is the part of a spares problem that extra stock can never solve.

Key takeaways

• Lead times have moved this year. A spares list built on last year's figures is carrying risk it was never designed for.
• Start from the machines whose failure would stop the line, then the parts that would stop those machines.
• Put three numbers against every critical part: today's lead time, what a day without the machine costs, and how much warning you would get.
• Four decisions cover almost every part: hold it, order on warning, repair or exchange, or risk it.
• Ordering on warning only works while the warning is longer than the lead time, and longer lead times shrink that list.
• Repeat failures are a reliability problem showing up in the stores. Engineering them out reduces spares exposure more than extra stock does.