Critical spares: what to hold and what to stop hoarding

It's 2am. The gearbox on the infeed conveyor has let go. Your fitter goes to the stores cage with a torch and finds three gearboxes on the bottom shelf.

None of them fit. One came off a line that was scrapped years ago. One is the right make but the wrong ratio. The third has no label, and nobody knows what it's for.

The right one is on a six-week lead time. The supplier's office opens at half eight.

That's the stores problem on most sites. Shelves full of parts nobody will ever fit, and a gap where the one part that matters should be.

Hoarding feels safe

Nobody throws a spare away. It might come in handy. The machine it fitted went years ago, but you never know. So the cage fills up, the labels fade, and nobody can find anything at 2am.

Hoarding costs you twice. There's the money tied up in stock that will never move. And there's the time lost hunting through it when a line is down. A full cage feels like cover. Often it's just clutter.

We'd go the other way. Every spare should earn its place. If you can't say which machine it fits, why you hold it and how long it would take to get another, it shouldn't be on the shelf.

Start with the machines that matter

You can't decide what to hold until you know which machines you can't afford to lose. If you haven't done it, start with criticality ranking. The top of that list is where your spares money should go.

For each of those machines, ask four questions about every part that could stop it:

1. How likely is it to fail?
2. If it fails, does the whole line stop, or just one lane?
3. How long would it take to get one, on a bad day?
4. Could we run without it for a shift, with a bypass or a workaround?

A part that fails often, stops the line and takes weeks to arrive goes on the shelf. No argument. A part that rarely fails and can be on site by lunchtime from a local supplier probably doesn't need to be held at all. Most parts land somewhere in between, and that's where you need judgement.

What usually belongs on the shelf

Every site is different, but some parts come up again and again on critical kit:

• wear parts you know will go, like belts, chains, seals, bearings and filters
• the drive or servo amplifier on a machine with no backup, with its parameters saved
• the sensors and proximity switches that fail most on your site
• the gearbox or motor on a single point of failure, if the lead time is long
• anything the maker has stopped making, while you can still get it

That last one catches people out. When a maker announces a part is going obsolete, that's your window. Our guide on what to do when the maker stops supplying covers the options.

For the wear parts, the manufacturer's manual is the place to start. HSE's code of practice for PUWER says maintenance should follow the manufacturer's recommendations, "for example periodic lubrication, replacement and adjustment of parts" (L22, paragraph 74). If the manual says change it every so many hours, you'll need one on the shelf when that time comes.

What to stop holding

Go through the cage with a list and be ruthless. Things to question:

• parts for machines that have gone
• five of something when you've used one in three years
• parts a local supplier can get you the same day
• anything with no label and no known home

Rubber, seals and some electronics don't last forever on a shelf. Check dates and condition, and rotate the stock. A drive belt that's sat in the sun by the window for years may crack the first time it's tensioned.

A spare is no good without someone to fit it

Here's the part most spares plans miss. Having the part on the shelf is only half of it.

Picture the servo drive failing on the case packer. You've got a spare, boxed and labelled. But the new drive needs parameters loaded. The only person who knows how is on holiday, and the backup file is on a laptop nobody can find. The line stays down for a shift, with the part sat on the bench.

So for every critical spare, write down three things. Where the part is. Where the settings or backups are. Who on site, or outside, can fit and set it up. If the honest answer to the last one is "only one person", that's a risk in its own right. Our guide to PLC program backups is a good place to start on the settings side.

Fitting the part can change the rules

Swapping a part isn't always the end of the job.

On some kit, a major repair is one of the things that can trigger a fresh inspection under PUWER. The code of practice lists "major modifications, refurbishment or repair work" as an example of exceptional circumstances (L22, paragraph 102). On lifting equipment the bar is clearer still. HSE lists the "replacement / repair of critical parts" as a reason for a thorough examination (HSE, thorough examinations). A new mast chain on a reach truck is the classic one. We cover the detail in LOLER thorough examinations.

And be careful with "near enough" parts. A sensor with a different range, or a motor with a different rating, may get the line running. It can also change how the guarding or the safety circuit behaves. If you fit something that isn't like for like, get someone competent to check it before you sign off the restart.

What to do on Monday

1. Take your top five critical machines and list every part that would stop each one.
2. For each part, note the lead time and whether you hold one. Fill the worst gaps first.
3. Walk the stores cage with that list. Bag and label anything with no known home, and decide what goes.
4. For every critical spare, write down who can fit it and where the settings live.

* Paragraph numbers refer to L22, fourth edition (2014, amended 2018), correct as of 3 September 2026. Check HSE's L22 page and HSE's thorough examination page for the latest.