Chain doesn't stretch, it wears
When a drive gets slack, the metal hasn't stretched like elastic. Every joint in a roller chain is a small bearing: a pin rotating inside a bush. As those surfaces wear, a tiny clearance opens up at each joint, and a chain has a lot of joints. A hundred pitches each worn by a tenth of a millimetre add 10 mm to the chain. That accumulated joint wear is called elongation, or just "stretch".
It matters because the chain and sprocket are a matched pair. As the chain elongates, its pitch no longer matches the sprocket's tooth spacing, so the rollers ride progressively higher on the teeth. Left long enough, the chain climbs and jumps teeth, hammers the tooth faces into hooks, and wears the sprockets to match its own worn pitch. At that point a new chain won't run properly on them either. Catch the wear early and you replace a chain; catch it late and you replace the drive.
Measuring elongation, step by step
- Take up the slack. Measure on the taut side of the drive, or lay the chain out and apply light tension so every joint is pulled to its worn limit. A slack chain hides its wear.
- Measure over as many pitches as practical. Pin-centre to pin-centre over a single pitch tells you almost nothing, because the wear per joint is fractions of a millimetre. Measure across at least 10–12 pitches with a steel rule or vernier; more pitches means more accuracy.
- Work out the nominal length. Number of pitches measured × the chain's nominal pitch. For example, 12 pitches of 08B chain (12.70 mm pitch) should measure 152.4 mm new.
- Calculate the elongation.
So if that 12-pitch section of 08B measures 155.0 mm: (155.0 − 152.4) ÷ 152.4 × 100 = 1.7% worn.
Check three or four sections around the chain, not just one. Wear is rarely even, and the worst section is the one that counts. A dedicated chain wear gauge does the same job faster if you check chains regularly.
The replacement thresholds
| Situation | Replace at |
|---|---|
| General industrial drives | 1.5–2% elongation |
| Drives with large sprockets (many teeth) | 200 ÷ N %, where N is the tooth count of the largest sprocket |
| Reversing, precision or high-shock drives | Toward 1%, or to the machine maker's figure |
The widely used industry rule is that a chain is worn out at around 2% elongation, and 1.5% is a sensible planning trigger. The large-sprocket rule exists because a worn chain sits higher up the tooth, and the bigger the sprocket, the less elongation it takes for the chain to ride over the top: 200/N gives 2% for a 100-tooth sprocket but only 1.5% for a 133-tooth one. If in doubt, use the lower figure.
Look at the sprockets while you're there. Hooked or sharply undercut tooth faces, a polished band high on the teeth, or the chain sitting visibly proud of the tooth root are all signs the sprockets have worn with the chain, and a new chain fitted to worn sprockets will wear out in a fraction of its normal life. Where wear is advanced, replace them as a set.
Why chains wear faster than they should
If a chain hits 2% well before its expected life, the wear itself is a symptom. The usual culprits, roughly in order: inadequate lubrication (the pin–bush joint is a bearing; run it dry and it machines itself), misalignment between sprockets (look for wear on the inner faces of the link plates and one side of the teeth), overload or shock loading beyond what the chain was selected for, and abrasive contamination: dust, grit and bark acting as grinding paste in the joints. Fixing the cause when you fit the new chain is the cheapest reliability upgrade there is; if the environment can't be fixed, the chain specification can be, with sealed, self-lubricating or corrosion-protected ranges.
Common questions
How much wear is too much for a roller chain?
Around 2% elongation is worn out for a general drive, and 1.5% is the sensible planning trigger. Drives with large sprockets need a tighter limit: 200 divided by the tooth count of the largest sprocket, as a percentage.
How do I measure chain wear without a gauge?
Measure pin centre to pin centre over 10 to 12 pitches with the chain under light tension, divide by the nominal length (pitch times the number of pitches), and read the percentage. Measure on the taut side and check several places around the chain.
Why does a worn chain damage sprockets?
As the chain elongates its pitch no longer matches the sprocket teeth, so the rollers ride higher and the load concentrates on fewer teeth. The chain hammers the tooth faces into hooks and wears the sprockets to its own worn pitch — after which a new chain won't run properly on them either.
Found wear past the limit?Send us the chain number, a photo or a worn sample and we'll match the replacement from UK stock.
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