A chain conveyor started jumping teeth on the drive sprocket. The chain was a 12B (3/4 inch pitch) roller chain, 2.2 meters long, running at 1 m/s on a 19-tooth drive. The chain had been on the line for 14 months. The customer’s first diagnosis was that the chain stretched. It had, but not the way they thought. Roller chains don’t stretch. The steel doesn’t elongate. The pins and bushings wear, and the pitch gets longer as material is removed. The “stretch” is wear. The sprocket hooks (worn, hooked tooth profile) were the visible evidence, and they were the symptom, not the cause.
Where the wear comes from
When a chain articulates over a sprocket, the pin rotates in the bushing through a small angle. That rotation, under load, is a sliding contact. Every pass around the sprocket removes a tiny amount of material from the pin and bushing. The pitch lengthens by the wear per articulation times the number of articulations. For a 12B chain at 1 m/s on a 19-tooth sprocket, the chain makes about 52 articulations per second. In 14 months of 8-hour shifts, that’s over 100 million articulations. A few microns of pin wear per million articulations gives a pitch growth of 1-2% over the chain’s life — that’s the “stretch” everyone sees.
Once the chain pitch grows beyond about 2-3%, the chain no longer fits the sprocket pitch properly. The rollers ride up on the tooth flank instead of seating in the root. The sprocket tooth takes the load on its tip instead of its flank, and the tooth starts to wear into a hook shape. The hooked sprocket then accelerates chain wear, because the chain can’t seat correctly. It’s a death spiral: worn chain wears the sprocket, worn sprocket wears the chain faster. The line gets noisier, the tension gets looser, and eventually the chain jumps a tooth on the drive.
The two mistakes that were made
First, the chain was run dry. It was a roller chain in a dusty area, and nobody lubricated it because “it was supposed to be maintenance-free.” A roller chain without oil on the pins runs maybe 1/5 of its lubricated life. The wear per articulation is five times higher. The 14-month failure was really a 3-year chain that got 1/5 of its life because it ran dry. The second mistake was replacing only the chain. The sprocket was left in service because the teeth “still had metal on them.” A worn sprocket with a new chain is a fast failure: the new chain immediately starts wearing against the hooked teeth. The rule is to replace the sprocket with the chain — or at least check it with a new-chain gauge.
The fix
The chain and sprockets (drive and driven) were replaced together. The new chain was lubricated with a drip oiler set to one drop every 30 seconds, which is a light continuous film. The tension was set to the manufacturer’s spec — for a 12B at 1 m/s, that’s about 1-2% of the chain’s breaking load as slack side tension, adjusted by checking the slack on the bottom run and taking it to about 2% of the span length. The line ran quiet. A chain that’s lubricated and correctly tensioned on a new sprocket gets its full life. The plant now measures chain wear monthly with a chain gauge and replaces at 2% elongation, before the sprocket gets hooked.
What the maintenance schedule should look like
Monthly: measure the chain length over a fixed number of links with a chain wear gauge. Replace at 2% elongation. Also check the sprocket teeth for hooking — a hooked profile means the sprocket is done, replace it with the chain. Quarterly: re-tension the chain. Chains relax as the joints wear in, and a loose chain whips, which accelerates wear and can throw the chain. Lubrication: continuous drip oil is ideal for most industrial chains. A heavy grease applied monthly is the fallback. Nothing is worse than a dry chain in a dusty environment — that’s the combination that killed this one.
One more thing worth checking: the alignment of the two sprockets. A misaligned chain drive puts side load on the side plates and wears the chain edges. The chain will still “stretch” but it will also wear unevenly. A straightedge across the two sprocket faces is a 5-minute check that doubles chain life if you catch a 2-degree misalignment.
The chain didn’t stretch — it wore. The pins and bushings lost material, the pitch grew, and the sprocket hooked. Run dry, a 3-year chain dies in 14 months. Lubricate the pins, replace the chain and sprocket together, and measure elongation monthly. The hooks on the sprocket aren’t a separate failure. They’re the receipt for running a worn chain on a worn sprocket.
