A timing chain that skipped teeth on an indexing drive. The chain was 06B, driving an index table that moved 90 degrees every 2 seconds. The chain tension was set by a fixed screw tensioner. After 3 months, the chain stretched 1%. The tensioner couldn’t compensate. At the start of each index, the chain slack took up before the load engaged. The table overshot by 0.5 degrees. The indexing accuracy was lost. The customer thought the chain was defective. It was normal chain stretch — but the fixed tensioner couldn’t track it. This is about chain tensioning in cyclic indexing applications.
The problem with fixed tensioners
A fixed tensioner sets the chain length once. As the chain wears (elongates), the slack appears. In a steady-speed conveyor, the slack is on the slack side and doesn’t matter. But in an indexing drive, the load reverses direction every cycle. The slack must be taken up on both sides. A fixed tensioner only works in one direction. When the table reverses, the slack on the other side causes overshoot.
The index table moved 90 degrees clockwise, then 90 degrees counterclockwise. The chain tension was set on the clockwise side. When the table reversed, the chain had 5 mm of slack. The table moved 5 mm before the chain engaged. At the chain pitch of 9.53 mm, that’s half a tooth. The overshoot was 0.5 degrees (measured at the table).
What was changed
1. Installed a spring-loaded tensioner arm. A spring-loaded arm pushed a jockey pulley against the chain slack side. The spring maintained 50 N tension regardless of chain stretch. As the chain elongated, the arm moved out — tension stayed the same. The overshoot disappeared. The table indexed accurately for the full chain life (3 years).
2. Used an idler sprocket on both sides. For critical indexing drives, two spring-loaded idlers — one on each slack side — maintain tension in both directions. The cost is higher, but the indexing accuracy holds for 5 million cycles. Used on high-speed indexers where 0.1 degrees overshoot is unacceptable.
3. Changed to a timing belt. The best solution for accuracy: replace the chain with a steel-reinforced timing belt. A timing belt doesn’t stretch (the steel cords are inextensible). No tensioner arm needed. The indexing accuracy is 0.05 degrees. The belt costs 3x more than the chain but eliminates the tensioner problem entirely. For new designs, a timing belt beats a chain for precision indexing.
The tensioner spring selection
The spring force must be enough to take up slack but not so high that it loads the bearings. For a 06B chain, the recommended tension is 15-25 N. The spring should provide this force at the working arm position. A too-strong spring (50 N) loads the shaft bearings. A too-weak spring (5 N) doesn’t take up slack. The spring rate should be soft — the force should stay within 15-25 N as the arm moves through its 20 mm travel. A long, soft spring is better than a short, stiff one.
Chain vs belt for indexing
| Factor | Roller chain | Timing belt |
|---|---|---|
| Elongation over life | 1-2% | <0.1% |
| Tensioner required | Yes (spring arm) | No (or fixed) |
| Indexing accuracy | ±0.5° (with tensioner) | ±0.05° |
| Speed limit | 10 m/s | 80 m/s |
| Maintenance | Lubrication + tension | None |
The tensioner rule: any indexing or reversing chain needs a spring-loaded arm, not a fixed screw. The skipping chain wasn’t defective — it stretched 1% and the fixed tensioner couldn’t compensate. For precision indexing, a timing belt eliminates the problem. For high-load applications, a spring-loaded idler on both slack sides maintains accuracy through the chain’s wear life.
