The number of teeth on the drive sprocket is the most important single design parameter in a roller chain drive. Too few teeth cause chordal action, accelerated wear, noise, and vibration. Too many teeth on the driven sprocket increase the impact of chain elongation. This guide covers the complete framework for tooth count specification.
The Minimum Tooth Count: Why 17 Is the Floor
The absolute minimum recommended tooth count for a drive sprocket in industrial applications is 17 teeth, established from the analysis of chordal action. On a 17-tooth sprocket the speed variation is 1.74 percent; on a 13-tooth sprocket it is 2.99 percent; on a 25-tooth sprocket it is 0.80 percent. The speed variation produces vibration, noise, and impact loading on the chain and driven equipment — all of which increase wear rate and reduce service life.
| Tooth Count | Speed Variation (%) | Recommended Use |
|---|---|---|
| 9 | 10.3% | Very slow drives only — not for powered primary drives |
| 13 | 4.5% | Absolute minimum for agricultural secondary drives |
| 17 | 2.6% | Standard minimum for all industrial and agricultural primary drives |
| 19 | 2.1% | Preferred for drives above 100 m/min or noise-sensitive applications |
| 21 | 1.7% | Good practice for drives above 150 m/min |
| 25 | 1.2% | Low-vibration and high-speed applications |
| 30+ | Below 0.8% | High-precision conveyors and noise-critical environments |
Practical Recommendations by Application Type
Primary Agricultural Drives
Minimum 17 teeth; 19 to 21 teeth preferred. Agricultural main drives run at moderate speed under shock load — a higher tooth count reduces per-engagement impact force and extends chain and sprocket life.
High-Speed Industrial Conveyors (Above 150 m/min)
Minimum 19 teeth; 21 to 25 teeth preferred. At high chain speeds, chordal action generates significant vibration. A 25-tooth drive sprocket at 300 m/min produces roughly one-quarter of the vibration of a 17-tooth sprocket.
Slow-Speed Heavy Conveyors (Below 30 m/min)
17 teeth is adequate. At very low speeds, chordal action produces negligible dynamic loading. Load capacity is the primary concern.
Gear Motor Direct Drive Outputs
19 to 21 teeth provides the best balance of chordal action, shaft load, and sprocket diameter for gear motor output shaft installations.
The Maximum Tooth Count on the Driven Sprocket
As chain elongates in service, rollers ride progressively higher on the sprocket tooth flanks. Maximum allowable chain elongation formula: Elongation % = 200 / N₂. For a 67-tooth sprocket: 200/67 = 2.99 percent — approximately equal to the standard 3 percent replacement threshold. For sprockets above 67 teeth, the chain must be replaced before the standard threshold. Keep the maximum driven sprocket tooth count below 67 teeth wherever possible.
Choosing an Odd Tooth Count for the Drive Sprocket
When drive and driven sprocket tooth counts share a common factor — for example 20T drive and 40T driven — the same chain links contact the same sprocket teeth on every revolution, concentrating wear. Choosing an odd tooth count for the drive sprocket (17, 19, 21, 23) distributes elongation evenly across the full chain and sprocket.
EverPower Roller Chains Australia can advise on the correct roller chain specification and sprocket tooth count for your application. Contact +61 2 9708 3322 or [email protected].
+61 2 9708 3322 | [email protected] | 27 Harley Crescent, Condell Park NSW 2201