Drive Pinion / Gear

Function

The drive pinion and gear ring must transmit full motor torque continuously for thousands of hours under the combined loading of drum weight overhang, gearbox side loads, and the cyclically varying torque imposed by the tumbling aggregate charge in the drum. Tooth profile accuracy, correct centre distance between pinion and gear ring, and an adequate open-gear lubrication regime are the primary factors governing noise, vibration, and wear life in this final drive stage.

Construction & quality

Pinion: alloy steel forging or bar (EN 36 / AISI 8620 or EN 19 / AISI 4140), rough-turned, carburised and case hardened to 58–62 HRC on tooth flanks and root fillet; core remains ductile. Gear quality ISO 1328 Grade 7–9. Precision hobbed and, on finer grades, profile-ground. Bore machined to H7/k6 fit with keyway. Bull gear: fabricated mild steel rim and web plate construction, or cast steel on smaller sizes; tooth ring in alloy steel, flame or induction hardened to 45–52 HRC; teeth precision machined after assembly. Split-gear construction (2- or 4-piece bolted segments) for in-situ drum mounting without drum removal.

When to replace

• Audible change in gear mesh noise from smooth whirring to knocking or irregular rattling
• Visible pitting, spalling, or scuffing on tooth flanks detectable by tactile inspection during maintenance
• Tooth profile wear visible as flat or hollow flanks, reducing the full involute contact ratio
• Backlash increasing beyond the acceptable limit (>0.5 mm measured at the pinion pitch circle)
• Cyclic variation in drive motor current detectable at one-revolution frequency, indicating bull gear eccentricity or runout