The choice between ductile iron and alloy steel is primarily driven by mill power and duty cycle:

Ductile iron advantages:

• Cost: 50-60% cheaper than alloy steel for the same gear size. The graphite structure in ductile iron provides self-lubricating properties, reducing wear on the mating pinion.
• Damping: The graphite nodules absorb vibration. Ductile iron gears run noticeably quieter than steel gears. This is important for mills near residential areas with noise restrictions.
• Machinability: Ductile iron cuts faster than alloy steel, reducing manufacturing lead time by 2-3 weeks for large gears.

Ductile iron limitations:

• Fatigue strength: Contact fatigue limit of ductile iron is approximately 700-900 MPa vs 1000-1200 MPa for ZG42CrMo. Under heavy load (especially SAG mills with shock loading from large rocks), ductile iron teeth pit faster.
• Notch sensitivity: Ductile iron is more susceptible to crack initiation from surface defects than steel. Any casting imperfection at the tooth root becomes a fatigue crack initiation point.

Choosing between them:

• Ductile iron (QT600-3): Best for ball mills up to 2,500 kW, rotary kilns, dryers — applications with steady load and no shock. Provides the best value per kW transmitted.
• Alloy steel (ZG35CrMo): Best for ball mills 1,500-3,500 kW — the standard choice for >3.0m diameter cement mills.
• Alloy steel (ZG42CrMo / 35CrMo Forged): Mandatory for SAG mills, large ball mills >3,500 kW, mills with variable load or frequent start-stop cycles. The higher cost ($1.5-2x vs ductile iron) is justified by preventing a single unplanned shutdown.

Yes. ZHILI has been reverse-engineering girth gears for 30+ years. The process is more complex than any other mill component because gear geometry must be precise to micron level:

Four critical gear parameters we must capture exactly:

• Tooth module (m): The ratio of pitch diameter to tooth count. Even a 0.5mm module error means the new gear will not mesh with the existing pinion — both must be replaced.
• Pressure angle (typically 20): Standard involute is 20, but some OEMs use 14.5 or 25 for specific applications. Must match exactly.
• Tooth profile modification: Many OEM gears use tip relief or root relief to compensate for tooth deflection under load. We must replicate these modifications, not just the nominal involute.
• Helix angle (for helical gears): Double-helical (herringbone) gears are common on large mills to eliminate axial thrust. Left-hand and right-hand helix angles must be matched within 0.1.

ZHILI reverse-engineering workflow:

• Option A — OEM drawing available: We manufacture to the drawing with our material recommendation. Lead time: 10-14 weeks.
• Option B — Existing gear available for measurement: We use a portable CMM (coordinate measuring machine) or laser tracker to capture the exact tooth geometry from your existing gear — even a worn one. From the scan data, we reconstruct the original (unworn) tooth profile. Lead time: 12-16 weeks.
• Option C — On-site measurement of pinion + gear housing: If your gear is too damaged to scan, we measure the mating pinion (which typically wears less) and the gear housing mounting points to calculate the required gear geometry. Lead time: 14-18 weeks.

Material upgrade opportunity: If your existing gear is gray cast iron (HT250/HT300) and you’re operating at >1,200 kW, upgrading to ductile iron (QT600-3) provides 100-140% higher tensile strength with only 50% cost increase. If you’re already running ductile iron and experiencing pitting, upgrading to ZG42CrMo alloy steel provides 40-50% higher contact fatigue resistance. Each material upgrade extends gear life by 30-50% under the same operating conditions.

Pinion replacement recommended: When replacing a girth gear, always replace the pinion simultaneously — a new gear running against a worn pinion will experience accelerated wear. ZHILI supplies matched gear + pinion sets with lapped tooth contact patterns.