Multiple possibilities are available to increase the transmissible power of girth gears. These solutions include: using a larger module, increasing of the gear diameter through the number of teeth, enlarging the face width, and increasing the hardness of the base material.

The first three parameters are mostly limited by cutting machine capability. Module, outside diameter, and face width (for a cast gear) can theoretically be increased to infinity, but not the cutting machine dimensions. There are also practical limits with respect to the installation of very large diameter/large face width gears.

The hardness is the sole parameter that is related to the base material.

Within the past decade, mining industry demand for gear-driven/high-powered grinding mills has pushed the installed power to levels previously thought to be unachievable or impractical. Girth gears are now being used to drive ball and SAG mills having total installed power in excess of 17,000 kW (23,000 HP).

The development of high-hardness materials suitable for these applications has resulted in the design and manufacturing of cast girth gears up to 350 HBW in steel and 340 HBW in ductile iron.

This paper intends to review the related impact in terms of design and manufacturing of such high-hardness gears and present a summary of results from a population of more than 170 gears manufactured from cast materials having hardness in excess of 300 HBW, including almost 20 gears manufactured from cast materials having hardness in excess of 340 HBW, with an approximately equal distribution between cast steel and ductile iron base materials.