Failure Analysis of the Girth Gear crack in Ball Mill, Pellet Plant

The ball mill of the Pellet Plant grinds the iron ore fines to reduce their size and prepare them for the Pelletizing process. The ball mill is rotated using a large Girth Gear and pinion assembly. This interim investigation report examines the failure of a girth gear in the ball mill at the Pellet Plant, manufactured by M/s FL Schmidt and installed in 2012. The gear failed after 13 years of operation, short of its expected 20-year lifespan. A radial crack, approximately 800 mm long, was observed emanating from the web to the rim, with axial propagation along the gear tooth.

Investigation methods included site observations, material analysis revealing a martensitic structure consistent with G35CrNiMo6-6, natural frequency checks, backlash and tooth thickness measurements, and finite element analysis (FEA) using ANSYS. The FEA highlighted high-stress concentrations (45-65 MPa) in areas corresponding to the observed crack, while the chemical composition matched the G35CrNiMo6-6 standard. Safety factors calculated using ASME Elliptical Failure Criteria ranged from 0.7 to 1.1, while ANSYS simulations indicated factors between 1.48 and 1.55, suggesting potential failure risks in high-stress zones.

The primary causes of failure were identified as stress concentrations in the gear web and rim, compounded by material fatigue and operational conditions, with stresses exceeding the calculated endurance limit.

Recommendations include immediate repair of the girth gear, preventative replacement of the entire assembly during scheduled maintenance, and a comprehensive design review by the OEM to rectify identified shortcomings. Regular inspections for micro-cracks and other defects are also advised. This report provides a thorough examination of the failure, integrating on-site observations, material analysis, FEA, and fatigue life calculations to determine root causes and propose corrective actions.