Measurements on the slow-rotating pinion drive gear, operating at 187 RPM, showed a significant increase in vibration velocity from 5.5 mm/s to 21.5 mm/s between April and October 2020. The Airius sensors, measuring three-axial vibration, temperature, and enveloping, revealed dominating gear mesh frequencies, indicative of gear damage. Additional enveloping technique readings identified increasing bearing noise, aligning with the Ball Pass Frequency Inner Race (BPFI) symptom.

Despite the observed deterioration, careful monitoring allowed the ball mill to operate until its planned November 2020 maintenance, during which a broken tooth on the girth gear was discovered. This case highlights the importance of predictive monitoring in industrial maintenance for timely interventions and preventing critical failures.