A mining contractor reported that a Hitachi ZX490LCH excavator made a loud grinding noise when slewing (swinging), and grease was leaking from the swing drive gearbox.
The technician removed the "Swing Drive Gearbox" (planetary reduction unit). He drained the gear oil and found it filled with metal shavings. He disassembled the planetary gear set. The "Needle Bearings" inside one of the planet gears had completely collapsed. The failed bearing allowed the planet gear to wobble on its pin. The wobbling gear meshed incorrectly with the ring gear, causing the grinding noise and chipping the gear teeth.
The grease leak was caused by the "Output Shaft Floating Seal" (a metal face seal). The metal shavings from the destroyed bearing had circulated in the oil and gotten between the sealing faces of the floating seal. The abrasive debris grooved the polished steel faces, destroying the seal and allowing oil to leak out past the shaft.
The repair involved replacing the entire planet carrier assembly (due to gear tooth damage), the output shaft floating seal, and the ring gear. The swing motor was flushed. The gearbox was refilled with new gear oil. After reassembly, the swing was quiet and leak-free.
