A mining contractor reported that a Volvo EC480D excavator had excessive side-to-side play in the upper structure. When slewing, a loud grinding noise came from the undercarriage, and the machine rocked violently when braking the swing.
The technician first checked the "Swing Gear Clearance" using a dial indicator. The backlash between the pinion and the internal ring gear was over 5 mm (normal is less than 1 mm). He removed the swing motor cover and inspected the pinion teeth-they were heavily worn on one side. He then crawled under the machine to inspect the massive "Swing Bearing" (slew ring). He found two of the 40 mounting bolts on the inner ring had sheared off completely, and three others were loose.
The broken bolts allowed the inner race of the swing bearing to lift slightly during slewing, misaligning the ring gear teeth and causing the grinding noise and excessive play. To find out why the bolts failed, he checked the "Auto-Lube System." The excavator is equipped with a centralized grease pump. He disconnected the grease line going to the swing bearing pinion. No grease flowed. He traced the line to the "Distribution Block" (a manifold with metering valves). The block was completely plugged with hardened, dried grease.
Because no fresh grease was reaching the swing bearing for an extended period, the lubrication cavity dried up. The resulting metal-to-metal friction created immense torque resistance on the bearing race. The heavy shock loads from dry slewing eventually sheared the high-tension bearing bolts.
The repair required using a crane to lift the entire upper structure off the base machine to access the swing bearing. The severely worn swing bearing and pinion were replaced. The auto-lube distribution block was disassembled, cleaned, and the metering valves were replaced. All grease lines were flushed. After reassembly, the swing backlash was reduced to 0.5 mm, and the grinding noise was eliminated.
