A utility contractor reported that a Komatsu PC200LC-8 excavator's upper house slowly drifted to the left when parked on a slope. Additionally, when slewing, the swing gearbox emitted a loud, grinding screech. The main implement hydraulics functioned normally, but swing speed was noticeably sluggish after the machine warmed up.
The technician first checked the central swivel joint (rotary manifold) for internal cross-leakage, but the seals were intact. He then focused on the "Swivel Motor." He disconnected the swivel motor case drain line and measured the flow while slewing. The drain flow was a massive 12 liters per minute (spec is under 3 L/min), indicating severe internal leakage in the motor. He removed the swivel motor and disassembled it. The "Valve Plate" face was severely scored with deep grooves, and the mating cylinder block was heavily burnt.
An inspection of the hydraulic tank revealed the oil was a milky, cloudy white, indicating severe water contamination. The water had destroyed the lubricity of the hydraulic oil film. Under high pressure (350 bar), the valve plate and cylinder block made direct metal-to-metal contact. The scored valve plate allowed high-pressure oil from the working ports to blow straight into the low-pressure case drain cavity. This bypassed the swing motor's mechanical torque output (causing the sluggish swing and drift) and generated extreme friction heat and metal screech. The overheated, degraded oil had also glazed the multi-disc swing parking brake, reducing its holding force.
The repair involved replacing the complete swivel motor assembly (the valve plate and cylinder block were too deeply scored to be lapped and reused) and replacing the swing parking brake friction discs. The hydraulic system was flushed twice to remove the emulsified oil, and fresh ISO 68 anti-wear hydraulic fluid was introduced. The clogged hydraulic tank breather was replaced, as a blocked breather creates internal condensation that leads to water ingress. Post-repair, the excavator slewed smoothly with zero drift and no grinding noise.
