A mass-excavation contractor reported that a Komatsu PC228US excavator had developed two related swing-system faults over a three-week period. First, when the operator stopped the upper structure at any angle - for instance, to hold a position while loading a truck - the upper structure would slowly drift backward, sometimes rotating 2 to 3 degrees over 30 seconds before stabilizing. Second, when the operator slewed under load (swinging the upper structure with a fully loaded bucket), a grinding noise was audible from the swing-motor area, and swing speed felt noticeably slower than normal.
The technician first understood the PC228US's swing architecture. The upper structure is driven by a "Swing Motor" - a hydraulic piston motor coupled to a swing reduction gearbox whose output pinion engages the slew ring's gear teeth. Between the swing motor and the gearbox sits a "Swing Brake" - a spring-applied, hydraulically released multi-disc brake that locks the upper structure in position when the swing function is not actively commanded. When the operator commands swing, hydraulic pressure releases the brake, allowing the motor to rotate the upper structure; when the swing command ceases, the brake springs back into engagement, holding the structure in position. Worn brake friction plates reduce the brake's holding capacity, causing the drift the operator described; contaminated hydraulic oil circulating through the swing motor's internal components causes scoring, grinding, and reduced motor efficiency.
He first checked the swing motor's "Case-Drain Oil" - the oil that leaks internally past the motor's pistons and returns to the reservoir through the case-drain circuit. Drawing a sample from the swing motor's case-drain port, he found the oil dark, viscous, and containing visible metallic particles - indicating internal wear within the swing motor or gearbox. He then tested the swing brake's holding capacity: with the upper structure slewed to a marked angle and the swing lever in neutral, he monitored the structure's rotation over 5 minutes. The structure drifted backward by approximately 8 centimeters measured at the boom tip - well beyond the 1-centimeter allowable limit, confirming swing brake failure.
He removed the swing motor and disassembled it on a bench. The "Swing Brake Friction Plates" showed severe wear, with the friction material on several plates worn down to the steel backing plates and one plate's friction material having broken off in pieces that had migrated into the brake's oil sump. The "Swing Motor's Piston Block and Cylinder Barrel" showed mild scoring consistent with the contaminated oil having circulated through the motor during operation, but the piston and barrel components were within service limits and did not require replacement.
The root cause was traced to a combination of swing brake wear and oil contamination. The swing brake's friction plates had worn normally over extended service (the machine had approximately 8,500 hours), but the brake's friction material had never been inspected or replaced, and the worn material's debris had circulated through the swing motor and gearbox, contaminating the oil and degrading the motor's efficiency - which explained the grinding noise and reduced swing speed.
The repair involved replacing the complete swing brake's friction plate set with new plates, thoroughly flushing the swing motor and gearbox to remove all friction-material debris and contaminated oil, and refilling the swing gearbox with the specified gear oil to the correct level. The swing motor was resealed with new O-rings and shaft seal, and the case-drain filter was replaced with a new element. After reassembly and bleeding, the upper structure held position with no measurable drift over a 5-minute test, the grinding noise was eliminated during loaded slewing, and swing speed was restored to specification across both directions. Swing brake friction plate inspection was added to the machine's annual inspection checklist.
