A heavy construction crew reported that a Kobelco SK350LC-10 excavator's swing speed was significantly slower than normal, and the upper structure would "drift" (rotate freely) when parked on a slope.
The technician tested the swing motor. He disconnected the "Case Drain Line" from the swing motor. A large volume of oil was flowing out, indicating severe internal leakage. He removed the swing motor. Kobelco uses an "Axial Piston Motor" for swing. Upon disassembling the motor, he found the "Swash Plate" (the angle plate that controls motor displacement) had deep scoring marks.
The scoring was caused by "Cavitation Erosion." The technician found the "Suction Line Flange" was loose. The loose flange allowed air to be sucked into the oil. When the air bubbles imploded inside the motor under high pressure, they blasted away the metal surface of the swash plate. The rough surface prevented the piston shoes from sealing, causing the leakage and speed loss.
The "Drift" issue was caused by the "Cross-Over Relief Valve." This valve protects the motor from shock loads. He removed the valve cartridge. The valve seat was pitted. The pitted seat allowed hydraulic oil to bypass the motor and flow directly from the A port to the B port (internal loop) when the machine was stopped, causing the drift.
The repair involved replacing the swing motor rotating group (swash plate and pistons). The cross-over relief valve seat was lapped (polished) to restore the seal. The suction line flange was tightened. The system was filled with fresh hydraulic oil and purged of air. The swing speed was restored, and the drift stopped.
