A contractor reported that a Kato HD820 excavator made a loud "grinding" noise when swinging, and the engine would stall if the operator swung too fast. The noise was rhythmic and increased with swing speed.
The technician first checked the hydraulic oil. It was clean. He then tested the swing motor. He removed the "Case Drain Line" from the swing motor. A large volume of oil was flowing out, indicating high internal leakage. He removed the swing motor. Kato excavators use an "Axial Piston Motor" for swing. Upon disassembling it, he found the "Swash Plate" (the angled plate that pushes the pistons) had severe scoring marks. The scoring was caused by a broken "Piston Shoe" (a small brass slipper).
The grinding noise was the metal piston shoes rubbing against the scored swash plate. The leakage reduced the motor's torque efficiency. When the operator tried to swing fast, the low torque caused the motor to stall, and the sudden pressure spike triggered the main relief valve to open, overloading the engine.
Furthermore, the "Cross-Over Relief Valve" (which protects the swing motor from shock loads) was stuck in the open position. This valve is located on the swing motor housing. It allows oil to bypass the motor if pressure exceeds 250 bar. If stuck open, it creates a short circuit for the oil flow.
The repair involved replacing the entire swing motor rotating group (pistons, shoes, swash plate) and cleaning the cross-over relief valve spool. The hydraulic filter was changed. After reassembly, the swing was smooth and quiet, and the engine no longer stalled.
