A landscaping contractor reported that a Sany SY55C Pro mini excavator's swing speed was very slow, and the machine made a "groaning" noise when turning the upper structure. The noise was louder when swinging to the left.
The technician first checked the pilot pressure. The gauge showed 30 bar, which is normal. He then checked the swing speed with a stopwatch. It was 50% slower than the specification. He removed the "Swing Motor Case Drain" line. A significant amount of oil was flowing out of the case drain even when the machine was not swinging, indicating internal leakage.
He removed the swing motor. The swing motor is a piston-type motor. He disassembled it and found that the "Thrust Bearing" (which supports the cylinder block) was worn flat. The wear debris had scored the "Valve Plate" (distribution plate). This scoring created a path for high-pressure oil to leak into the case drain, reducing the torque delivered to the gearbox.
Furthermore, the "Swing Pilot Filter" (a small inline filter) was completely plugged with mud. This filter protects the pilot valve that releases the swing parking brake. The blockage meant the brake was only partially releasing. The "groaning" noise was the brake pads dragging against the disc while the motor tried to turn.
The repair involved replacing the swing motor rotating group (pistons, valve plate, thrust bearing). The pilot filter was replaced, and the pilot line was flushed. The swing gearbox oil was also changed. After reassembly, the swing speed returned to normal, and the groaning noise disappeared.
