A landscaping contractor reported that a Sany SY55C Pro mini excavator was making a loud "grinding" noise whenever the upper structure was swung. The noise was rhythmic, and the swing felt "heavy" or resistant, as if the brakes were partially applied. The swing speed was also noticeably slower than normal.
The technician began by testing the swing motor's hydraulic pressures. The pilot pressure for the "brake release" circuit was reading 35 bar, which was sufficient. However, he noted that the noise occurred immediately upon moving the joystick, before the machine had even started moving. This pointed to a mechanical issue in the brake assembly.
He removed the swing motor cover. The SY55C uses a multi-disc parking brake that is released by hydraulic pilot pressure. Upon disassembling the brake housing, he found that the friction discs were blued and scored, indicating they had been running against the separator plates while the machine was in motion. The root cause was found in the "Brake Release Orifice."
The pilot line feeds into a small orifice fitting before entering the brake piston chamber. This orifice restricts the flow to ensure a smooth release. The technician found that the orifice fitting had sheared off inside the threaded port, creating a blockage. The pilot pressure was reading 35 bar at the test port *before* the blockage, but the pressure was effectively zero at the brake piston itself. The brake was never fully disengaging.
Furthermore, the grinding noise was amplified because the worn brake plates had contaminated the hydraulic oil with fine metallic particles. These particles had started to score the swash plate of the swing motor's internal piston pump.
The repair involved extracting the broken orifice fitting (using a screw extractor), replacing the entire brake disc pack, and flushing the swing motor case. The hydraulic filter was changed. After reassembly, the brake released instantly when the joystick was moved, the grinding noise ceased, and the swing speed returned to full performance.
