A heavy civil contractor reported that a Hitachi ZX350LC-6 excavator had an erratic swing function. When the operator moved the swing joystick, the house would hesitate, then accelerate rapidly, making it difficult to stop the bucket accurately over a truck. The swing brake was also making a grinding noise when engaged.
The technician tested the swing pilot pressure, which was stable at 50 bar. The main pump pressure was also normal. The erratic movement and grinding noise pointed to a mechanical issue in the swing motor or the parking brake assembly. The swing motor was removed and disassembled.
Inside the swing motor housing, the brake release orifice (a tiny hole that sends pilot oil to release the spring-applied brake) was found to be clogged with a tiny piece of rubber seal. This blockage prevented the brake from releasing fully and instantly when the joystick was moved. The "grinding" noise was the brake pads dragging against the disc as the motor tried to turn.
The erratic speed was caused by the brake releasing partially, then fully, causing the swing to lurch. Additionally, the brake valve return spring was found to be broken, preventing the brake piston from retracting smoothly.
The technician cleaned the orifice with a precision wire, replaced the broken return spring, and installed a new set of brake friction discs. After reassembly and bleeding the air from the swing circuit, the swing function became smooth and predictable, and the grinding noise ceased.
