A demolition contractor reported that a Hitachi ZX330-7 excavator's upper structure would slowly drift to the left when parked on a slight grade, even with the swing brake engaged. Additionally, when slewing a heavy load, the swing speed was noticeably slower, and the machine struggled to maintain swing momentum.
The technician first tested the "Swing Holding Brake." He raised the track off the ground, slewed the house, and released the lever. The house coasted to a stop but then continued to slowly creep. He knew the mechanical brake was either worn or the hydraulic circuit was bypassing. He disconnected the swing motor's case drain line and measured the flow. The flow was 15 liters per minute (spec is under 2 L/min), indicating severe internal leakage in the swing motor.
He removed the "Swing Motor Control Valve Block" from the motor. The "Relief Valve Cartridge" (which protects the swing circuit from pressure spikes) was stuck open. The internal poppet spring had broken, allowing hydraulic oil to continuously bypass from the high-pressure side to the case drain. This prevented the system from building full swing torque and also prevented the brake-release pressure from holding the mechanical brake firmly.
To find why the spring broke, he inspected the "Swing Bearing Ring Gear" (the large internal gear inside the base machine). He found severe "pitting corrosion" (deep rust craters) on the gear teeth. The machine had been parked outdoors for months without being greased. The rust created massive friction shock loads when slewing, which was transmitted back through the pinion into the swing motor, ultimately snapping the relief valve spring.
The repair involved replacing the swing motor relief valve cartridge and the mechanical brake discs. The swing bearing was manually greased repeatedly until fresh grease pushed the rust-contaminated grease out of the mesh. The internal gear teeth were dressed with a hand grinder to remove the worst spalling. After reassembly, the swing held firm on the grade, and slewing speed and power were restored.
