A construction site reported that a Komatsu PC200-8M0 excavator made a loud "squealing" noise when swinging to the right, and the upper structure would not stop immediately when the joystick returned to neutral.
The technician first tested the pilot pressure at the swing motor. The pressure was stable. He then focused on the "Swing Parking Brake." The brake is a spring-applied, hydraulically-released multi-disc brake located inside the swing motor housing.
He removed the swing motor cover and inspected the brake assembly. He found that one of the large "Brake Piston Return Springs" was broken. The swing motor uses multiple springs to push the piston against the friction plates when the machine is off. When the pilot pressure acts on the piston, it compresses the springs to release the brake. The broken spring allowed the piston to tilt slightly in the bore. This caused the piston to drag against the friction plates even when hydraulic pressure was applied, creating the squealing noise and heat.
Furthermore, the dragging brake caused the friction plates to glaze (become smooth). The "glazing" reduced the coefficient of friction. When the operator released the joystick, the springs pushed the piston back, but the glazed plates could not generate enough braking torque to stop the upper structure quickly.
The repair involved replacing the entire set of return springs and friction plates. The technician also found air bubbles in the pilot line to the brake release port, indicating a loose fitting. The fitting was tightened, and the pilot system was bled. After reassembly, the swing brake engaged sharply, holding the machine securely on a 15-degree slope, and the squealing noise disappeared.
