A heavy earthmoving contractor faced a puzzling issue on a Hitachi ZX350LC-6 excavator. When swinging the house, the rotation would accelerate uncontrollably, and the machine's "Overload Warning" alarm would trigger randomly, even with a light load. The swing motion was jerky, and the machine would not stop precisely when the joystick was released.
The diagnostic started with the swing motor. A pressure gauge was installed to check the brake release pressure. The brake pressure was within spec, but the brake holding pressure was dropping rapidly when the joystick returned to neutral. The swing motor was removed, and the mechanical parking brake pack was inspected. The friction discs were glazed and polished smooth, significantly reducing their holding capacity. This caused the house to drift or "overspeed" as gravity acted on the boom.
However, the erratic motion and false overload alarm pointed to a mechanical issue. The Hitachi ZX350LC-6 uses a gear-driven swing motor. The technician removed the swing motor cover to inspect the pinion gear. The pinion gear teeth were heavily worn and chipped, and the mating slew bearing ring gear was found to be missing two teeth entirely. The broken teeth were jamming in the gear mesh, causing the violent jerking motion during swing.
The false overload alarm was caused by the sudden shock loads generated when the broken teeth jammed. The machine's strut pressure sensors detected these massive, instantaneous spikes and interpreted them as a static overload condition, triggering the alarm.
The repair was extensive. The broken pieces of the slew ring gear were retrieved from the swing housing to prevent further damage. The swing motor was rebuilt with new brake discs and a new pinion gear. Because the slew bearing ring gear had missing teeth, the entire swing bearing had to be replaced-a major undertaking requiring the entire upper structure of the excavator to be lifted off the track frame using a crane. After reassembly, the excavator swung smoothly and quietly, and the false overload alarms ceased completely.
