A civil contractor reported that a Liebherr R 926 excavator had developed a severe, jerky motion when swinging the house to the right. The swing to the left was smooth, but any attempt to swing right resulted in a violent stuttering motion that shook the entire cab. The operator also noted a loud grinding noise coming from the base of the house during the swing.
The diagnostic process began by checking the swing motor pressures. Gauges were installed on both the left and right swing motor supply lines. The right swing circuit spiked to the relief pressure of 4,000 psi instantly, but the house barely moved. This indicated a mechanical binding or a brake that was failing to release, rather than a pure hydraulic flow issue.
The technician checked the swing brake solenoid valve on the main control valve. The solenoid coil was receiving proper voltage from the ECM, but the valve spool was seized in the closed position. The spool had rusted internally. Because the brake fluid couldn't return to the tank, the multi-disc swing brake remained partially applied at all times, dragging against the housing and causing the violent stutter as the hydraulic motor tried to overcome the mechanical friction.
However, the rusted solenoid was a symptom of a larger contamination issue. The technician drained the oil from the swing reduction gearbox. Instead of golden gear oil, a thick, milky-white emulsion poured out. A torn dust seal on the swing bearing had allowed rainwater and high-pressure wash water to ingress the swing gear cavity over months. The water emulsified the gear oil, destroying its lubricity. The resulting metal-on-metal friction had shed iron shavings into the swing bearing teeth, causing the loud grinding noise.
The repair involved replacing the seized swing brake solenoid valve and flushing the brake control circuit. The swing reduction gearbox was removed, disassembled, and thoroughly cleaned. The heavily scored planetary gears had to be replaced. The swing bearing seal was replaced, and the swing circle was manually packed with fresh, extreme-pressure lithium complex grease. After reassembly, the excavator swung smoothly in both directions, and the grinding noise was entirely eliminated.
