A road construction crew reported that a Hyundai R210W-9 wheeled excavator suddenly lost all ability to swing left or right. The boom, stick, and bucket functions operated normally, and the machine drove forward and backward without issue. The operator noted that right before the swing failed, there was a loud mechanical "pop" from under the cab, followed by a complete loss of rotational control.
The diagnostic process began by checking the hydraulic side of the swing circuit. A pressure gauge was connected to the swing motor supply line. When the operator moved the joystick, the pressure spiked to the system relief setting of 3,800 psi, confirming the main control valve and pilot circuit were delivering flow to the swing motor. Because the hydraulics were functioning but the house wasn't moving, the failure was isolated to either the swing motor itself or the mechanical swing reduction gear.
The technician drained the gear oil from the swing reduction gearbox. The oil was heavily contaminated with a thick, metallic grey sludge, indicating severe internal mechanical grinding. The swing motor and reduction gear assembly were unbolted from the undercarriage and lowered to the ground.
Upon removing the swing motor from the reduction gear housing, the root cause was immediately apparent. The splined adapter that connects the hydraulic swing motor shaft to the sun gear of the planetary reduction drive had completely sheared off. The motor was spinning freely, but it was no longer transferring torque to the swing gear.
The sheared splines were a secondary failure. The technician inspected the hydraulic drain line coming from the swing motor. The swing motor case drain line is crucial; it carries internal leakage away from the motor to prevent pressurizing the motor housing. The drain line was found to be completely blocked by a hardened chunk of RTV silicone gasket maker. Someone had applied excessive silicone to a fitting during a prior repair, and a piece had broken loose and lodged in the drain orifice.
With the drain line blocked, the internal leakage had nowhere to go. The pressure inside the motor housing built up until it blew out the output shaft oil seal. Once the seal blew, the pressurized hydraulic oil flooded the planetary reduction gearbox, displacing the gear oil and washing away all lubrication. The sun gear splines, running dry under high torque, overheated and sheared off.
The repair required installing a new swing motor and a new splined adapter. The planetary gears were inspected, cleaned of the metallic sludge, and reassembled with fresh EP gear oil. The blocked drain line was cleared, the restrictive fitting was replaced, and a new shaft seal was installed. After reassembly, the excavator swung smoothly in both directions, and the case drain pressure was verified to be within the normal 5-10 psi range.
