A mass excavation contractor reported that a Doosan DX225LC excavator had developed a loud grinding noise from the machine's swing (slewing) mechanism whenever the upper structure rotated. The operator also noted a "notchy" feel during slow slewing - the upper structure would move in small jerks rather than smoothly - and an intermittent resistance that occasionally locked the swing entirely until additional hydraulic pressure was applied. A visual inspection revealed fine metallic dust accumulating on the slew ring's external gear teeth and around the swing reducer's mounting flange.
The technician first understood the excavator's swing architecture. The upper structure rotates on a "Swing Bearing" (slew ring) - a large-diameter slewing bearing comprising an inner race (bolted to the undercarriage) and an outer race (bolted to the upper structure), separated by rolling elements (balls or rollers) that support the upper structure's weight and transmit the swing torque. Drive torque is applied through the "Swing Reducer" - a planetary gearbox coupled to a hydraulic swing motor, whose output pinion engages the slew ring's external gear teeth to rotate the upper structure. When either the bearing's internal raceway or the reducer's output pinion wears, the upper structure develops play, produces grinding noises, and can intermittently bind under load.
He measured the slew ring's radial and axial play using dial indicators mounted at the upper structure's edge. With a pry bar applying lateral pressure, the upper structure shifted axially by 4 millimeters (spec maximum is roughly 1.5 millimeters) and radially by 2.5 millimeters - both well beyond service limits, confirming significant slew ring wear. He then inspected the slew ring's external gear teeth - the teeth showed heavy wear on one quadrant, with visible scoring and metal removal, consistent with the upper structure's weight having shifted toward one side of the bearing and concentrated wear in that quadrant.
He removed the swing reducer from the upper structure's deck and inspected its output pinion. The pinion showed severe wear on its teeth, with two teeth exhibiting chipping at their tips and a pronounced scuffing pattern along the flank surfaces. The reducer's gear oil, when drained, was heavily contaminated with metallic particles - consistent with the pinion's wear debris migrating into the reducer's internal gear set.
The root cause was traced to a combination of slew ring wear and inadequate lubrication. Slew ring bearings require periodic greasing at the proper interval to maintain the grease film that separates the rolling elements from their raceways. The machine's service history showed slew ring greasing had been performed at extended intervals (500 hours rather than the specified 250 hours), and the machine had been operating in an abrasive, dusty quarry environment that accelerated grease contamination. As the slew ring's raceway wore, the upper structure's weight distribution shifted, concentrating wear in one quadrant of the bearing and further stressing the reducer's output pinion, which had to transmit torque across an increasingly uneven bearing surface.
The repair involved replacing the complete slew ring bearing with a new unit (the internal raceway wear was beyond repair), replacing the swing reducer with a remanufactured unit including a new output pinion, and thoroughly cleaning both the undercarriage's and upper structure's slew ring mounting surfaces to remove all metallic debris. The slew ring's greasing system was inspected - all grease zerks and distribution lines were verified functional, and the slew ring was greased thoroughly until fresh grease purged from the bearing's seal faces. After reassembly and proper alignment torque on all slew ring mounting bolts, the upper structure rotated smoothly through full 360-degree turns without grinding, notchy feel, or binding. Slew ring greasing at 250-hour intervals was added to the machine's preventive maintenance checklist.
