A scrap metal recycling yard reported that a Liebherr A924 Litronic material handler's upper structure would not rotate under load. When the operator attempted to swing with a loaded grapple, the upper structure shuddered, a loud metallic clicking emanated from the slewing ring area, and the structure only rotated when the grapple was empty and positioned near the machine's centerline.
The technician first performed a "Rocking Test" on the slewing ring bearing-a standard industry diagnostic where a dial indicator is mounted to the upper structure and the bearing is rocked under controlled load to measure axial and radial play. The measurement showed 3.5 millimeters of axial play (spec is under 1.5 millimeters), indicating severe internal bearing wear. He then removed the slewing ring's inspection cover. The internal ring gear teeth and the slewing reducer's pinion were coated with a hard, dry, black grease that had completely lost its lubricating properties. Scattered around the gear mesh were flakes of spalled bearing steel, indicating the bearing race surfaces had begun to disintegrate.
Further inspection revealed the root cause. The slewing ring's automatic greasing system had failed months earlier-a broken grease line at the bearing's grease inlet fitting had gone unnoticed, and the bearing had been operating essentially dry. The dry grease had hardened into an abrasive paste, and the steel debris from the failing bearing races had contaminated the remaining grease, accelerating the gear-tooth wear. The "Slewing Reducer Pinion Lock Bolt" had sheared from the excessive shock loading of the degraded gear mesh, allowing the pinion to back off its spline and intermittently disengage from the ring gear under load-this was the source of the loud clicking and the failure to rotate under load.
The repair involved replacing the complete slewing ring bearing with a new unit, replacing the slewing reducer assembly (the pinion and internal gearing had suffered wear from the metallic debris), and installing a new pinion lock bolt with thread-locking compound and correct torque specification. The automatic greasing system was repaired with a new grease line and fitting, and the greasing interval was verified with the machine's onboard diagnostic system. The upper structure was rotated through multiple full revolutions to verify smooth slewing under full grapple load, with no clicking or shudder, and the bearing play was measured at 0.8 millimeters-well within specification.
