A mass excavation contractor reported that a Hitachi ZX200-6 excavator's bucket developed audible "clunk" sounds each time the bucket curl direction changed-from curl-in to curl-out, or vice versa. The operator also noted that the bucket would shift laterally by several millimeters at the cutting edge when transitioning between curl directions, reducing placement precision when grading or trimming. Over several months, the clunking had become progressively more pronounced.
The technician first visually inspected the bucket's four-bar linkage-the series of pins, bushings, and link arms that connect the bucket cylinder to the bucket, the arm, and the idler linkage. He used a pry bar to check for play at each pin joint. At the "Bucket-to-Arm Pin" and the "Link-to-Bucket Pin," he measured 4 millimeters of lateral play and 6 millimeters of radial play-far beyond the 0.5-millimeter service limit. The bushings' inner surfaces were visibly worn oval, and the pins showed deep scoring along their bearing surfaces.
He further inspected the remaining linkage pins-the "Link-to-Arm Pin" and the "Bucket Cylinder Rod Pin." These pins showed moderate wear with approximately 1.5 millimeters of radial play, indicating they too were approaching their service limits. The bucket cylinder's rod-end bushing also showed wear.
The root cause was traced to grease starvation. The bucket linkage operates in one of the most abrasive environments on the excavator-continually exposed to dirt, rock dust, and water. When the linkage pins are greased at the proper interval (every 50 operating hours in abrasive conditions), the grease forms a hydrodynamic film that separates the pin from the bushing's inner surface, preventing metal-to-metal contact. The customer had extended grease intervals to 250 hours, allowing the grease film to break down. The pins had then operated in boundary-lubrication contact with the bushings, progressively wearing both surfaces.
The repair involved replacing all four bucket linkage pins and their matching bushings with new hardened-pin and induction-hardened-bushing sets. The linkage bores were cleaned and inspected for ovality (all within specification). New grease zerks were installed on all pins, and the entire linkage was thoroughly greased with an EP-2 lithium-complex grease until clean grease purged from each joint. The operator was advised to grease the bucket linkage every 50 hours in abrasive conditions. After reassembly, the clunking disappeared, bucket lateral play was reduced to under 1 millimeter at the cutting edge, and the bucket's placement precision was restored.
