A mass excavation contractor reported that a Hitachi ZX350LC-7 excavator's right track developed a loud, rhythmic "rattle" during travel and swing-under-load operations. The technician noted that the track's upper span - between the sprocket and the front idler - was visibly sagging and bouncing rather than running taut over the top of the roller frame. The track's guide lugs showed accelerated wear, and the drive sprocket's teeth were wearing unevenly on their right-side flanks.
The technician first inspected the undercarriage. The ZX350's carrier roller - also called a "top roller" or "upper roller" - supports the track's upper span and keeps it taut between the sprocket and the idler. When a carrier roller fails, the track's upper span loses its support and begins to flap, especially at higher travel speeds and during the shock loads of swing-under-load operations. The flapping track produces the characteristic rattle and accelerates wear on the track's guide lugs, which repeatedly slap against the roller frame's support surfaces.
He elevated the right side of the machine and inspected the carrier roller. The roller's outer surface showed flat-spot wear, indicating it had stopped rotating freely at some point. He attempted to rotate it by hand - it turned with heavy resistance and produced a gritty grinding sound, indicating bearing damage. He then removed the carrier roller's mounting bolts and disassembled the roller. The "Carrier Roller Bearing" - a sealed double-row tapered roller bearing - had completely lost its grease, and the bearing race surfaces were brinelled and pitted. The roller's internal seal had failed, allowing the grease to escape and allowing clay and water to enter the bearing cavity.
The root cause was traced to a failed roller seal. Carrier rollers in excavator undercarriages are sealed-for-life units, but seal failure from abrasive material ingress or heat can cause grease loss and bearing damage. The failed seal had allowed abrasive clay from the job site to enter the bearing, destroying the rollers and races. With the bearing seized, the carrier roller stopped rotating and became a stationary wear surface that the track's guide lugs ground against - accelerating lug wear. The track's upper span then sagged and flapped, producing the rattle and causing the uneven sprocket tooth wear from the track's irregular engagement.
The repair involved replacing the failed carrier roller with a new unit, inspecting all remaining carrier rollers (the left-side rollers were healthy), and inspecting the track's guide lugs for wear. The track's guide lugs showed moderate wear but were serviceable. The drive sprocket's teeth were inspected - the right-side flank wear was within acceptable limits, and the sprocket was retained. The track tension was adjusted to the correct specification. After reassembly, the track's upper span ran taut over the new carrier roller, the rattle disappeared, and the machine operated through a full shift without abnormal track wear or noise.
