A mass excavation contractor reported that a Komatsu PC490LC-11 excavator suddenly lost bucket curling force. When the operator attempted to curl the bucket under a heavy load, the bucket would not move. Furthermore, when pulling the arm (stick) in toward the machine rapidly, a loud clanking noise was heard from the bucket linkage, and the bucket would spontaneously open up.
The technician first checked the bucket cylinder pressure. When the lever was moved to the curl position, the pressure only reached 150 bar (spec is 350 bar). The main relief valve and the bucket control valve spool tested normally. He then disconnected the bucket cylinder rod-end line and found that oil was being pushed back out of the cylinder when the arm was pulled in, indicating the cylinder was being forced to retract.
He disassembled the "Bucket Cylinder Port Logic" (a pilot-to-open check valve integrated into the cylinder head). The check valve is designed to allow oil to fill the rod end of the cylinder when the arm is pulled in rapidly, preventing the cylinder from cavitating and collapsing. He found the check valve cartridge was installed backwards (rod end facing the closed port).
This reversed installation meant the check valve was blocking the make-up oil flow to the rod end. When the operator pulled the arm in rapidly, the geometry of the linkage physically forced the bucket cylinder to retract. Because the make-up oil was blocked, the cylinder rod end developed a massive vacuum (cavitation). The vacuum pulled the piston rod backward so violently that it bent the bucket linkage connecting rods (the clanking noise was the bent rods binding against the bucket pins). The loss of curl force was because the cylinder piston was physically jammed against the rod-end cap due to the vacuum, preventing any pressure from being applied to the head end.
The repair involved removing the bucket cylinder, extracting the reversed check valve, and reinstalling it in the correct orientation. The bent bucket linkage connecting rods were straightened and reinforced. The cylinder internals were inspected for cavitation damage; the piston seal was replaced. After reassembly, the bucket curled to full pressure, and the arm-in operation was smooth with no binding or clanking.
