A mining contractor reported that a Doosan DX800LC-7 large excavator experienced a violent shuddering in the boom when lifting a full bucket of overburden. Simultaneously, the main hydraulic pump emitted a loud, rhythmic knocking sound that synchronized with the boom shudder. The oil temperature was running 15 degrees above normal.
The technician first isolated the noise. By dead-heading each hydraulic function, he confirmed the knocking originated from the main pump when the boom was raised. He installed a pressure gauge on the boom raise line. When the boom was raised under load, the pressure fluctuated wildly between 150 bar and 320 bar (relief pressure) in rapid cycles. This pulsation was the cause of the mechanical shudder.
He suspected the "Main Pump Servo Piston" (the actuator that tilts the pump swashplate to change displacement). He removed the servo piston housing. The piston was seized solid in the bore due to varnish buildup and lack of lubrication. Because the servo piston couldn't stroke smoothly, the pump regulator could not adjust the flow to match the load. The pump slammed to maximum displacement, hit relief pressure, slammed back to minimum, and repeated this cycle-the source of the "pump knocking."
To find out why the boom specifically triggered the violent feedback, he disconnected the boom cylinder lines. He found the "Boom Cylinder Piston Rod" had a deep circumferential score mark near the fully retracted position. The score had shredded the main piston seal and the rod wiper seal. When raising a heavy load, the high-pressure oil bypassed the piston internally, dropping the cylinder pressure. The pump regulator sensed this pressure drop and commanded maximum flow, but the seized servo piston caused the pump to over-shoot, creating the hydraulic oscillation.
The repair required replacing the main pump servo piston and cylinder block kit, as the seizure had scored the bore. The boom cylinder was removed, the rod was chrome-plated and polished to remove the score, and a complete seal kit was installed. The hydraulic system was flushed and refilled. Post-repair, the boom lifted smoothly under full load, and the pump operated quietly.
