2. BOOM DOWN OPERATION 2. BOOM DOWN OPERATION ARM CYLINDER BUCKET CYLINDER BOOM CYLINDER SWING MOTOR ps1 ps2 T3 T1 T2 T4 TRAVEL MOTOR A1 A2 FRONT PUMP REAR PUMP PILOT PUMP HYDRAULIC TANK 480A3HC11 When the RH control lever is pushed forward, the boom spools in the main control valve are moved to the down position by the pilot oil pressure (pa3, pa7) from the remote control valve. The oil from the A2 pump flows into the main control valve and then goes to the small chamber of boom cylinders. At the same time, the oil from the large chamber of boom cylinders returns to the hydraulic tank through the boom spools in the main control valve.
When the down speed of boom is faster, the oil returned from the large chamber of boom cylinder combines with the oil from the A2 pump, and flows into the small chamber of the boom cylinders. This prevents cylinder cavitation by the negative pressure when the A2 pump flow can not match the boom down speed. And the excessive pressure in the boom cylinder rod end circuit is prevented by the relief valve. 3-15 3-15 A5 pb5 ARM 1 pa5 B4 A4 pb4 pa4 A3 B3 pb3 B2 pa3 A2 OPTION Anti-drift val Relief valve BOOM 2 pb2 SWING DR3 pa2 B1 pc6 TRAVEL(LH A1 pb1 pa1 TRAVEL(RH) TRAVEL STRAIGHT pr1 P1 PSPT PP PA PH PS SWING TRAVEL( RH) TRAVEL(LH TRAVEL STRAIGHT A6 pb6 pa6 ) P3 P2