2. BOOM DOWN OPERATION ARM CYLINDER BUCKET CYLINDER BOOM CYLINDER SWING MOTOR TRAVEL MOTOR pa5 pc1 (34,2) pa9 pb8 (34,1) P2 pa11 pa8 TURNING JOINT A7 B7 pc2 pc7 pb3 pb7 pa3 pa7 B6 pc6 A6 PT PS P1 P2 PH PA PP PS SH A1 A2 FRONT PUMP REAR PUMP PILOT PUMP HYDRAULIC TANK 900L3HC11 When the RH control lever is pushed forward, the boom 1 and boom 2 spools in the main control valve are moved to the down position by the pilot oil pressure 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 1 and boom 2 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 cylinder. 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.
3-13 ps1 pa11 DR1 pc1 B5 A5 pb9 pb5 1 1 1 Relief valve pa PO B A B B1 A A1 DR pc A3 B3 pb ARM1 5 4 4 OPTION 7 7 3 pb9 ARM2 B8 A8 BUCKET Boom holding valve DR2 pc2 3 BOOM2 2 SWING 3 2 6 11 TRAVEL-LH