3. ARM IN OPERATION ARM CYLINDER BUCKET CYLINDER BOOM CYLINDER SWING MOTOR TRAVEL MOTOR pa5 pc1 pa9 (34,2) P2 pb8 (34,1) A7 pa11 pa8 TURNING JOINT pb3 pb7 pa3 pa7 B7 pc2 pc7 A6 B6 pc6 PT PS P1 P2 PH PA PP PS SH A1 A2 FRONT PUMP REAR PUMP PILOT PUMP HYDRAULIC TANK 900L3HC12 When the LH control lever is pulled back, the arm spools in the main control valve are moved the to roll in position by the pilot oil pressure from the remote control valve. The oil from the A1 and A2 pump flows into the main control valve and then goes to the large chamber of arm cylinder.
At the same time, the oil from small chamber of arm cylinder returns to the hydraulic oil tank through the arm 1 spool in the main control valve. When this happens, the arm rolls in. The excessive pressure in the arm cylinder head side is prevented by relief valve. The cavitation which will happen to the head side of the arm cylinder is also prevented by the make- up valve in the main control valve. 3-14 Arm holding valve 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 DR2 pc2 3 BOOM2 2 SWING 3 2 6 11 TRAVEL-LH