2. BOOM DOWN OPERATION ARM CYLINDER BUCKET CYLINDER BOOM CYLINDER TRAVEL MOTOR P5 P4 Pi2 Rs MU Pi1 Pn1 Pn2 SWING MOTOR PBa P02 P04 PAb1 PDb2 A2 PUMP A1 PUMP PILOT PUMP HYDRAULIC TANK 330S3HC10 When the right control lever is pushed forward, the boom 1 spool in the main control valve is 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 spool 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 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 side is prevented by the relief valve.
3-15 BC1 NRV2 Cck DR9 Co P05 Do PCo PDo OPTION Ca Da BC2 NRV1 Cco CoR AkR LCk1 LCk2 DoR OPTION LCo CaR DaR BUCKET BbkR DR7 CRa PaL ARM1 LCa LCa2 ARM2 PAa2 DR6 Csp Db2 BOOM2 Ccb SWING P/R LCar LCbk ARM REGEN P02 Abk PBrc PAbk DR4 Cs Ds PCs PDs DR5 SWING P03 DR3 Ctl Dtl PCtl PDtl SH Patt ( TR RIG AVH ET L) PS DR1 P01 PtrPS TS Cs Ds PCs PDs DR5 SWING Relief valve LCs BbR AbR LCb CRb BOOM1 P03 DR3 Ctl Dtl PCtl PDtl Patt (RIGHT) TRAVEL DR1 BtR AtR (RIGHT) TRAVEL (LEFT) TRAVEL MRV