2. BOOM DOWN OPERATION R/G R/G Ps Ps Pm1 Pm2 Pm2 Pm1 P2 P1 T1,T2 T1,T2 P2 P1 Pn2 Pn1 D B A C DR P Rs CK1 NRV1 Pk BC COMP.
NRV2 B5 S/Motor DB Pc5 Pd5 D5 C5 A5 Pb5 SRG ARG C4 OPTION-B BUCKET CK2 Pa5 (RG) SGo Dr SH PG DR3 Pc4 D4 1 REGENERATIION CUT Pa4 Pb4 B4 Au Dr(R7) Pd4 0 Pc4 0 ARM1 OPTION-A BREAKER Pc4 2 GB GA DR2 Pc3 Pa2 1 ARM2 Pd4 Pb3 1 B(CW) Mu A(CCW) C2 D2 V3 BOOM2 Pb2 1 B2 A2 Pd2 Pc2 SWING BOOM1 P02 DR1 Pa2 Pb2 0 0 Pc1 D1 Pd1 C1 Patt A1 Pb1 B1 Pa1 P01 DR0 TRAVEL (LEFT) TRAVEL (RIGHT) Ptr STRAIGHT TRAVEL T1 P2 P1 Pu MRV SWING ARM BUCKET BOOM TRAVEL (2) (4) (3) (1) (3) (1) (2) (4) Psv A B D E G F H J SH Pd1 Pc1 Pb1 Pa1 (1) (2) (3) (4) a2 a1 A1 A2 a4 Pd2 Pc2 Pc4 0 Pa2 0 Pb2 0 T P Pd4 0 Pa5 Pb5 Dr#,dr#,R# Pm1 (Pump) Pm2 (Pump) A1 Pk (T/Joint) A1 A2 ACC Pi1 (Fl,MCV) Pi2 (Fr,MCV) Pm1 Pm2 A2 P2 P3 P4 PG(S/Motor) Po1(MCV) M Dr a3 T(dr2) T T P1 B1 B3 A3 D21073HS10 When the right control lever is pushed forward, the boom 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 rear 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 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 rear pump, and flows into the small chamber of the cylinder. This prevents cylinder cavitation by the negative pressure when the rear 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