jcb JS300 Service Manual page 696

Section E - Hydraulics
Hydraulic System
Hydraulic Operation and Schematics
Boom Down
K Fig 7. ( T
For schematic, E-36).
Servo pressure from the RH hand controller 51 at port 4
enters the servo shuttle valve 46 at port and is distributed
to port B4 to port E of the cushion control valve 49. The oil
flows across the unrestricted part of the spool and out of
port G to the main control valve 14 at port Pb8, moving the
spool to the down position, also selecting the boom holding
valve and the upper pressure switch.
Note: If HBCV's 12 are fitted a signal is also sent from port
E1 of the cushion control valve 49 to the P ports of the
HBCV's to move the HBCV spool over and allow oil to
return from the rams.
Flow from pump 1 enters the main control valve at port P2
and travels through the neutral gallery to the boom (1)
spool. The flow is restricted allowing a reduced pressure at
port Ps2 which is sensed at Pm2 of the pump to put the
pump to full flow. Oil flow also passes through the parallel
working gallery to the Boom (1) spool and is directed to the
boom rams 16 via port B8.
Exhaust oil passes through the HBVC's 12 (if fitted) to the
main control valve 14 at port A8, lifts the boom holding
valve BHV off it's seat and passes through the boom (1)
spool and exhausts to tank.
Exhaust oil passing through the spool can be regenerated
into the feed side of the rams. A check valve in the spool
allows oil to enter the pressure side of the spool, If the
returning oil is at a higher pressure, due to the weight of the
K Fig 7. ( T
boom or load, Item A E-36). The restrictor
creates back pressure, causing the check valve to open.
E-35 9813/5100-1 E-35