jcb JS300 Service Manual page 700

Section E - Hydraulics
Hydraulic System
Hydraulic Operation and Schematics
Dipper In
K Fig 9. ( T E-40)
For schematic,
Servo pressure from the hand controller 50 is sent to port
A6 of the servo shuttle valve 46 and is distributed to:
Port B6 to port A on the cushion valve 49, through the
valve leaving at port C (if machine is fitted with HBCV,
pressure is also sent via port A1 to port P on the HBCV to
allow trapped oil in the ram to exit). From port C to port Pb5
on the main control valve 14 to the dipper (1) spool, dipper
(2) spool, dipper holding valve the upper pressure switch.
Flow from pump A1 enters main control valve 14 at P1 and
is available at the dipper (1) spool via the left hand neutral
gallery, or after passing over the straight line travel spool
via the parallel working passage. At the dipper (1) spool
the flow is diverted to the dipper ram 3. Pressure is sensed
at the regeneration spool RGS and moves it to the lesser
restricted position.
Flow from pump A2 is blocked at the dipper (2) spool and
joins the flow of pump A1 before the dipper (1) spool. The
pressure from both pumps is now cut off from the negative
control ports Ps1 and Ps2. This reduced pressure is
sensed at both pumps which now come into full flow.
Exhaust oil from the ram passes (through the HBCV if
fitted) to the main control valve 14 at port A5 through the
dipper hold check valve DHV (that has been released by
servo pressure) to the dipper (1) spool.
Exhaust oil from the dipper ram 3 can be at a higher
pressure than the feed oil due to the effect of gravity on the
dipper arm when first selected. This causes the
regeneration check valve RG to open, feeding a proportion
of exhaust oil into the feed side. The remaining exhaust oil
returns to tank passing over the regeneration spool RGS.
The regeneration spool has two restrictors, one more
restricted than the other. The restrictors cause back
pressure encouraging the regeneration check valve to
open. As the pressure drops the regeneration spool moves
across to the less restricted position. When the feed
pressure is greater than the exhaust pressure the
regeneration check valve will move to the unrestricted
position allowing return oil to free flow to tank.
E-39 9813/5100-1 E-39