Operation - jcb JS300 Service Manual

Middle Speed
Note: Second stage MRV is not selected during travel on
this machine.
Oil from the valve block enters the motor at either P1 or P2.
In this case the oil enters at P1.
Pressure entering the motor crosses the counterbalance
spool A and is directed to the travel motor B and the
auxiliary relief valve (ARV) ports C. At this point the motor
cannot move as the brakes D are applied.
The pressure also travels to the end of the counterbalance
via:
1 The restrictor which restricts the flow to the spool
2 The check valve which will allow full flow to the spool
end. This is the normal path for the oil to take.
The counterbalance spool is selected at 6 - 13bar (87- 188
lb in 2 ) which in turn diverts the flow of oil to the travel motor
in 2 directions:
1 Oil can flow over the spool and direct to the motor.
This is also available to the motor ARVs and the 2
speed travel spool E that is in the neutral position.
Once the brake is lifted the motor will start to turn.
2 Oil is also diverted to the travel motor brake via a
restrictor.
Once the pressure in the brake rises to 14bar (203lb in
the brake is released. This flow of oil also crosses the
brake piston and is available at the end of the two speed
travel spool and along with the assistance of a spring,
holds the two speed spool in the low speed position. With
the two speed spool in this position the swash plate angle
piston head is vented to the tank port and the motor stays
on maximum displacement. (Low speed).
Once the pressure rises to a 319bar (4627lb in
motor's ARVs will open and allow excess pressure to be
exhausted to the return side. The ARVs are of the
shockless type to reduce harsh starting and stopping.
When travel is de-selected by the driver the P1 port loses
pressure. Pressure is also lost to the end of the
counterbalance spool which is cushioned to reduce the
shock when stopping. The cushioning is achieved by the
F-5

Operation

K Fig 2. ( T
F-6).
2
)
2 ) the
9813/5100-1
Section F - Transmission
Traction Motor / Reduction Gear
check valve trapping the pressure at the spool end and
only allows it to vent through the restrictor, allowing the
counterbalance spool to return slowly. Returning oil from
the motor is slowly blocked in its path to tank and as the
back pressure rises, braking occurs.
Once the counterbalance spool is in the neutral position
any additional pressure within the motor due to inertia
(motor pumping function) above 319bar (4627lb in
relieved through the ARV and is fed back to the opposite
side to prevent cavitation. The pressure in the brake piston
is also released slowly due the restrictor in the brake line
restricting the flow, so the brake remains off initially to allow
the motor to stop by hydraulic breaking, to protect the
brakes components from damage.
Slow Speed Travel (Creep)
When slow travel speed is selected the electronic control
unit (MECU) controller sends a signal to the max flow cut
solenoid CT4 on the 8-spool solenoid block. This then
allows servo pressure to the ports Pm1 and Pm2 of the
main hydraulic pump and reduces the pump flow to the
drive motors.
The drive motors work in exactly the same way as in
middle speed travel. Only the pump flow is reduced to
achieve slow-speed travel.
Operation
2
) is
F-5