jcb JS300 Service Manual page 686

Hydraulic Pump Regulation
Note: The pressures quoted are for illustration purposes
only - the pressure figures will be dependent on machine
power band selection.
Hydraulic Pump Regulation
For this description pump A2 has been used. For
K Fig 2. ( T
Schematic, E-26).
On start up, the swash plate piston B is held in the
maximum flow position by the spring A and also by servo
pressure which enters via check valve H and is available
to the small area side of swash plate piston B to hold the
pump in the maximum flow position.
Once oil as passed through the valve block a 40 bar (580
Ib/in 2 ) negative control signal enters port Pi2 and is
available to the negative control piston D. This acts against
spring A to move the spool across allowing pump pressure
to cross the spool and pressurise the large diameter side
of the swash plate piston B. Pressure is now available to
both sides of the swash piston B, but due to the difference
in surface areas the piston moves to the minimum flow
position.
When a service is selected the negative control pressure
drops. Piston D now moves back due to the spring
pressure. This allows the oil in the larger area side of the
swash plate piston B to vent to tank, but pump pressure oil
is still available to the smaller diameter side so the pump
moves over to the maximum flow position.
As the pump pressure increases, the pressure seen at the
small diameter end of the swash plate piston B is also seen
at the piston E. At the same time any pressure generated
at pump A1 is also seen at pump A2 at the piston. As the
pressure increases at piston E via line F it starts to push
against spring A. When the pressure has increased to
approximately 160 bar (2320 Ib/in
select. This will now allow pump pressure to the large
diameter side of the swash plate piston B which will
proportionally start to select minimum flow.
When a Low Power mode is selected a 40 bar (580 Ib/in
secondary pressure is also seen at piston G. This signal
will add to the force of the pump pressure at pistons E and
G so that minimum flow is selected earlier.
In a High Power mode the secondary pressure drops to
approximately 4.3 bar (62.3 Ib/in
secondary pressure at piston G has to be made up for by
E-25
2 ) the spool will start to
2 )
2 ). This reduction in
9813/5100-1
Section E - Hydraulics
Hydraulic Operation and Schematics
higher pump pressure at pistons E and F before the
sumater spool starts to select, hence more hydraulic
horsepower. The regulator on pump A1 works the same as
above.
Hydraulic System
E-25