Copyright Goodheart-Willcox Co., Inc.
452 Hydraulic Systems for Mobile Equipment
Summary
✓ A load-sensing hydraulic system is designed
to operate at a prescribed pressure value
above the highest working pressure.
✓ Pressure compensation enables a hydraulic
actuator to maintain a specifi c speed based
on how far the operator has positioned the
DCV spool.
✓ Pressure compensation is accomplished
by using a primary shuttle valve to sense
actuator working pressure and directing
the actuator working pressure to a pressure
compensator valve.
✓ An LSPC system has a pressure compensator
in the variable-displacement pump.
✓ An LSPC system also has a pressure
compensator within the DCV prior to its
spool, an arrangement known as pre-spool
compensation.
✓ The pressure compensator valve counteracts
changes to system pressures and fl ow to help
maintain the prescribed actuator speed as
dictated by the position of the DCV spool that
is set by the operator.
✓ When the DCVs in an LSPC fi xed-
displacement hydraulic system are in a
neutral position, the system operates in
the standby mode, and the pump delivers
maximum fl ow at a low pressure value, for
example 300 to 500 psi.
✓ When the DCVs, in an LSPC variable-
displacement hydraulic system are in a
neutral position, the system operates in the
standby mode. The system generates very
little fl ow and low pressure, wasting very
little hydraulic horsepower.
✓ When an LSPC hydraulic system is in the
working mode, an actuator will be moving
and the system pressure will equal the
highest signal pressure plus margin pressure.
✓ When a LSPC fi xed-displacement system
is in the stall mode, the unloading valve
blocks the pump’s fl ow, forcing the oil fl ow
to dump over the main system relief valve.
In this mode, the pump generates maximum
pressure and fl ow, equaling maximum
hydraulic horsepower.
✓ When a LSPC variable-displacement pump
is in the stall mode, the fl ow control spool
will be closed and the PC spool will open,
directing oil to the control piston to destroke
the pump, resulting in high system pressure
with little fl ow.
✓ PQ graphs consist of mapping a hydraulic
pump’s fl ow rate (Q) at different operating
pressures (P).
✓ Torque limiting is a pump control that
destrokes the pump to prevent the engine
from stalling any time the operator
simultaneously requests high fl ow and high
pressure.
✓ Jammer solenoid valves in variable-
displacement LSPC systems will connect
pump outlet pressure to signal pressure
when activated. This puts the system in
stall mode, so that high pressure is available
immediately when the operator activates a
control.