Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 419
Copyright Goodheart-Willcox Co., Inc.
At least one gear pump supplier, Concentric (formerly Haldex), manufac-
tures a LS gear pump that places the unloading valve inside the pump housing.
They advertise that this design has the pump directly controlling the fl ows and
pressures, which will reduce valving losses and circuit ineffi ciencies.
Some individuals might be tempted to call the hydraulic system in
Figure 18-2 an open-center hydraulic system. When the DCV is in the neutral
position, the fi xed-displacement pump delivers a maximum amount of fl ow
at a relatively low pressure of 300 psi (21 bar). Technically, the DCV is closed
center and the DCV sends a signal to an unloading valve. The hydraulic system
is therefore more accurately described as a constant fl ow closed-center LS
hydraulic system. This style of hydraulic system still wastes hydraulic horse-
power when the DCV is in the neutral position. In this case, if the gear pump is
delivering a constant 30 gpm (113.5 lpm) of fl ow at 300 psi (21 bar), the system
will consume a certain amount of engine horsepower:
30 gpm (113.5 lpm) × 300 psi (21 bar) × 0.000583 = 5.2 hp
This style of hydraulic system is still found in mobile machinery and exam-
ples of machines using this design are listed later in this chapter.
Figure 18-2. A fixed-displacement pump can be used in a load-sensing application by adding a shuttle valve and an
unloading valve. The unloading valve keeps the pump operating at a set low system pressure when the DCV is in a
neutral position.
Shuttle valve
High-pressure
relief (3000 psi)
Unloading
valve (300 psi)