Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 431
Note
Some personnel will label constant-flow LSPC systems (for example,
Figure 18-6) as an open-center hydraulic system. They would point
out that the system uses a fixed-displacement pump that delivers a
constant flow at a relatively low pressure anytime the DCVs are in
a neutral position. However, this system uses closed-center DCVs;
has pressure compensator valves installed in series prior to the DCV
spools; and uses an unloading valve that enables the hydraulic system
to operate at a preset value above the actuator working pressure.
Technically, the DCVs in this system are identical to the DCVs used in
a variable flow LSPC system.
One drawback to a fi xed-displacement LSPC system is that the pump
produces a constant fl ow when the DCVs are in a neutral position. Even
though the fl ow is at a relatively low pressure, this still results in wasted
power. Variable-displacement LSPC hydraulic systems can reduce the
hydraulic pump’s horsepower consumption to only a fraction, sometimes as
little as 0.15 hp.
Examples of LSPC with Fixed-Displacement Hydraulic Pumps
LSPC systems that use a fixed-displacement pump are not as popular as LSPC
variable-displacement pump systems. Some manufacturers use fixed-displacement
LSPC systems in their economy model tractors and variable-displacement LSPC
systems in their premium model tractors. A fixed-displacement LSPC system
was used on older machines such as the Case IH 9390 Steiger 4WD tractor
equipped with the high-flow hydraulic system, Caterpillar D6H and D7H track-
type tractors, and Caterpillar 926A, 936A, 936E wheel loaders. Some examples
of newer machines using this system design are Caterpillar D3K–D5K Series I
and II track-type tractors and John Deere 6030, 7030, and 6M economy model
tractors.
Case IH 9390 High Flow Steiger
The Case IH 9390 High Flow tractor contained two separate LSPC systems,
one with a fixed-displacement pump and one with a variable-displacement pump.
The tractor was designed so that DCVs 3 and 4 used the fixed-displacement
30 gpm steering gear pump, while DCVs 1 and 2 used the variable-displacement
30 gpm piston pump. The fixed-displacement gear pump contained an unloading
valve that received a load-sensing signal pressure from DCV 3 and DCV 4.
DCV 1 and DCV 2 sent a LS signal to the variable-displacement LSPC pump.
Variable-displacement LSPC systems are discussed later in this chapter.
Caterpillar D3K–D5K Series I and II Dozers
The Caterpillar D3K–D5K dozers used a fixed-displacement pump and LSPC
DCVs to control the implement hydraulic system. The unloading valve dumped
the gear pump’s constant flow at a low pressure value when the DCVs were in a
neutral position. The DCVs sent a signal pressure to enable the unloading valve
to build the pressure required for the DCVs to operate.