Copyright Goodheart-Willcox Co., Inc.
420 Hydraulic Systems for Mobile Equipment
Fixed-Displacement Pump Standby Mode
Figure 18-3 illustrates a cross-sectional drawing of an unloading valve that
is used in a fi xed-displacement LS hydraulic system. The spool valve has a cross-
drilled passageway, which enables pump pressure to act on the left-hand side
of the spool. Pump pressure opposes the margin spring. When the DCV is in
the neutral position, signal pressure will equal zero. Chapter 25 will explain
that some steering circuits will have a residual steering signal pressure that will
cause the standby pressure to be a little higher than the margin spring value.
Pump pressure will build to achieve the margin spring value—300 psi
(21 bar) in this example—causing the spool to shift to the right, compressing
the margin spring. As the spool shifts to the right, it opens the passageway
allowing the pump’s constant fl ow of oil to be dumped to the reservoir at the
margin spring value of 300 psi (21 bar). While the DCV is in neutral, the system
is in the standby mode, also called low-pressure standby. LS systems normally
operate at a pressure between 300 to 500 psi (21 to 34 bar) while in the standby
mode. Standby mode as it relates to variable-displacement LSPC systems will
be explained later in this chapter.
Fixed-Displacement Pump Working Mode
When a DCV is actuated, its shuttle valve directs the hydraulic cylinder’s
working pressure, also known as signal pressure, to the unloading valve so that the
signal pressure can work in conjunction with the margin spring. See Figure 18-4.
Anytime an actuator is moving, the hydraulic system is in the working mode.
While in the working mode, hydraulic pump outlet pressure will equal the
Figure 18-3. When the DCV is in a neutral position, the unloading valve receives no signal pressure. The pump will
build enough pressure, such as 300 psi (21 bar), to overcome the unloading valve’s margin spring pressure. This causes
the unloading spool valve to shift to the right, allowing the constant flow of oil to be dumped to the reservoir at a relative
low pressure value.
Oil exiting
to tank
No signal
pressure applied
Margin
spring
Oil from fixed-
displacement
pump into
unloading valve
Cross-drilled
passageway
Capped
passageway