Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 421
signal pressure plus the margin pressure. For example, if the cylinder requires
1000 psi (69 bar) to extend, and the margin pressure equals 250 psi (17 bar),
the pump outlet pressure will equal 1250 psi (86 bar).
Differential Spring Value
A margin spring has a static spring pressure value, known as standby pres-
sure, and a dynamic spring pressure value, known as margin pressure. The
standby pressure is usually a higher value than the margin pressure. Some
service literature will only specify adjusting the standby pressure, while other
literature will specify only adjusting the margin pressure. Some service man-
uals will provide specifi cations for adjusting both the standby and the margin
pressure. This topic will be discussed in more detail later in this chapter.
Referring to Figure 18-4, the function of the LS system’s working mode is
the heart of LS hydraulics. The system builds only the necessary pressure to do
the work, plus the set pressure value of the margin spring.
Stall Pressure
When the cylinder reaches its end of travel, the LS system will be in its fi nal mode,
called the stall mode. It is also called the high-pressure cutoff mode or high-pressure
standby mode. In this mode, the hydraulic pump cannot build enough pressure to
overcome both the signal pressure and the margin spring. As a result, the margin
spool remains closed, blocking off pump fl ow, which causes the system pressure to
climb to the highest system pressure value. In a fi xed-displacement system, a high-
pressure relief valve is used to set the pump’s stall pressure.
Figure 18-4. In the working mode, the DCV’s shuttle valve sends the working signal pressure to the unloading
valve. The system pressure builds only to the necessary pressure value, which equals signal pressure plus
margin pressure. The remaining pump flow that is not used by the DCV is dumped over the unloading valve
back to the reservoir.
Oil exiting
to tank
Signal
pressure
applied
Oil from fixed-
displacement
pump into
unloading valve
Cross-drilled
passageway
Capped
passageway
Margin
spring