Copyright Goodheart-Willcox Co., Inc.
418 Hydraulic Systems for Mobile Equipment
Load-Sensing System
In Figure 18-1, a closed-center DCV valve is used with a fi xed-displacement
pump. In this scenario, anytime the DCV is in a neutral position, maximum
hydraulic horsepower will be generated by constantly fl owing oil over the main
system relief valve at a high system pressure. As a result, the system will generate
unnecessary heat, pressure, and noise. This is one reason this style of hydraulic
system is not used in machinery today.
A load-sensing (LS) hydraulic system is designed to operate at a prescribed
pressure value above the highest working pressure. See Figure 18-2. Two items
have been inserted into the previous closed-center system schematic in order
to convert the hydraulic system into a LS hydraulic system: an unloading valve
and a shuttle valve. The term unloading valve means that the valve dumps the
hydraulic pump’s fl ow to the reservoir at a set low-pressure value anytime the
closed-center DCV is in the neutral position. In this case, the unloading valve’s
spring value is set at 300 psi (21 bar).
When the DCV is actuated, the shuttle valve senses the load’s working
pressure and sends that signal pressure to the unloading valve. If the cylinder
requires 1000 psi (69 bar) to extend, the shuttle valve sends that 1000 psi (69 bar)
signal pressure to work in conjunction with the unloading valve’s spring pres-
sure. The combination of the spring pressure and signal pressure causes the
system to operate at 1300 psi (90 bar). The difference between pump outlet pres-
sure and signal pressure is known as differential pressure or margin pressure.
Margin pressure as it relates to variable-displacement LSPC systems is explained
later in this chapter.
Note
The unloading valve’s spring can be called the differential spring,
margin spring, standby spring, low-pressure standby spring, or the
dump valve spring.
Figure 18-1. The fixed-displacement pump in this schematic operates at a constant high pressure. It wastes too
much energy and generates excess heat when the DCV is moved to a neutral position.
Main system relief valve