Copyright Goodheart-Willcox Co., Inc.
422 Hydraulic Systems for Mobile Equipment
LS systems can also use variable-displacement pumps. Systems using
variable-displacement LS pumps are even more effi cient than fi xed-
displacement LS systems. In variable-displacement LS systems, the unloading
valve becomes the fl ow control valve or the margin spool. These systems are
described later in the chapter.
Signal Network
LS hydraulic systems require a group of shuttle valves to sense actuator
working pressure. As mentioned in Chapter 9, a shuttle valve is a T-shaped
valve that determines the higher pressure of two inlet pressures and sends the
higher pressure to a new destination within the system. See Figure 18-5. That
destination can be another shuttle valve, a pump compensator assembly, or an
unloading valve.
Primary Shuttle Valve
Shuttle valves can be designated as primary or secondary shuttle valves.
A primary shuttle valve chooses an actuator’s higher working pressure. This
means the primary shuttle determines if the double-acting cylinder is extending
or retracting based on the higher working pressure and sends the appropriate
signal pressure to a secondary shuttle valve, as shown in Figure 18-6.
Secondary Shuttle Valve
A secondary shuttle valve is used to choose the higher working pressure
between two different DCVs’ working pressures. A series of secondary shuttle
valves is used to send the highest working pressure to the margin spool. The
group of primary shuttle valves and secondary shuttle valves makes up the
hydraulic system’s signal network.
Figure 18-5. Most LS DCVs have the shuttle valves incorporated inside the DCV’s valve block. Individual shuttle
valves are also available for building a LS system when using simple closed-center DCVs.