Copyright Goodheart-Willcox Co., Inc.
424 Hydraulic Systems for Mobile Equipment
For example, refer to Figure  18-7 and consider the following actuator
requirements:
• DCV 3 requires 1500 psi (103 bar) to retract its cylinder.
• DCV 2 requires 1000 psi (69 bar) to extend its cylinder.
• DCV 1 requires 500 psi (34 bar) to extend its cylinder.
The following sequence of events would occur if the operator requested the
previous actions from the cylinders:
1. The primary shuttle inside DCV 3 would direct 1500 psi (103 bar) to
DCV 2 secondary shuttle.
2. The primary shuttle in DCV 2 would direct 1000 psi (69 bar) of signal
pressure to the DCV 2 secondary shuttle.
3. The secondary shuttle in DCV 2 would choose the higher working
pressure of 1500 psi (103 bar) and send it to the secondary shuttle located
in DCV 1.
4. The primary shuttle in DCV 1 would direct 500 psi (34 bar) to the
secondary shuttle inside DCV 1.
5. The secondary shuttle in DCV 1 would choose 1500 psi (103 bar) and send
the 1500 psi (103 bar) to the unloading valve.
6. The unloading valve would maintain a system pressure of 1500 psi
(103 bar), plus the value of margin pressure, 300 psi (21 bar).
Note that DCV 3 in Figure 18-7 does not contain a secondary shuttle valve.
The last DCV does not have to distinguish the difference in signal pressure
of a downstream valve that does not exist. However, if a customer wanted to
add another DCV (such as DCV 4, power beyond, or a three-point hitch), a
secondary shuttle valve must be installed inside DCV 3.
The signal network must have an orifi ce that allows the network to have a
controlled drain back to the reservoir. If the signal network has no controlled
drain back to tank it can cause one of two problems, depending on whether the
system uses a variable-displacement pump or a fi xed-displacement pump. In
a fi xed-displacement LS hydraulic system, a plugged LS network orifi ce will
cause the signal network to hydrostatically lock, resulting in the unloading
valve blocking system oil fl ow. When the unloading valve blocks off the fi xed-
displacement pump, the constant fl ow of oil must dump across the main
system relief at high pressure. In a variable-displacement LS hydraulic system,
a plugged orifi ce in the LS network will cause the variable-displacement pump
to achieve high-pressure standby. This mode will be discussed later in the
chapter.
Note
In John Deere agricultural machines, the shuttle valves are called
dime valves because they resemble the shape of a dime, as
shown in Figure 18-8. The shuttle valves are called isolators in
John Deere construction machines. Caterpillar calls their shuttle
valves resolvers because the shuttle valves resolve which pressure
is the higher pressure.