Copyright Goodheart-Willcox Co., Inc.
438 Hydraulic Systems for Mobile Equipment
The key to understanding the working mode shown in Figure  18-17 is
that the cylinder’s movement must be visualized. To be in the working mode,
an actuator must be performing some work, such as a cylinder moving or a
hydraulic motor rotating. Once an actuator stops, the system is no longer in a
working mode, and the pump enters either the standby mode or the stall mode.
For this example, consider that the cylinder needs 1000 psi (69 bar) to extend.
While the pump fl ow is directed to extend the cylinder, a pressure drop occurs
across the DCV spool. The pressure drop across the DCV spool equals margin
pressure, 300 psi (21 bar).
Margin Pressure
Caterpillar and John Deere construction equipment use the term margin
pressure to describe a differential pressure. Stated another way, while in the
working mode, the difference between the hydraulic pump’s output pres-
sure and the highest system working pressure is margin pressure. Chapter 19
expands on the margin pressure defi nition as it relates to fl ow sharing or post-
spool compensation.
Both the low-pressure standby value and margin pressure value are
changed by adjusting one fl ow control spring value, which is the smaller spool
in Figure 18-16. It has the responsibility of setting the pump’s reaction.
Two methods are used for adjusting the spool. The fi rst method is a
static adjustment made with the DCVs in the neutral position. The second, or
dynamic, method is to operate a hydraulic actuator slowly while measuring the
margin pressure. Both adjustments alter the pump’s reaction. As more spring
tension is placed on the spool, the pump’s reaction time is shortened.
The fl ow control spool’s spring setting is usually one adjustment that can
make a substantial improvement to an LSPC hydraulic system. However,
there are negative effects to adjusting the fl ow compensator spring value too
high. As the spool’s spring pressure increases, system ineffi ciency normally
increases minimally. As an example, consider an LSPC system that is operating
at 300 psi (21 bar) in low-pressure standby and presume the system is fl owing
approximately 1 gpm. The hydraulic horsepower could be determined with the
following equation:
300 psi × 1 gpm × 0.000583 = .17 hp
If the standby pressure was adjusted up to 500 psi, it would increase the
hydraulic horsepower to the following value:
500 psi × 1 gpm × 0.000583 = .29 hp
In addition, if the spool spring is adjusted too high, the machine can
become too sensitive and overly reactive. For example, a loader backhoe oper-
ator complains that the tractor’s hydraulic system is sluggish. The technician
servicing the machine is in a hurry and adjusts the low-pressure standby
value from 200 psi (14 bar) to 650 psi (45 bar) before going home for the day.
The next day, the operator tries to slowly operate the boom swing while gin-
gerly lowering a pipe into a trench. Now the operator fi nds that the machine
is too sensitive and too hard to control. The operator calls the technician back
to fi x the new problem—a hydraulic system that is overly reactive. In addition