Copyright Goodheart-Willcox Co., Inc.
436 Hydraulic Systems for Mobile Equipment
Figure 18-15. Newer schematic symbols sometimes detail the spool valves (pump compensator spool and load-
sensing spool) housed within the pump assembly.
DCV
LS
PC
When the DCVs are in the neutral position, pump fl ow deadheads at the
DCV, and pump pressure begins to build. The pressure increases to the value
of the fl ow control spool’s spring value. This spring is adjustable, and is com-
monly adjusted to a value between 300–500 psi (21–34 bar) for the low-pressure
standby mode.
If the low-pressure standby is set at 300 psi (21 bar) and the DCVs are in
the neutral position, pump pressure increases until it reaches 300 psi (21 bar).
At this point, pump pressure overcomes the fl ow control spring value, causing
the fl ow control spool to shift. Once the spool shifts, oil pressure is sent to the
control piston, which destrokes the pump.
As the control piston pushes the pump’s swash plate back to a neutral
position, the control piston meters a small amount of oil into the pump’s case
through an orifi ce that becomes exposed as the control piston is extended.
While in the standby mode, the swash plate will never reach a complete neu-
tral angle, but will maintain a minimal amount of fl ow to overcome leakage
and maintain the standby pressure. The orifi ce effectively stops the swash plate
from returning to a completely neutral angle. As the oil dumps through the
orifi ce, the oil pressure can push no further on the swash plate. The oil is com-
monly routed through a case drain back to the reservoir.