Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 433
Relating Constant-Flow LSPC to Variable-Flow LSPC
Referring back to Figure 18-6, if the high-pressure relief and the unloading
valve were relocated to the compensator assembly inside a variable-displacement
pump and relabeled as the pressure cutoff spool and the fl ow control spool
respectively, the fi xed-displacement LSPC system could be converted to an LSPC
system that uses a variable-displacement pump. This LSPC system, Figure 18-12,
is commonly used in thousands of old and new machines today.
A compensator assembly used on an inline axial variable-displacement
LSPC piston pump is shown in Figure 18-13. The compensator assembly has
two control spools. The smaller of the two spools is the fl ow control spool.
The larger one is the pressure compensator spool. The fl ow control spool must
receive the working pressure from the signal line, which is connected via the
fl at-face O-ring elbow fi tting.
LSPC Variable-Displacement Hydraulic Pump Symbols
LSPC variable-displacement pump symbols can be depicted in different
styles (or formats). Two simplifi ed symbols used to represent the pump in a
basic hydraulic system are shown in Figure 18-14. These two symbols are com-
monly found in older schematics.
Newer schematic symbols used to depict LSPC variable-displacement
pumps sometimes show the pump’s individual pressure compensator spool
and load-sensing (fl ow control) spool in greater detail. See Figure 18-15.
LSPC Variable-Displacement Pump Modes of Operation
An LSPC variable-displacement piston pump will operate in one of three
modes depending on the status of the DCV and the actuator:
• Low-pressure standby.
• Working mode.
• Stall mode.
Low-Pressure Standby
Low-pressure standby is the mode of operation when the DCVs are in the
neutral position. For example, when the tractor is started and no DCVs have
been actuated, the pump fl ow is destroked in the low-pressure standby mode.
Refer to Figure 18-16.
The oil fl ow from the hydraulic piston pump is low, 0.5 to 1 gpm (1.9 to 3.8 lpm),
which is the minimum amount of oil needed to maintain the minimal leakage
inside the pump. The pump operates at a relatively low pressure, 300–500 psi
(21–34 bar). With practically no fl ow of oil and a low system pressure, the pump
uses very little horsepower while in the standby mode.
500 psi × 0.5 gpm ÷ 1714 = .15 hp
The fl ow control spool establishes the low-pressure standby pressure setting.
On older machines, the fl ow control spool is usually the smaller of the two spools.
This spool must sense the actuator’s working pressure from the signal network.