Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 441
Chapter 21 will discuss hydraulic test equipment, including a differential
pressure gauge. This type of gauge is essential for measuring margin pressure.
Margin pressure requires reading two different pressures simultaneously
while oil fl ows through a circuit and subtracting one pressure reading from
the other pressure reading.
An example is measuring margin pressure in a bulldozer hydraulic
system while actuating the dozer blade to lift. The problem is that the load
on the blade will cause the pressure value to constantly change as the blade
is moving. Operating the DCV at a relatively slow speed rather than a faster
speed helps minimize this issue. As the pump outlet pressure varies so will
the signal pressure change during the test. It is diffi cult to simultaneously
watch two pressure gauges with fl uctuating pressures and attempt to sub-
tract the difference.
A differential pressure gauge solves this problem. The gauge displays the
difference in pressure between two separate input pressure values, which,
in this case, are the pump outlet pressure and signal pressure. One pressure
line is connected to the left side of the gauge and the other is connected to
the right.
Figure 18-18. A PQ curve is a graphic representation of the relationship between a hydraulic system’s flow rate and
operating pressure. This graph has two different curves, one with a low value for low-pressure standby (LPSB) and
one with a higher value. The higher value LPSB allows a greater pump flow rate over a longer operating pressure
range compared to the lower value LPSB.
900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900
Operating Pressure (psi)
200 psi LPSB
550 psi LPSB
Pump
Flow
Rate
(gpm)
25
30
20
15
10
5
0
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Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 441
Chapter 21 will discuss hydraulic test equipment, including a differential
pressure gauge. This type of gauge is essential for measuring margin pressure.
Margin pressure requires reading two different pressures simultaneously
while oil fl ows through a circuit and subtracting one pressure reading from
the other pressure reading.
An example is measuring margin pressure in a bulldozer hydraulic
system while actuating the dozer blade to lift. The problem is that the load
on the blade will cause the pressure value to constantly change as the blade
is moving. Operating the DCV at a relatively slow speed rather than a faster
speed helps minimize this issue. As the pump outlet pressure varies so will
the signal pressure change during the test. It is diffi cult to simultaneously
watch two pressure gauges with fl uctuating pressures and attempt to sub-
tract the difference.
A differential pressure gauge solves this problem. The gauge displays the
difference in pressure between two separate input pressure values, which,
in this case, are the pump outlet pressure and signal pressure. One pressure
line is connected to the left side of the gauge and the other is connected to
the right.
Figure 18-18. A PQ curve is a graphic representation of the relationship between a hydraulic system’s flow rate and
operating pressure. This graph has two different curves, one with a low value for low-pressure standby (LPSB) and
one with a higher value. The higher value LPSB allows a greater pump flow rate over a longer operating pressure
range compared to the lower value LPSB.
900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900
Operating Pressure (psi)
200 psi LPSB
550 psi LPSB
Pump
Flow
Rate
(gpm)
25
30
20
15
10
5
0

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