Copyright Goodheart-Willcox Co., Inc.
Chapter 18 Load-Sensing Pressure-Compensating (LSPC) Hydraulic Systems 429
control or steering the tractor along a line of trees, the hydraulic system flow fl
and pressures will vary. As the fl ow and pressures vary, the planter’s hydraulic fl
motor speed will be negatively affected. As a result, the speed of the planter’s
hydraulic motor will fl uctuate, causing seed population problems. Inserting a fl
compensator valve in-line prior to the DCV spool can alleviate this concern.
Most planter motors are unidirectional. However, the schematic in
Figure 18-10 shows a bidirectional hydraulic motor to illustrate how the pri-
mary shuttle is used to direct signal pressure of a double-acting actuator to the
compensator valve.
Pre-spool compensation has the following attributes:
• The compensator valve must sense the actuator’s working pressure.
Therefore, a primary shuttle valve is used to send the actuator’s signal
pressure to the compensator valve.
• The compensator’s spring value establishes the pressure drop across
the DCV spool.
• The DCV spool must be closed center.
The pressures in Figure 18-10 will equal the following values:
• Working pressure/signal pressure.
• 1000 psi (determined by the load on the actuator)
• Pump outlet pressure.
• 300 psi (unloading spring) + 1000 psi (working pressure) = 1300 psi
• Pressure drop across the compensator valve.
• 1300 psi – (1000 psi + 40 psi spring) = 260 psi
Figure 18-10. Pre-spool pressure compensation places the compensator valve prior to the DCV spool. The
compensator valve senses the actuator’s working pressure through the use of a shuttle valve to maintain a fixed
implement speed.
Primary
shuttle valve
Unloading
valve (300 psi)
High-pressure
relief (3000 psi)
Pressure
compensator
valve (40 psi)
Bidirectional
hydraulic
motor
(1000 psi)