HUB AS 1717093 - Caterpillar

Pilot Hydraulic Actuators
The pilot hydraulic actuators control pilot oil flow from the ends of the main control valve spools to the tank.Pilot Hydraulic Actuator – Solenoids Not Energized
Illustration 1 g03667202This image shows a cross-sectional view of a pilot hydraulic actuator. In the HOLD position, both the left actuator solenoid (A) and the right actuator solenoid (B) are de-energized. When both solenoids are de-energized, each pilot spool (C) directs pilot oil to the ends of the main spool (D). The pilot pressure at each end of the main spool and the force of each centering spring (E) keep the main spool centered in the control valve.Pilot Hydraulic Actuator – Left Solenoid Energized
Illustration 2 g03667203When the left actuator solenoid (A) is energized, the solenoid moves the left pilot spool (C) to the right. The pilot spool directs the pilot oil on the left side of the main spool (D) to the tank, which decreases the pilot oil pressure at the left end of the main spool.When the right actuator solenoid (B) is de-energized, the feedback spring (K) holds the right pilot spool (J) to the right. The right pilot spool directs pilot oil to the right end of the main spool. The pilot oil pressure at the right end of the main spool moves the main spool to the left. As the main spool moves to the left toward the left pilot spool, the centering springs (E) and the feedback spring are compressed.Pilot Hydraulic Actuator – Left Actuator Balanced
Illustration 3 g03667204When the left actuator solenoid (A) is energized, the main spool (D) moves to the left, which compresses the centering springs (E) and the feedback spring (H). The force of the feedback spring is equal to the force of the actuator solenoid and the stabilizing spring (G). When the pilot spool moves far enough to the left, the pilot oil port (L) and the tank oil port (M) are blocked. When the oil in the actuator is blocked, the main spool stops moving. The main spool will stay in this balanced position until the current to the actuator solenoid changes.If the operator moves the implement control lever farther in the same direction, the Implement ECM will increase the current to the actuator solenoid. The increase of current increases the force on the pilot spool (C). The pilot spool will move to the right, which opens the tank oil port. The decreased oil pressure inside the left actuator will allow the main spool to move farther to the left due to the pilot oil pressure on the right end of the main spool. As the main spool moves to the left, the feedback spring will be compressed more, and the feedback sequence begins again.If the operator moves the implement control lever toward the centered position, the Implement ECM will decrease the current to the actuator solenoid. The decrease of current decreases the force on the pilot spool. The pilot spool will move to the left, which opens the pilot oil port and blocks the