558 Forest Machine Machine Systems Pilot Valve (Joystick) Caterpillar


Pilot Valve (Joystick)
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Illustration 1g03506930
Cab
(1) Joystick (left)
(2) Joystick (right)

When joystick (1) and/or joystick (2) are operated, the pilot control valves send pilot pump oil through the pilot lines to pilot ports at the main control valve in order to shift the spools in the main control valve.



Illustration 2g02959677
Pilot control valve
(1) Joystick
(2) Rod
(3) Return passage
(4) Passage (return pressure to valve)
(5) Passage
(6) Spool
(7) Plate
(8) Rod
(9) Spring
(10) Seat
(11) Seat
(12) Spring
(13) Spring
(14) Return chamber
(15) Return passage
(16) Passage
(17) Spool
(18) Port (reduced pressure to valve)
(19) Port (pilot supply pressure)
(20) Port (tank)

When joystick (1) is moved to the right, plate (7) tilts to the right. Plate (7) pushes down on rod (8). Seat (11) moves down against the force of metering spring (12) and spring (13). The force of metering spring (12) shifts spool (17) downward. Passage (16) opens. The pilot oil flows through passage (19), passage (16), and port (18) to the main control valve. The pilot oil pressure shifts the spool of the main control valve. This enables the implement operation or swing operation.

The return pilot oil at the opposite end of the spool in the main control valve returns to the pilot control valve through port (4). Since rod (2) is not pushed down by plate (7), return passage (3) is open and passage (5) is closed. The return pilot oil flows through return passage (3), return chamber (14), and port (20) to the hydraulic tank.

The force of metering spring (12) varies with the position of the joystick. Since spool (17) is moved by the force of metering spring (12), the pilot oil pressure that flows through passage (16) to the main control valve directly corresponds with the position of the joystick. Spool modulation in the main control valve directly corresponds with the amount of movement of the joystick.

When the joystick is moved slightly from the NEUTRAL position, metering spring (12) moves spool (17) slightly. Low pilot oil pressure is sent to the spool of the main control valve. The main control valve spool shifts a slight amount. The volume of oil delivery to the cylinders and/or motors is small. The speed of the cylinders and/or motors is slow. As the joystick is moved farther from the NEUTRAL position, the force of metering spring (12) on spool (17) increases. The pilot oil pressure that is sent to the main control valve increases. The spool in the main control valve shifts farther and the speed of the cylinders and/or motors increases. Thus, cylinder speed and motor speed is controlled by the amount of movement and the position of the joystick.

When the joystick is moved slightly from the NEUTRAL position, only metering spring (12) acts on spool (17). Fine control of the cylinders and/or motors is accomplished since the pilot oil pressure that is sent to the main control valve is decreased. As the joystick is moved farther from the NEUTRAL position, the bottom of rod (8) comes in contact with spring (9). Now, the combined force of metering spring (12) and spring (9) act on spool (17). The pilot oil pressure increases rapidly. The cylinders and/or motors respond more rapidly.

When the joystick is released, the joystick will return to the NEUTRAL position due to the force of spring (13).

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