1. Basic functions
The pilot valve controls the stroke, direction and main control valve of the spool. The spring at one end of the main control valve spool performs the basic function of the pilot valve, and the output pressure (secondary pressure) of the pilot valve is supplied to the other end of the main control valve spool.
In order to perform this function correctly, the pilot valve has the following functions.
① The oil inlet (P) supplies oil from the hydraulic pump.
② The oil outlet sends the pressure of the oil inlet to the end of the control valve spool.
③ The oil tank port (T) controls the above output pressure.
④ The valve core connects the oil outlet to the oil inlet or the tank oil inlet.
⑤ The pilot valve controls the output pressure, including the spring acting on the above spool.
2. Functions of main parts
The function of the spool is to receive the oil pressure from the hydraulic pump at port P, and switch the oil circuit to determine whether the oil pressure of port P can reach the output port, or whether the pressure output of port P reaches the tank port T.
The spring acting on the spool determines the output pressure.
Variation of spring deviation, such as insufficient spring force of the spring grate, can cause the push rod to be inserted and slide in the plug.
The displacement of the push rod is changed by the switch board and the adjusting nut. The displacement is controlled by the handle. The handle can be tilted in any direction with the axis of the universal joint as the center.
The spring acts on the valve housing and spring seat and returns the push rod to the zero-discharge position as much as possible, ensuring that it is reset to the intermediate position.
Modern excavator operators feel the correct sense of control through the response of the spring.
3. Operation
Now explain the working principle of the pilot valve based on the hydraulic circuit diagram shown in Figure 1-85 and the cross-sectional diagram shown in 1-83 (b).
(1) The handle is in the middle position
As shown in Figure 1-86, the force of the spring determines that the output pressure of the pilot valve will not be applied to the spool. Therefore, the spool is pushed up to the port position by the spring. Because the output port is only connected to the tank port T, the output port pressure and the tank pressure become equal.
(2) The handle is tilted
As shown in Figure 1-87, when the push rod is pushed down, the spool moves downwards. Then, the oil port P is connected to the oil port, and the oil supplied by the pilot pump flows through the oil port to generate oil pressure.
When the pressure increase at the port is equivalent to the spring force set by the handle tilt, the pressure is balanced with the spring force. If the pressure at the port increases above the set pressure, port P will be disconnected from the port and port T will be connected to the port. If that port.
The pressure decreases below the set pressure, port P is connected to port, and port T is disconnected from port.
In this way, the secondary pressure always maintains a stable value. Moreover, when the handle tilts beyond--a fixed angle, the upper end of the spool contacts the inner bottom of the push rod, and the output pressure is connected to the oil port P.
