Function; Basic Function; Functions Of Major Parts - Kubota U20-3 Workshop Manual

U20-3, U25-3 WSM
c. Function

1. Basic function

The pilot valve is intended to control the stroke and direction of the control valve spool. To do this, the
output pressure of the pilot valve is applied to the end of the control valve spool.
To achieve this function, the pilot valve consists of the following elements.
(1) Inlet port (P) used to supply oil from the hydraulic pump
(2) Multiple outlet ports (1, 2, 3 and 4) used to apply the feed pressure of the inlet port to the end of
the control valve spool.
(3) Tank port (T) required to control the output pressure discussed above
(4) Spool used to connect the output ports to the inlet port or tank port
(5) Mechanism (including the spring for the above spool) used to control the output pressure

2. Functions of major parts

The spool (201) is designed to switch between two oil passages: one passage is for the pressure oil
from the hydraulic pump to go to the port P first and then to the output ports (1, 2, 3 and 4). The other
is for the pressure oil to go from the output port to the port T. The secondary pressure setting spring
(241) exerts upon this spool (201) to determine the output pressure.
The push rod (212) is integrated in the plug (211) and ready to slide to change the deflection of the
secondary pressure setting spring (241).
The return spring (221) exerts upon the casing (101) and spring seat (216) to keep the push rod (212)
free of the output pressure and pressed in the zero-displacement direction. This ensures the neutral
return of the spool (201). This spring serves also as reaction spring to give the operator proper oper-
ating feel.
Hydraulic system (Mechanism section)
IV-M-30