U20-3, U25-3 WSM Hydraulic system (Mechanism section)
IV-M-20
(3) Boom down (with port Pb under pressure)
When the boom is lowered, the oil from the pilot valve is introduced into the port Pb and the control valve
spool is moved to the right. The hydraulic oil from the pump flows through parallel passage, pushing load
check valve. The oil further goes through the port B.
The oil from the pilot valve, on the other hand, is also introduced to the port Pi, pushing down the piston,
pin and poppet "d". The oil in the chamber Z flows through the poppet "d" and the pin's internal hole and
then via the drain port back into the tank.
Now the pressure in the chamber Z goes down, and the return oil from the port A pushes open the anti-
drift poppet. It further flows through the passage a, the spool’s throttle and the tank passage back into the
tank.
From P/V
Piston
To drain
Pin
Flowing via this hole into the pin
Poppet d
Flowing out of this hole
Flowing through this pin hole
Oil flowing from here
Section B-B
Port Pi
Port DR
Poppet d
Passege a
From port "A"
Anti-drift poppet
Chamber Z
Section A-A
Port B
Parallel Passage
Pa
Pb
Load check valve
Port T
Port A
To tank
From pilot valve
BUCK
BOOM
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