H Y D R A U L I C
S Y S T E M
TM 5-3805-258-24-1
S Y S T E M S
O P E R A T I O N
Oil from the implement pump section goes
through inlet passage (10) around tilt valve spool
(11) before it goes to lift valve spool (16). The lift
valve spool is a spring-centered, open center, pilot
operated spool with three positions: RAISE, HOLD
and LOWER. Make-up valve (19) provides the
FLOAT operation of the lift circuit. Springs (18)
keep lift valve spool (16) in HOLD position when
there is no pilot pressure oil in chambers (17) and
(27). Movement of the spool is controlled by pilot
pressure oil from the lift and tilt pilot valve in
chambers (17) and (27).
When lift valve spool (16) is in HOLD position,
implement oil goes from inlet passage (10), through
passage (14) and outlet passage (15) to the hydraulic
tank. The position of the spools holds (blocks) the oil
in both ends of the lift cylinders and keeps the cy-
linders from movement.
RAISE Operation
When the lift control lever is moved to RAISE posi-
tion, pilot oil is sent to chamber (17) and lift valve
spool (16) moves to the right. Implement oil is sent
from inlet passage (10) to load check valve (25). The
load check valve does not open until the oil pressure
becomes greater than the force of spring (26) and the
head end pressure of the lift cylinders. The oil opens
load check valve (25), goes through passage (24) and
out passage (23) to the head end of the lift cylinders.
This causes the bucket (lift arms) to lift.
Oil from the rod end of the lift cylinders comes
through passage (22), goes around lift valve spool
(16), through passage (13) and outlet passage (15) to
the hydraulic tank.
LOWER Operation
When the lift control lever is moved to LOWER
position, pilot oil goes to chamber (27) and lift valve
spool (16) moves to the left. Oil flow to the lift
cylinders is now the reverse of oil flow in RAISE
position. Implement oil is sent from inlet passage
(10) to load check valve (25). The load check valve
opens when inlet pressure becomes greater than the
force of spring (26) and the rod end pressure of the
lift cylinders. The oil now goes through passage (24)
and out passage (22) to the rod end of the lift cy-
linders. This causes the bucket (lift arms) to lower.
During this operation when the lift cylinder rods
retract faster than the pump can provide oil to the rod
ends, make-up valve (19) moves against spring (21)
and the pressure of the oil in vent passage (20). This
permits oil from the tank to add to the implement oil
and prevents cavitation (vacuum) in the system.
Oil from the head end of the lift cylinders comes
through passage (23), goes around lift valve spool
(16) and through outlet passage (15) to the hydraulic
tank.
FLOAT Operation
When the lift control lever is moved to FLOAT
position, the movement of lift valve spool (16) is the
same as in LOWER position. The lift and tilt pilot
valve opens vent passage (20) to a tank passage and
make-up valve (19) is free to open. Implement oil
pressure holds the make-up valve open. The pump oil
from passage (24) and the oil in passage (22) now
goes through passage (13) and outlet passage (15) to
the hydraulic tank. The lift cylinders can now move
freely in either direction according to the amount and
direction of force on the bucket. As the cylinder rods
move, all extra oil goes out passage (15) to the hy-
draulic tank and pump oil comes in from inlet pas-
sage (10).
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