TM 5-3805-290-23-1
THEORY OF OPERATION - CONTINUED
0003 00
HYDRAULIC SYSTEM - CONTINUED
c.
LOWER Position.
(1)
When the lift circuit is the only hydraulic circuit in operation, the tilt circuit will have the highest signal oil
pressure in the hydraulic system. When the pilot valve is moved to the LOWER position, pilot oil enter the
control valve through port (24). The pilot oil flows through the shock reducing check valve (23). The
shock reducing check valve moves to the right.
(2)
Pilot oil flows into chamber (25) and passage (28). As the pilot oil moves spool (27) to the right, spring (9)
is compressed. When the spool moves to the right, pump supply oil flows over the metering slots in the
spool and into passage (17). Also, pilot oil opens the pilot operated check valve (12).
(3)
The hydraulic oil in passage (17) flows to pressure compensator valve (6) and passage (29). In the pressure
compensator valve, the hydraulic oil flows into internal passages. The pressure compensator valve moves
up. As the pressure compensator valve moves up, the cross-drilled passage is opened to passage (4). Some
of the hydraulic oil flows around the pressure compensator valve to spring (3). The hydraulic oil that flows
to spring and passage (4) is signal oil. Spring and the signal oil balance the pressure compensator valve
against the hydraulic oil pressure in passage (17). This controls the flow across spool (27).
(4)
The signal oil in passage (4) flows to the pressure compensator valve in each control valve. The signal oil
also flows to the pressure and flow compensator valve on the hydraulic pump. The signal oil helps to con-
trol the output of the hydraulic pump. The hydraulic pump upstrokes and the hydraulic oil pressure in pas-
sage (17) increases. When the hydraulic oil pressure in passage raises above circuit pressure, the hydraulic
oil unseats load check valve (21) and flows to passages (14 and 19).
(5)
In passage (14), spool (27) blocks the flow of oil. The hydraulic oil flows to passage (20) and through port
(2) to the rod end of the lift cylinders.
(6)
Hydraulic oil from the head end of the lift cylinders flows through port (5) to the pilot operated check
valve (12). The hydraulic oil flows through the pilot operated check valve to passage (13). The hydraulic
oil flows around spool (27) into passages (8 and 15).
(7)
The hydraulic oil in passage (8) flows to all control valves as makeup oil. The hydraulic oil in passage also
flows through passage (30) to back pressure check valve (1).
(8)
Back pressure check valve (1) allows hydraulic oil pressure to develop in passages (8 and 15). This
hydraulic oil pressure allows the regeneration check valve to operate. When the oil pressure overcomes the
force of the spring in the back pressure check valve, the back pressure check valve opens. The hydraulic
oil flows through the valve. Then, the hydraulic oil flows to the tank.
(9)
The hydraulic oil in passage (15) flows to the regeneration check valve (18). When the pressure in passage
overcomes the force of the spring in the regeneration check valve, the regeneration check valve opens.
Hydraulic oil flows into passage (19).
(10)
In passage (19), the hydraulic oil from passage (15) combines with hydraulic oil from passage (29). Then,
the hydraulic oil flows around spool (27) into passage (20). The hydraulic oil flows through port (2) to the
rod end of the lift cylinders.
(11)
When the operator releases the pilot valve, the flow of pilot oil to the pilot valve moves the shock reducing
check valve to the left. The hydraulic oil flows through an orifice. This dampens the flow of hydraulic oil
to the pilot valve. The spool slowly closes off the hydraulic oil to the circuit. Pilot operated check valve
(12) closes. Spring (9) moves spool (27) to the HOLD position. Pilot operated check valve and load check
valve (21) block the hydraulic oil in the hydraulic lines.
0003 00-180
