THEORY OF OPERATION - CONTINUED
HYDRAULIC SYSTEM - CONTINUED
When the lift circuit is the only hydraulic circuit in operation, the lift circuit will have the highest signal oil
pressure in the hydraulic system. When the pilot valve is moved to the RAISE position, pilot oil enters the
control valve through port (10). The pilot oil flows into chamber (11). As the pilot oil moves spool (27) to
the left, spring (26) is compressed.
Pump supply oil flows from passage (16) over the metering slots in the spool to passage (17). The hydrau-
lic oil from passage flows to the pressure compensator valve (6) and passage (29). In the pressure compen-
sator 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).
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 (20) and flows to passages (14 and 19). In passage, the hydraulic oil flows
around spool (27) to the regeneration check valve (18). The regeneration check valve blocks the flow of
the hydraulic oil.
In passage (14), hydraulic oil flows around the spool (27) into passage (13). The hydraulic oil flows to the
pilot operated check valve (12). The hydraulic oil flows through the pilot operated check valve. Then, the
hydraulic oil flows through port (5) to the head end of the lift cylinders.
Hydraulic oil flows from the rod end of the lift cylinder flows through port (2) into passage (20). The
hydraulic oil flows around spool (27) into passage (22). Then, the hydraulic oil flows to the tank.
When the operator releases the pilot valve, the pilot oil flows to the right. Pilot operated check valve (12)
closes. Spring (26) moves spool (27) to the HOLD position. Pilot operated check valve and load check
valve (21) block the hydraulic oil in the hydraulic lines.