16-2. PRINCIPLES OF OPERATION (CONT)
A two section gear pump draws hydraulic oil from the hydraulic reservoir through the suction strainers
whenever the engine is operating. The small section (1.48 cu. in.) of the gear pump supplies oil for the brake
system, frame tilt function, and charge pressure oil for the axial-piston pump.
From the small section of the gear pump, oil flows past the relief valve. If system pressure exceeds 1750 psi
(12066 kPa), oil pressure unseats the relief valve and allows excess oil to flow through the hydraulic filter
back to the reservoir.
Past the relief valve, oil flows through the brake valve, through the frame tilt valve and finally to the axial-
piston pump inlet port where the oil is used as charge pressure oil. The remaining oil flows back to the
The large section (3.94 cu. in.) of the gear pump supplies oil to the priority valve for the steering system and
to the main valve for boom hoist and extend functions. From the main valve, oil flows back through the
hydraulic filter and is returned to the reservoir.
Oil flows from the large section of the gear pump to the priority valve. The steering system is first priority,
receiving a portion of the total oil flow first. When the steering system is satisfied, oil pressure increases and
moves the spool to return the total oil flow to the main valve. A load sensing line from the steering valve to
the spring end of the priority spool monitors the pressure in the steering valve, to insure proper oil flow and
pressure is provided to the steering valve. When the steering cylinder bottoms, pressure in the steering valve
increases. The pilot pressure is relieved through the 2500 psi (17236 kPa) relief valve in the priority valve and
all the remaining pump flow is directed to the main valve.
The variable displacement, axial-piston pump supplies oil flow for attachment control valve which controls
fork, carriage, and attachment functions. The piston pump also supplies oil flow to the hydraulic joystick
through the shuttle valve. The shuttle valve closes the port from the emergency steer pump during normal
piston pump operation.
The piston pump utilizes specifically designed control valves which is the "brain" of the piston pump and its
related hydraulic functions. The valves control delivery of oil flow and pressure in amounts only as required
by the function that is operated. Both flow and pressure are tailored to the specific load. All adjustments by
the control valves occur in fractions of a second.
When functions of the attachment control valve are idle, the pump is in "stand-by" mode. The load sensing
line from the attachment control valve senses no load (zero pressure). Therefore, pump output pressure is
controlled only by spring force of the compensator (1ow pressure) spool spring. The pump will continue its low
output until the spring force of the compensator is overcome by the load sensing pressure. Oil from the
compensator spool holds the swash plate to near vertical position. The pump is allowed to supply only enough
oil flow to meet normal attachment control valve leakage and output pressure is maintained at approximately
400 psi (2758 kPa).
When one or more of the attachment control valves are a actuated, the pump goes into "pumping" mode. The
load sensing line from the attachment control valve is pressurized by the function experiencing the highest
pressure. This pressure is fed back to the compensator spool. The pump continues its output until the load
sensing pressure overcomes the compensator spool spring force. Oil from the compensator spool moves the
swash plate to an angle that will provide the pump with an output equal to the load sense pressure plus
approximately 400 psi (2758 kPa). Therefore the pump will supply only enough flow and pressure to perform