The pump is in "maximum high pressure" mode when load sensing line pressure reaches approximately
3625 psi (24994.4 kPa). The high pressure spool compresses the spring and allows oil to move the swash plate
to near vertical position. Oil flow from the compensator spool is stopped and now the pump will supply oil flow
to meet circuit leakage and output pressure is maintained at approximately 3625 psi (24994.4 kPa).
The piston pump is connected to the reservoir so any additional oil required is readily available. Any by-pass
oil in the pump can be returned to the reservoir through the drain line.
Return oil from the main valve, steering valve, relief valve of the priority valve, brake valve, and relief valve
passes through the hydraulic filter before entering the reservoir. The filter is equipped with a bypass which
has a visual indicator showing when bypass is occurring.
b. Frame Tilt System. The frame tilt valve has three operating positions, tilt right, tilt left, and hold. The
center position is the hold position.
With the frame tilt valve lever in the hold (center) position, the hydraulic oil is trapped in the cylinder and
lines. The cylinder cannot move. Oil flow from the brake valve, which originates from the small section of the
gear pump, continues to the piston pump inlet port and returns to the reservoir through the frame tilt valve.
By moving the frame tilt valve lever forward, the cylinder is extended and tilts the machine to the left. Oil
flows to the counterbalance valve for the head end of the cylinder, unseats the ball check, and enters the
cylinder. Pressure in the head end begins to increase because oil in the rod end of the cylinder is trapped; the
rod end counterbalance valve is closed and ball is seated. The pressure in the head end pilots open the
counterbalance valve for the rod end. Oil can now flow from the rod end of the cylinder, allowing the cylinder
to extend. Oil from the rod end of the cylinder continues on to the piston pump inlet port and returns to the
Moving the frame tilt valve rearward, the cylinder is retracted and tilts the machine to the right. Oil flows to
the rod end of the cylinder and pilots open the head end counterbalance valve. Oil flows through the tilt
cylinder in the opposite direction of the tilt left function. Oil from the head end is routed to the piston pump
inlet port and the reservoir.
All counterbalance valves in hydraulic cylinders also serve as a safety device in case of hose burst or power
loss. The counterbalance valves require pressure to pilot them open which will then allow oil to flow. If
pressure is removed the counterbalance valve closes and stops the oil from flowing; the cylinder is "locked up".
c. Boom Hoist System. The boom hoist function is controlled by the hydraulic joystick and the main
valve. Pilot oil for the hydraulic joystick is routed through the shuttle valve from the piston pump. The pilot
return oil is routed to the hydraulic reservoir.
As the hydraulic joystick is moved towards the rear, pilot pressure oil is allowed to flow into the left bottom
pilot port of the main valve. The pilot pressure oil pushes the left (boom) spool up, allowing oil to flow through
the main valve to the lower work port for the hoist cylinders. Oil flows to the counterbalance valve in the
cylinders, unseats the ball check, and enters the head end of the cylinders. The oil pressure pushes the
pistons up extending the cylinders, thereby raising the boom. Oil in the rod end of the cylinders is routed
through ball check of the flow control, the main valve and to the reservoir through the hydraulic filter.
With the hydraulic joys tick in the center position, the pilot oil flow is blocked and each valve in the joystick is
connected to the return line to the hydraulic reservoir. The spools of the main control valve are kept in the
center position by springs which provides passage for the oil through the valve and back to the reservoir
through the hydraulic filter.