Power Down Operation - Komatsu 930E-2 Shop Manual

POWER DOWN OPERATION (Figure 7-8)
When the operator moves the lever to lower the body,
the hoist pilot valve is positioned to direct the pilot
supply oil in ports (12) through ports (15) to the top of
the rod end spool (7).
Pilot pressure increases to move the spool down com-
pressing the bottom spring. Movement of the spool
connects the high pressure passage (19) to the rod
end (annulus area) of the hoist cylinders. At the same
time, the flow control valve (2) is forced to close as pilot
pressure increases thus directing the incoming pump
oil to the hoist cylinders through Spool (7) and check
valve in the overcenter manifold rather than back to
the tank. Oil flow to the brake cooling circuit ceases
when the flow control valve closes.
If the body is at the maximum up position, the hoist limit
switch has the hoist limit solenoid activated, therefore
closing the raise port (14) on the hoist valve. Power
down pilot pressure in ports (15) pushes open the pilot
operated check valve (21) located in the bleeddown
manifold so the pilot pressure in ports (14) is open to
tank through the Pilot valve spool.
L7-12
As oil attempts to return from the head end of the hoist
cylinders, it initially encounters the closed head end
spool (8). Pressure increases on the bottom end of the
spool causing it to move upward. This allows the
returning oil to go into the low pressure passage (20),
build up 26 psi (179 kPa) to open the primary low
pressure relief (3), and exit the hoist valve through
Port (10) to the tank.
As the body descends and the hoist limit solenoid is
no longer activated, the pilot operated check valve is
no longer necessary.
Hoist Circuit L07023