POWER UP OPERATION (Figure 7-6)
The hoist pilot valve spool is moved to the Power Up
position when the operator moves the lever rearward
in the cab. The pilot supply oil coming in port (12) is
prevented from returning to the tank and, instead, is
directed out port (14) through hoist limit solenoid (13)
in the bleeddown manifold and into port (14) of the
hoist valve.
From there it goes to the top of the head end spool (8),
builds pressure on the end of the spool, causes the
spool to move down compressing the bottom spring,
and connects the high pressure passage (19) to head
end port (9). Working oil flow in the high pressure
passage is now allowed to flow through the spool (8)
and out port (9) to extend the hoist cylinders.
Even though a small amount of oil flows through the
check poppet in the top of Spool (8), raise pilot pres-
sure at ports (14) increases to slightly higher pressure
than the required hoist cylinder pressure. As a result,
the pilot supply pressure in ports (12) also increases
causing back pressure to occur in the spring area of
the flow control valve (2). This overcomes the pilot
pressure on the other end of the flow control valve
causing it to close and direct the incoming pump oil
through the head end spool (8) to the hoist cylinders
to extend them. At this time, oil flow to the brake cooling
circuit is stopped.
If at any time the resistance to the flow of the pump oil
coming into the inlet section causes the pressure to
increase to 2500 psi (17,238 kPa), the pilot pressure
against hoist relief valve (1) causes it to open and allow
flow to exit out port (23) to the brake cooling circuit and
to return to the tank.
As the hoist cylinders extend, oil in the annulus area
of all three stages must exit sequentially from the
cylinders. Initially, the rod end spool (7) ports are
closed. As the returning oil entering port (5) builds low
pressure, it flows through the check-poppet in the top
of the spool, through Ports (15), through the pilot valve
spool, and out port (10) of the pilot valve to the bleed-
down manifold and to the tank.
No pressure is present on the top of spool (7). Cylinder
return pressure passes through the check-poppet in
the bottom of spool (7) to build pressure under the
spool which moves the spool upward compressing the
top spring. This movement allows the returning cylin-
der oil to flow into the low pressure passage (20) to
the primary low pressure relief valve (22). Approxi-
mately 26 psi (179 kPa) causes this valve to open,
allowing the oil to flow out port (10) to the tank.
If the load passing over the tail of the body during
dumping attempts to cause the body to raise faster
than the oil being supplied by the pump, the oil return-
ing from the annulus area of the hoist cylinders passing
through the conterbalance valve manifold controls how
fast the hoist cylinders can extend due to the external
force of the load. The speed is controlled by counter-
balance valve.
During the normal raise process, the raise pilot pres-
sure supplied to the counterbalance valve (24) will
open the cylinder return ports fully any time raise
pressure is above 666 psi (4,600 kPa). As the load
shifts toward the tail of the body, the raise pressure
decreases. As it drops below 666 psi (4,600 kPa), the
valve starts restricting oil flow causing a backpressure
to the annulus area. If raise pressure were to drop to
0 psi (kPa), return pressure would increase to 3000 psi
(20,400 kPa) maximum.
When the operator releases the hoist lever, the valves
change to the HOLD position. If the body raises to the
position that activates the hoist limit switch located
above the right rear suspension before the operator
releases the lever, the hoist limit solenoid (13) is ener-
gized. The solenoid valve closes the raise pilot Port
(14) on the hoist and releases the hoist pilot valve raise
pilot pressure at port (14) to tank, allowing the head
end spool (8) to center and shut off supply of oil to the
hoist cylinders. This prevents maximum extension of
the hoist cylinders.
L7-8 Hoist Circuit L07023
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