1.21.3. Screed Lift System. The screed lift system
consists of the screed lift switch on the operator switch
panel, right auxiliary hydraulic pump, 50/50 flow divider,
solenoid valves, and two hydraulic cylinders.
Power for the screed lift circuit is supplied from CBS.
From CB5 the power is sent to the screed lift switch on
the operator switch panel. The screed lift switch is a
momentary contact in the raise position and maintains
contact in the lower position. When the switch is placed
in the up, LIFT position, power is sent to the screed lift
valve solenoid and the power beyond valve solenoid.
When the screed lift switch is placed in the down,
FLOAT position, power is sent to a bleed valve solenoid
to allow the screed to float.
Hydraulic oil for the screed lift system is supplied from
the right auxiliary hydraulic pump. The right auxiliary
hydraulic pump contains two pumps, 0.7 and 0.58 cu in.
(11,5 and 9,5 cc) outputs, mounted on a common shaft.
The 0.58 cu in.
(9,5 cc) output pump supplies oil to a 50/50 flow divider.
The flow divider splits the hydraulic flow, sending half to
the stack valve.
Stack valve pressure is regulated at 2500 psi (16 472
kPa) by a pressure relief valve internal to the stack valve.
This pressure can be read at the test manifold. The
relief valve ports excess hydraulic oil to the return
manifold and back to the hydraulic reservoir. The oil that
is not routed through the relief valve is
routed to the power beyond valve and the screed lift
valve. In the normal, deenergized position, the power
beyond valve routes all hydraulic oil out of the stack
valve and to the track tensioning system. The power
beyond valve solenoid energizes whenever any hydraulic
function on the stack valve is operated. When the power
beyond valve solenoid is energized, the valve shifts and
does not allow any oil to leave the stack valve by way of
the track tensioning system. All hydraulic oil entering the
stack valve is therefore directed to the screed lift valve.
This temporarily removes hydraulic oil from the track
The screed lift valve is normally deenergized, allowing all
hydraulic oil to flow to the stack valve supply header.
When the screed lift switch on the operator switch panel
is placed in the up, LIFT position, the screed lift valve
shifts and redirects all hydraulic oil to the screed lift
system. The hydraulic oil is sent to the screed lift
cylinders causing the screed to raise. When the screed
lift switch is placed in the down, FLOAT position, a bleed
valve is opened. This allows the hydraulic oil in the
screed lift cylinders to return to the return manifold and
then to the hydraulic reservoir. The screed travel lock
valve installed in the supply line for the screed lift
cylinders can be used to prevent the screed from
lowering. When the valve is shut, the bypass check
valve will seat when hydraulic oil from the cylinders
attempts to flow to the return manifold. The screed may
not be lowered until the screed travel lock valve is
opened. The bypass check valve on the screed travel
lock valve allows hydraulic pressure from the stack valve
to raise the screed even when the screed travel lock
valve is shut.